AU2015227493A1 - Ultrasonic device for cutting and coagulating with stepped output - Google Patents

Abstract

Various embodiments are directed to an apparatus, system, and method for driving an end effector coupled to an ultrasonic transducer in a surgical instrument. The method comprises 5 generating a first ultrasonic drive signal by a generator coupled to an ultrasonic drive system, actuating the ultrasonic transducer with the first ultrasonic drive signal for a first period, generating a second ultrasonic drive signal by the generator, and actuating the ultrasonic transducer with the second ultrasonic drive signal for a second period, subsequent to the first period. The first drive signal is different from the second drive signal over the respective first 10 and second periods. The first and second drive signals define a step function waveform over the first and second periods. The apparatus comprises a generator configured to couple to an ultrasonic instrument. The system comprises a generator coupled to an ultrasonic instrument. The ultrasonic instrument comprises an ultrasonic drive system comprising an ultrasonic transducer coupled to a waveguide and an end effector coupled to the waveguide, and wherein 15 the ultrasonic drive system is configured to resonate at a resonant frequency.

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Ultrasonic device for cutting and coagulating with stepped output The invention is described in the following statement: ULTRASONIC DEVICE FOR CUTTING AND COAGULATING WITH STEPPED OUTPUT CROSS REFERENCE TO RELATED APPLICATION This application claims the benefit under Title 35, United States Code § 119(e), of (1) U.S. 5 Provisional Patent Application Ser. No. 61/086619, filed August 6, 2008 and entitled "Ultrasonic Device for Cutting and Coagulating with Stepped Output" and (2) U.S. Provisional Patent Application Ser. No. 61/188790, filed August 13, 2008 and entitled "Ultrasonic Device for Cutting and Coagulating with Stepped Output," both of which are hereby incorporated by reference in its entirety. 10 TECHNICAL FIELD The present disclosure generally relates to ultrasonic surgical systems and, more particularly, to an ultrasonic system that allows surgeons to perform cutting and coagulation. BACKGROUND Ultrasonic surgical instruments are finding increasingly widespread applications in 15 surgical procedures by virtue of the unique performance characteristics of such instruments. Depending upon specific instrument configurations and operational parameters, ultrasonic surgical instruments can provide substantially simultaneous cutting of tissue and homeostasis by coagulation, desirably minimizing patient trauma. The cutting action is typically realized by an end effector, or blade tip, at the distal end of the instrument, which transmits ultrasonic energy to 20 tissue brought into contact with the end effector. Ultrasonic instruments of this nature can be configured for open surgical use, laparoscopic, or endoscopic surgical procedures including robotic-assisted procedures. Some surgical instruments utilize ultrasonic energy for both precise cutting and controlled coagulation. Ultrasonic energy cuts and coagulates by using lower temperatures than those used 25 by electrosurgery. Vibrating at high frequencies (e.g., 55,500 times per second), the ultrasonic blade denatures protein in the tissue to form a sticky coagulum. Pressure exerted on tissue with the blade surface collapses blood vessels and allows the coagulum to form a hemostatic seal. The precision of cutting and coagulation is controlled by the surgeon's technique and adjusting the power level, blade edge, tissue traction, and blade pressure. 30 A primary challenge of ultrasonic technology for medical devices, however, continues to be sealing of blood vessels. Work done by the applicant and others has shown that optimum -1vessel sealing occurs when the inner muscle layer of a vessel is separated and moved away from the adventitia layer prior to the application of standard ultrasonic energy. Current efforts to achieve this separation have involved increasing the clamp force applied to the vessel. Furthermore, the user does not always have visual feedback of the tissue being cut. 5 Accordingly, it would be desirable to provide some form of feedback to indicate to the user that the cut is complete when visual feedback is unavailable. Moreover, without some form of feedback indicator to indicate that the cut is complete, the user may continue to activate the harmonic instrument even though the cut is complete, which cause possible damage to the harmonic instrument and surrounding tissue by the heat that is generated exponentially when 10 activating a harmonic instrument with nothing between the jaws. It would be desirable to provide an ultrasonic surgical instrument that overcomes some of the deficiencies of current instruments. The ultrasonic surgical instrument described herein overcomes those deficiencies. SUMMARY 15 In one general aspect, an ultrasonic surgical instrument assembly embodying the principles of the described embodiments is configured to permit selective dissection, cutting, coagulation, and clamping of tissue during surgical procedures. In one embodiment, an end effector is coupled to an ultrasonic drive system of a surgical instrument. A generator coupled to an ultrasonic drive system generates a first ultrasonic drive signal. The ultrasonic drive system 20 comprises an ultrasonic transducer coupled to a waveguide and an end effector coupled to the waveguide. The ultrasonic drive system is configured to resonate at a resonant frequency. The ultrasonic transducer is actuated with the first ultrasonic drive signal for a first period. The generator generates a second ultrasonic drive signal. The ultrasonic transducer is actuated with the second ultrasonic drive signal for a second period, subsequent to the first period. The first 25 drive signal is different from the second drive signal over the respective first and second periods. The first and second drive signals define a step function waveform over the first and second periods. FIGURES The novel features of the described embodiments are set forth with particularity in the 30 appended claims. The described embodiments, however, both as to organization and methods of operation, may be best understood by reference to the following description, taken in conjunction with the accompanying drawings in which: -2- FIG. 1 is a perspective view illustrating one embodiment of an ultrasonic surgical instrument. FIG. 2 is a perspective assembly view of one embodiment of an ultrasonic surgical instrument. 5 FIG. 3 is a schematic of one embodiment of a clamp arm illustrating force calculations. FIG. 4 is a graphical representation of current, voltage, power, impedance, and frequency waveforms of a conventional oscillator at high power and lightly loaded. FIG. 5 is a graphical representation of current, voltage, power, impedance, and frequency waveforms of a conventional oscillator at high power and heavily loaded. 10 FIG. 6 is a graphical representation of a current step function waveform and voltage, power, impedance, and frequency waveforms of one embodiment of an oscillator and unloaded. FIG. 7 is a graphical representation of a current step function waveform and voltage, power, impedance, and frequency waveforms of one embodiment of an oscillator and lightly loaded. 15 FIG. 8 is a graphical representation of a current step function waveform and voltage, power, impedance, and frequency waveforms of one embodiment of an oscillator and heavily loaded. FIG. 9 illustrates one embodiment of a drive system of a generator, which creates the ultrasonic electrical signal for driving an ultrasonic transducer. 20 FIG. 10 illustrates one embodiment of a surgical system comprising an ultrasonic surgical instrument and a generator comprising a tissue impedance module. FIG. 11 illustrates one embodiment of a drive system of a generator comprising a tissue impedance module. FIG. 12 illustrates one embodiment of a clamp arm assembly that may be employed with a 25 surgical system. FIG. 13 is a schematic diagram of a tissue impedance module coupled to a blade and a clamp arm assembly with tissue located therebetween. FIG. 14 illustrates one embodiment of a method for driving an end effector coupled to an ultrasonic drive system of a surgical instrument. 30 FIG. 15A illustrates a logic flow diagram of one embodiment of determining a change in tissue state and activating an output indicator accordingly. FIG. 15B is a logic flow diagram illustrating one embodiment of the operation of the frequency inflection point analysis module. -3- FIG. 15C is a logic flow diagram 900 illustrating one embodiment of the operation of the voltage drop analysis module. DESCRIPTION Before explaining various embodiments of ultrasonic surgical instruments in detail, it 5 should be noted that the illustrative embodiments are not limited in application or use to the details of construction and arrangement of parts illustrated in the accompanying drawings and description. The illustrative embodiments may be implemented or incorporated in other embodiments, variations and modifications, and may be practiced or carried out in various ways. Further, unless otherwise indicated, the terms and expressions employed herein have been chosen 10 for the purpose of describing the illustrative embodiments for the convenience of the reader and are not for the purpose of limitation thereof. Further, it is understood that any one or more of the following-described embodiments, expressions of embodiments, examples, can be combined with any one or more of the other following-described embodiments, expressions of embodiments, and examples. 15 Various embodiments are directed to improved ultrasonic surgical instruments configured for effecting tissue dissecting, cutting, and/or coagulation during surgical procedures. In one embodiment, an ultrasonic surgical instrument apparatus is configured for use in open surgical procedures, but has applications in other types of surgery, such as laparoscopic, endoscopic, and robotic-assisted procedures. Versatile use is facilitated by selective use of ultrasonic energy. 20 The various embodiments will be described in combination with an ultrasonic instrument as described herein. Such description is provided by way of example, and not limitation, and is not intended to limit the scope and applications thereof For example, any one of the described embodiments is useful in combination with a multitude of ultrasonic instruments including those described in, for example, U.S. Patent Nos. 5,938,633; 5,935,144; 5,944,737; 5,322,055; 25 5,630,420; and 5,449,370. As will become apparent from the following description, it is contemplated that embodiments of the surgical instrument described herein may be used in association with an oscillator unit of a surgical system, whereby ultrasonic energy from the oscillator unit provides the desired ultrasonic actuation for the present surgical instrument. It is also contemplated that 30 embodiments of the surgical instrument described herein may be used in association with a signal generator unit of a surgical system, whereby electrical energy in the form of radio frequencies (RF), for example, is used to provide feedback to the user regarding the surgical instrument. The ultrasonic oscillator and/or the signal generator unit may be non-detachably integrated with the -4surgical instrument or may be provided as separate components, which can be electrically attachable to the surgical instrument. One embodiment of the present surgical apparatus is particularly configured for disposable use by virtue of its straightforward construction. However, it is also contemplated that other 5 embodiments of the present surgical instrument can be configured for non-disposable or multiple uses. Detachable connection of the present surgical instrument with an associated oscillator and signal generator unit is presently disclosed for single-patient use for illustrative purposes only. However, non-detachable integrated connection of the present surgical instrument with an associated oscillator and/or signal generator unit is also contemplated. Accordingly, various 10 embodiments of the presently described surgical instruments may be configured for single use and/or multiple use with either detachable and/or non-detachable integral oscillator and/or signal generator unit, without limitation, and all combinations of such configurations are contemplated to be within the scope of the present disclosure. With reference to FIGS. 1-3, one embodiment of a surgical system 19 including an 15 ultrasonic surgical instrument 100 is illustrated. The surgical system 19 includes an ultrasonic generator 30 connected to an ultrasonic transducer 50 via a suitable transmission medium such as a cable 22, and an ultrasonic surgical instrument 100. Although in the presently disclosed embodiment, the generator 30 is shown separate from the surgical instrument 100, in one embodiment, the generator 30 may be formed integrally with the surgical instrument 100 to form 20 a unitary surgical system 19. The generator 30 comprises an input device 406 located on a front panel of the generator 30 console. The input device 406 may comprise any suitable device that generates signals suitable for programming the operation of the generator 30 as subsequently described with reference to FIG. 9. Still with reference to FIGS. 1-3, the cable 22 may comprise multiple electrical conductors for the application of electrical energy to positive (+) and negative 25 (-) electrodes of the ultrasonic transducer 50. It will be noted that, in some applications, the ultrasonic transducer 50 may be referred to as a "handle assembly" because the surgical instrument 100 of the surgical system 19 may be configured such that a surgeon may grasp and manipulate the ultrasonic transducer 50 during various procedures and operations. A suitable generator 30 is the GEN 300 sold by Ethicon Endo-Surgery, Inc. of Cincinnati, Ohio as is 30 disclosed in one or more of the following U.S. Patents, all of which are incorporated by reference herein: U.S. Patent No. 6,480,796 (Method for Improving the Start Up of an Ultrasonic System Under Zero Load Conditions); U.S. Patent No. 6,537,291 (Method for Detecting a Loose Blade in a Handle Connected to an Ultrasonic Surgical System); U.S. Patent No. 6,626,926 (Method for Driving an Ultrasonic System to Improve Acquisition of Blade Resonance Frequency at Startup); -5- U.S. Patent No. 6,633,234 (Method for Detecting Blade Breakage Using Rate and/or Impedance Information); U.S. Patent No. 6,662,127 (Method for Detecting Presence of a Blade in an Ultrasonic System); U.S. Patent No. 6,678,621 (Output Displacement Control Using Phase Margin in an Ultrasonic Surgical Handle); U.S. Patent No. 6,679,899 (Method for Detecting 5 Transverse Vibrations in an Ultrasonic Handle); U.S. Patent No. 6,908,472 (Apparatus and Method for Altering Generator Functions in an Ultrasonic Surgical System); U.S. Patent No. 6,977,495 (Detection Circuitry for Surgical Handpiece System); U.S. Patent No. 7,077,853 (Method for Calculating Transducer Capacitance to Determine Transducer Temperature); U.S. Patent No. 7,179,271 (Method for Driving an Ultrasonic System to Improve Acquisition of Blade 10 Resonance Frequency at Startup); and U.S. Patent No. 7,273,483 (Apparatus and Method for Alerting Generator Function in an Ultrasonic Surgical System). In accordance with the described embodiments, the ultrasonic generator 30 produces an electrical signal of a particular voltage, current, and frequency, e.g. 55,500 cycles per second (Hz). The generator is 30 connected by the cable 22 to the handle assembly 68, which contains 15 piezoceramic elements forming the ultrasonic transducer 50. In response to a switch 312a on the handle assembly 68 or a foot switch 434 connected to the generator 30 by another cable the generator signal is applied to the transducer 50, which causes a longitudinal vibration of its elements. A structure connects the transducer 50 to a surgical blade 79, which is thus vibrated at ultrasonic frequencies when the generator signal is applied to the transducer 50. The structure is 20 designed to resonate at the selected frequency, thus amplifying the motion initiated by the transducer 50. In one embodiment, the generator 30 is configured to produce a particular voltage, current, and/or frequency output signal that can be stepped with high resolution, accuracy, and repeatability. Referring to Fig. 4, in current systems a conventional oscillator is activated at time 0 25 resulting in current 300 rising to a desired set point of approximately 340mA. At approximately 2 seconds a light load is applied resulting in corresponding increases to voltage 310, power 320, impedance 330, and changes in resonant frequency 340. Referring to Fig. 5, in current systems a conventional oscillator is activated at time 0 resulting in the current 300 rising to a desired set point of approximately 340mA. At 30 approximately 2 seconds an increasing load is applied resulting in corresponding increases to the voltage 310, power 320, impedance 330, and changes in resonant frequency 340. At approximately 7 seconds, the load has increased to the point that the oscillator enters into a flat power mode where further increases in load maintain the power at 35W as long as the oscillator stays within voltage limits of the power supply. The current 300 and therefore, displacement, -6varies during flat power mode. At approximately 11.5 seconds, the load is reduced to the point where the current 300 returns to the desired set point of approximately 340mA. The voltage 310, power 320, impedance 330, and resonant frequency 340 vary with the load. With reference now back to FIGS. 1-3, the ultrasonic surgical instrument 100 includes a 5 multi-piece handle assembly 68 adapted to isolate the operator from the vibrations of the acoustic assembly contained within the ultrasonic transducer 50. The handle assembly 68 can be shaped to be held by a user in a conventional manner, but it is contemplated that the present ultrasonic surgical instrument 100 principally be grasped and manipulated by a trigger-like arrangement provided by a handle assembly of the instrument, as will be described. While a multi-piece 10 handle assembly 68 is illustrated, the handle assembly 68 may comprise a single or unitary component. The proximal end of the ultrasonic surgical instrument 100 receives and is fitted to the distal end of the ultrasonic transducer 50 by insertion of the transducer 50 into the handle assembly 68. In one embodiment, the ultrasonic surgical instrument 100 may be attached to and removed from the ultrasonic transducer 50 as a unit. In other embodiments, the ultrasonic 15 surgical instrument 100 and the ultrasonic transducer 50 may be formed as an integral unit. The ultrasonic surgical instrument 100 may include a handle assembly 68, comprising a mating housing portion 69, a housing portion 70, and a transmission assembly 71. When the present instrument is configured for endoscopic use, the construction can be dimensioned such that the transmission assembly 71 has an outside diameter of approximately 5.5mm. The elongated 20 transmission assembly 71 of the ultrasonic surgical instrument 100 extends orthogonally from the instrument handle assembly 68. The transmission assembly 71 can be selectively rotated with respect to the handle assembly 68 by a rotation knob 29 as further described below. The handle assembly 68 may be constructed from a durable plastic, such as polycarbonate or a liquid crystal polymer. It is also contemplated that the handle assembly 68 may alternatively be made from a 25 variety of materials including other plastics, ceramics, or metals. The transmission assembly 71 may include an outer tubular member or an outer sheath 72, an inner tubular actuating member 76, a waveguide 80, and an end effector 81 comprising, for example, the blade 79, a clamp arm 56, and one or more clamp pads 58. As subsequently described, the outer sheath 72, the actuating member 76, and the waveguide 80 or transmission 30 rod may be joined together for rotation as a unit (together with the ultrasonic transducer 50) relative to the handle assembly 68. The waveguide 80, which is adapted to transmit ultrasonic energy from the ultrasonic transducer 50 to the blade 79 may be flexible, semi-flexible, or rigid. The waveguide 80 also may be configured to amplify the mechanical vibrations transmitted through the waveguide 80 to the blade 79 as is well known in the art. The waveguide 80 may -7further have features to control the gain of the longitudinal vibration along the waveguide 80 and features to tune the waveguide 80 to the resonant frequency of the system. In particular, the waveguide 80 may have any suitable cross-sectional dimension. For example, the waveguide 80 may have a substantially uniform cross-section or the waveguide 80 may be tapered at various 5 sections or may be tapered along its entire length. In one expression of the current embodiment, the waveguide diameter is about 0.113 inches nominal to minimize the amount of deflection at the blade 79 so that gapping in the proximal portion of the end effector 81 is minimized. The blade 79 may be integral with the waveguide 80 and formed as a single unit. In an alternate expression of the current embodiment, the blade 79 may be connected by a threaded 10 connection, a welded joint, or other coupling mechanisms. The distal end of the blade 79 is disposed near an anti-node in order to tune the acoustic assembly to a preferred resonant frequency fo when the acoustic assembly is not loaded by tissue. When the ultrasonic transducer 50 is energized, the distal end of the blade 79 is configured to move longitudinally in the range of, for example, approximately 10 to 500 microns peak-to-peak, and preferably in the range of about 15 20 to about 200 microns at a predetermined vibration frequency fo of, for example, 55,500 Hz. With particular reference to FIGS. 1-3, therein is illustrated one embodiment of the clamp member 60 for use with the present ultrasonic surgical instrument 100 and which is configured for cooperative action with the blade 79. The clamp member 60 in combination with the blade 79 is commonly referred to as the end effector 81, and the clamp member 60 is also commonly referred 20 to as the jaw. The clamp member 60 includes a pivotally movable clamp arm 56, which is connected to the distal end of the outer sheath 72 and the actuating member 76, in combination with a tissue engaging pad or clamp pad 58. The clamp arm 56 is pivotally movable by a trigger 34 and the end effector 81 is rotatably movable by the rotation knob 29. In one expression of the embodiment, the clamp pad 58 is formed from TEFLON@ a trademark name of E. I. Du Pont de 25 Nemours and Company, a low coefficient of friction polymer material, or any other suitable low friction material. The clamp pad 58 mounts on the clamp arm 56 for cooperation with the blade 79, with pivotal movement of the clamp arm 56 positioning the clamp pad 58 in substantially parallel relationship to, and in contact with, the blade 79, thereby defining a tissue treatment region. By this construction, tissue is grasped between the clamp pad 58 and the blade 79. As 30 illustrated, the clamp pad 58 may be provided with a non-smooth surface, such as a saw tooth-like configuration to enhance the gripping of tissue in cooperation with the blade 79. The saw tooth like configuration, or teeth, provide traction against the movement of the blade 79. The teeth also provide counter traction to the blade 79 and clamping movement. As would be appreciated by one skilled in the art, the saw tooth-like configuration is just one example of many tissue engaging -8surfaces to prevent movement of the tissue relative to the movement of the blade 79. Other illustrative examples include bumps, criss-cross patterns, tread patterns, a bead, or sand blasted surface. Due to sinusoidal motion, the greatest displacement or amplitude of motion is located at 5 the most distal portion of the blade 79, while the proximal portion of the tissue treatment region is on the order of 50% of the distal tip amplitude. During operation, the tissue in the proximal region of the end effector 8 will desiccate and thin, and the distal portion of the end effector 81 will transect tissue in that distal region, thereby allowing the desiccated and thinned tissue within the proximal region to slide distally into the more active region of the end effector 81 to complete 10 the tissue transection. FIG. 3 illustrates a force diagram and the relationship between the actuation force FA (provided by the actuating member 76) and transection force FT (measured at the midpoint of the optimal tissue treatment area). FT = FA (X2/X1) (1) 15 Where FA equals the spring preload of a proximal spring 94 (less frictional losses), which, in one embodiment, is about 12.5 pounds, and FT equals about 4.5 pounds. FT is measured in the region of the clamp arm/blade interface where optimal tissue treatment occurs as defined by tissue marks 61a and 61b. The tissue marks 61a, b are etched or raised on the clamp arm 56 to provide a visible mark to the surgeon so the surgeon has a clear 20 indication of the optimal tissue treatment area. The tissue marks 61a, b are about 7mm apart in distance, and more preferably about 5mm apart in distance. FIG. 9 illustrates one embodiment of a drive system 32 of the generator 30, which creates an ultrasonic electrical signal for driving an ultrasonic transducer. The drive system 32 is flexible and can create an ultrasonic electrical drive signal 416 at a desired frequency and power level 25 setting for driving the ultrasonic transducer 50. In various embodiments, the generator 30 may comprise several separate functional elements, such as modules and/or blocks. Although certain modules and/or blocks may be described by way of example, it can be appreciated that a greater or lesser number of modules and/or blocks may be used and still fall within the scope of the embodiments. Further, although various embodiments may be described in terms of modules 30 and/or blocks to facilitate description, such modules and/or blocks may be implemented by one or more hardware components, e.g., processors, Digital Signal Processors (DSPs), Programmable Logic Devices (PLDs), Application Specific Integrated Circuits (ASICs), circuits, registers and/or software components, e.g., programs, subroutines, logic and/or combinations of hardware and software components. -9- In one embodiment, the generator 30 drive system 32 may comprise one or more embedded applications implemented as firmware, software, hardware, or any combination thereof The generator 30 drive system 32 may comprise various executable modules such as software, programs, data, drivers, application program interfaces (APIs), and so forth. The 5 firmware may be stored in nonvolatile memory (NVM), such as in bit-masked read-only memory (ROM) or flash memory. In various implementations, storing the firmware in ROM may preserve flash memory. The NVM may comprise other types of memory including, for example, programmable ROM (PROM), erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), or battery backed random-access memory (RAM) such as 10 dynamic RAM (DRAM), Double-Data-Rate DRAM (DDRAM), and/or synchronous DRAM (SDRAM). In one embodiment, the generator 30 drive system 32 comprises a hardware component implemented as a processor 400 for executing program instructions for monitoring various measurable characteristics of the ultrasonic surgical instrument 100 (FIG. 1) and generating a step 15 function output signal for driving the ultrasonic transducer 50 in cutting and/or coagulation operating modes. It will be appreciated by those skilled in the art that the generator 30 and the drive system 32 may comprise additional or fewer components and only a simplified version of the generator 30 and the drive system 32 are described herein for conciseness and clarity. In various embodiments, as previously discussed, the hardware component may be implemented as a 20 DSP, PLD, ASIC, circuits, and/or registers. In one embodiment, the processor 400 may be configured to store and execute computer software program instructions to generate the step function output signals for driving various components of the ultrasonic surgical instrument 100, such as the transducer 50, the end effector 81, and/or the blade 79. In one embodiment, under control of one or more software program routines, the processor 25 400 executes the methods in accordance with the described embodiments to generate a step function formed by a stepwise waveform of drive signals comprising current (I), voltage (V), and/or frequency (f) for various time intervals or periods (T). The stepwise waveforms of the drive signals may be generated by forming a piecewise linear combination of constant functions over a plurality of time intervals created by stepping the generator 30 drive signals, e.g., output 30 drive current (I), voltage (V), and/or frequency (f). The time intervals or periods (T) may be predetermined (e.g., fixed and/or programmed by the user) or may be variable. Variable time intervals may be defined by setting the drive signal to a first value and maintaining the drive signal at that value until a change is detected in a monitored characteristic. Examples of monitored characteristics may comprise, for example, transducer impedance, tissue impedance, -10tissue heating, tissue transection, tissue coagulation, and the like. The ultrasonic drive signals generated by the generator 30 include, without limitation, ultrasonic drive signals capable of exciting the ultrasonic transducer 50 in various vibratory modes such as, for example, the primary longitudinal mode and harmonics thereof as well flexural and torsional vibratory modes. 5 In one embodiment, the executable modules comprise one or more step function algorithm(s) 402 stored in memory that when executed causes the processor 400 to generate a step function formed by a stepwise waveform of drive signals comprising current (I), voltage (V), and/or frequency (f) for various time intervals or periods (T). The stepwise waveforms of the drive signals may be generated by forming a piecewise linear combination of constant functions 10 over two or more time intervals created by stepping the generator's 30 output drive current (I), voltage (V), and/or frequency (f). The drive signals may be generated either for predetermined fixed time intervals or periods (T) of time or variable time intervals or periods of time in accordance with the one or more stepped output algorithm(s) 402. Under control of the processor 400, the generator 30 steps (e.g., increment or decrement) the current (I), voltage (V), and/or 15 frequency (f) up or down at a particular resolution for a predetermined period (T) or until a predetermined condition is detected, such as a change in a monitored characteristic (e.g., transducer impedance, tissue impedance). The steps can change in programmed increments or decrements. If other steps are desired, the generator 30 can increase or decrease the step adaptively based on measured system characteristics. 20 In operation, the user can program the operation of the generator 30 using the input device 406 located on the front panel of the generator 30 console. The input device 406 may comprise any suitable device that generates signals 408 that can be applied to the processor 400 to control the operation of the generator 30. In various embodiments, the input device 406 includes buttons, switches, thumbwheels, keyboard, keypad, touch screen monitor, pointing device, remote 25 connection to a general purpose or dedicated computer. In other embodiments, the input device 406 may comprise a suitable user interface. Accordingly, by way of the input device 406, the user can set or program the current (I), voltage (V), frequency (f), and/or period (T) for programming the step function output of the generator 30. The processor 400 then displays the selected power level by sending a signal on line 410 to an output indicator 412. 30 In various embodiments, the output indicator 412 may provide visual, audible, and/or tactile feedback to the surgeon to indicate the status of a surgical procedure, such as, for example, when tissue cutting and coagulating is complete based on a measured characteristic of the ultrasonic surgical instrument 100, e.g., transducer impedance, tissue impedance, or other measurements as subsequently described. By way of example, and not limitation, visual feedback -11comprises any type of visual indication device including incandescent lamps or light emitting diodes (LEDs), graphical user interface, display, analog indicator, digital indicator, bar graph display, digital alphanumeric display. By way of example, and not limitation, audible feedback comprises any type of buzzer, computer generated tone, computerized speech, voice user interface 5 (VUI) to interact with computers through a voice/speech platform. By way of example, and not limitation, tactile feedback comprises any type of vibratory feedback provided through the instrument housing handle assembly 68. In one embodiment, the processor 400 may be configured or programmed to generate a digital current signal 414 and a digital frequency signal 418. These signals 414, 418 are applied 10 to a direct digital synthesizer (DDS) circuit 420 to adjust the amplitude and the frequency (f) of the current output signal 416 to the transducer 50. The output of the DDS circuit 420 is applied to an amplifier 422 whose output is applied to a transformer 424. The output of the transformer 424 is the signal 416 applied to the ultrasonic transducer 50, which is coupled to the blade 79 by way of the waveguide 80 (FIG. 2). 15 In one embodiment, the generator 30 comprises one or more measurement modules or components that may be configured to monitor measurable characteristics of the ultrasonic instrument 100 (FIG. 1). In the illustrated embodiment, the processor 400 may be employed to monitor and calculate system characteristics. As shown, the processor 400 measures the impedance Z of the transducer 50 by monitoring the current supplied to the transducer 50 and the 20 voltage applied to the transducer 50. In one embodiment, a current sense circuit 426 is employed to sense the current flowing through the transducer 50 and a voltage sense circuit 428 is employed to sense the output voltage applied to the transducer 50. These signals may be applied to the analog-to-digital converter 432 (ADC) via an analog multiplexer 430 circuit or switching circuit arrangement. The analog multiplexer 430 routes the appropriate analog signal to the ADC 432 for 25 conversion. In other embodiments, multiple ADCs 432 may be employed for each measured characteristic instead of the multiplexer 430 circuit. The processor 400 receives the digital output 433 of the ADC 432 and calculates the transducer impedance Z based on the measured values of current and voltage. The processor 400 adjusts the output drive signal 416 such that it can generate a desired power versus load curve. In accordance with programmed step function 30 algorithms 402, the processor 400 can step the drive signal 416, e.g., the current or frequency, in any suitable increment or decrement in response to the transducer impedance Z. To actually cause the surgical blade 79 to vibrate, e.g., actuate the blade 79, the user activates the foot switch 434 (FIG. 1) or the switch 312a (FIG. 1) on the handle assembly 68. This activation outputs the drive signal 416 to the transducer 50 based on programmed values of -12current (I), frequency (f), and corresponding time periods (T). After a predetermined fixed time period (T), or variable time period based on a measurable system characteristic such as changes in the impedance Z of the transducer 50, the processor 400 changes the output current step or frequency step in accordance with the programmed values. The output indicator 412 5 communicates the particular state of the process to the user. The programmed operation of the generator 30 can be further illustrated with reference to FIGS. 6, 7, and 8, where graphical representations of current 300, voltage 310, power 320, impedance 330, and frequency 340 are shown for the generator 30 in an unloaded state, a lightly loaded state, and a heavily loaded state, respectively. FIG. 6 is a graphical representation of 10 current 300, voltage 310, power 320, impedance 330, and frequency 340 waveforms of one embodiment of the generator 30 in an unloaded state. In the illustrated embodiment, the current 300 output of the generator 30 is stepped. As shown in FIG. 6, the generator 30 is initially activated at about time 0 resulting in the current 300 rising to a first set point I1 of about 100mA. The current 300 is maintained at the first set point I1, for a first period T 1 . At the end of the first 15 period T 1 , e.g., about 1 second in the illustrated embodiment, the current 300 set point I1 is changed, e.g., stepped, by the generator 30 in accordance with the software, e.g., the step function algorithm(s) 402, to a second set point 12 of about 175mA for a second period T 2 , e.g., about 2 seconds in the illustrated embodiment. At the end of the second period T 2 , e.g., at about 3 seconds in the illustrated embodiment, the generator 30 software changes the current 300 to a 20 third set point 13 of about 350mA. The voltage 310, current 300, power 320, and frequency respond only slightly because there is no load on the system. FIG. 7 is a graphical representation of the current 300, voltage 310, power 320, impedance 330, and frequency 340 waveforms of one embodiment of the generator 30 under a lightly loaded state. Referring to Fig. 7, the generator 30 is activated at about time 0 resulting in the current 300 25 rising to the first current 300 set point I1 of about 1 00mA. At about 1 second the current 300 set point is changed within the generator 30 by the software to 12 of about 175mA, and then again at about 3 seconds the generator 30 changes the current 300 set point to 13 of about 350mA. The voltage 310, current 300, power 320, and frequency 340 are shown responding to the light load similar to that shown in FIG. 4. 30 FIG. 8 is a graphical representation of the current 300, voltage 310, power 320, impedance 330, and frequency 340 waveforms of one embodiment of the generator 30 under a heavily loaded state. Referring to Fig. 8, the generator 30 is activated at about time 0 resulting in the current 300 rising to the first set point I1 of about 100mA. At about 1 second the current 300 set point is changed within the generator 30 by the software to 12 of about 175mA, and then again at about 3 -13seconds the generator 30 changes the current 300 set point to 13 of about 350mA. The voltage 310, current 300, power 320, and frequency 340 are shown responding to the heavy load similar to that shown in FIG. 5. It will be appreciated by those skilled in the art that the current 300 step function set points 5 (e.g., I1, 12, 13) and the time intervals or periods (e.g., T 1 , T 2 ) of duration for each of the step function set points described in FIGS. 6-8 are not limited to the values described herein and may be adjusted to any suitable value as may be desired for a given set of surgical procedures. Additional or fewer current set points and periods of duration may be selected as may be desired for a given set of design characteristics or performance constraints. As previously discussed, the 10 periods may be predetermined by programming or may be variable based on measurable system characteristics. The embodiments are not limited in this context. Having described operational details of various embodiments of the surgical system 19, operations for the above surgical system 19 may be further described in terms of a process for cutting and coagulating a blood vessel employing a surgical instrument comprising the input 15 device 406 and the transducer impedance measurement capabilities described with reference to FIG. 9. Although a particular process is described in connection with the operational details, it can be appreciated that the process merely provides an example of how the general functionality described herein can be implemented by the surgical system 19. Further, the given process does not necessarily have to be executed in the order presented herein unless otherwise indicated. As 20 previously discussed, the input device 406 may be employed to program the stepped output (e.g., current, voltage, frequency) to the ultrasonic transducer 50/blade 79 assembly. Accordingly, with reference now to FIGS. 1-3 and 6-9, one technique for sealing a vessel includes separating and moving the inner muscle layer of the vessel away from the adventitia layer prior to the application of standard ultrasonic energy to transect and seal the vessel. 25 Although conventional methods have achieved this separation by increasing the force applied to the clamp member 60, disclosed is an alternative apparatus and method for cutting and coagulating tissue without relying on clamp force alone. In order to more effectively separate the tissue layers of a vessel, for example, the generator 30 may be programmed to apply a frequency step function to the ultrasonic transducer 50 to mechanically displace the blade 79 in multiple 30 modes in accordance with the step function. In one embodiment, the frequency step function may be programmed by way of the user interface 406, wherein the user can select a stepped-frequency program, the frequency (f) for each step, and the corresponding time period (T) of duration for each step for which the ultrasonic transducer 50 will be excited. The user may program a -14complete operational cycle by setting multiple frequencies for multiple periods to perform various surgical procedures. In one embodiment, a first ultrasonic frequency may be set initially to mechanically separate the muscle tissue layer of a vessel prior to applying a second ultrasonic frequency to cut 5 and seal the vessel. By way of example, and not limitation, in accordance with one implementation of the program, initially, the generator 30 is programmed to output a first drive frequency fi for a first period T 1 of time (for example less than approximately 1 second), wherein the first frequency fi is significantly off resonance, for example, fo/2, 2fo or other structural resonant frequencies, where fo is the resonant frequency (e.g., 55.5 kHz). The first frequency fi 10 provides a low level of mechanical vibration action to the blade 79 that, in conjunction with the clamp force, mechanically separates the muscle tissue layer (subtherapeutic) of the vessel without causing significant heating that generally occurs at resonance. After the first period T 1 , the generator 30 is programmed to automatically switch the drive frequency to the resonant frequency fo for a second period T 2 to transect and seal the vessel. The duration of the second period T 2 may 15 be programmed or may be determined by the length of time it actually takes to cut and seal the vessel as determined by the user or may be based on measured system characteristics such as the transducer impedance Z as described in more detail below. In one embodiment, the tissue/vessel transection process (e.g., separating the muscle layer of the vessel from the adventitia layer and transecting/sealing the vessel) may be automated by 20 sensing the impedance Z characteristics of the transducer 50 to detect when the transection of the tissue/vessel occurs. The impedance Z can be correlated to the transection of the muscle layer and to the transection/sealing of the vessel to provide a trigger for the processor 400 to generate the frequency and/or current step function output. As previously discussed with reference to FIG. 9, the impedance Z of the transducer 50 may be calculated by the processor 400 based on the current 25 flowing through transducer 50 and the voltage applied to the transducer 50 while the blade 79 is under various loads. Because the impedance Z of the transducer 50 is proportional to the load applied to the blade 79, as the load on the blade 79 increases the impedance Z of the transducer 50 increases, and as the load on the blade 79 decreases the impedance Z of the transducer 50 decreases. Accordingly, the impedance Z of the transducer 50 can be monitored to detect the 30 transection of the inner muscle tissue layer of the vessel from the adventitia layer and can also be monitored to detect when the vessel has been transected and sealed. In one embodiment, the ultrasonic surgical instrument 110 may be operated in accordance with a programmed step function algorithm responsive to the transducer impedance Z. In one embodiment, a frequency step function output may be initiated based on a comparison of the -15transducer impedance Z and one or more predetermined thresholds that have been correlated with tissue loads against the blade 79. When the transducer impedance Z transitions above or below (e.g., crosses) a threshold, the processor 400 applies a digital frequency signal 418 to the DDS circuit 420 to change the frequency of the drive signal 416 by a predetermined step in accordance 5 with the step function algorithm(s) 402 responsive to the transducer impedance Z. In operation, the blade 79 is first located at the tissue treatment site. The processor 400 applies a first digital frequency signal 418 to set a first drive frequency fi that is off resonance (e.g., fo/2, 2fo or other structural resonant frequencies, where fo is the resonant frequency). The drive signal 416 is applied to the transducer 50 in response to activation of the switch 312a on the handle assembly 10 68 or the foot switch 434. During this period the ultrasonic transducer 50 mechanically activates the blade 79 at the first drive frequency fi. A force or load may be applied to the clamp member 60 and the blade 79 to facilitate this process. During this period, the processor 400 monitors the transducer impedance Z until the load on the blade 79 changes and the transducer impedance Z crosses a predetermined threshold to indicate that the tissue layer has been transected. The 15 processor 400 then applies a second digital frequency signal 418 to set a second drive frequency f 2 , e.g., the resonant frequency fo or other suitable frequency for transecting, coagulating, and sealing tissue. Another portion of the tissue (e.g., the vessel) is then grasped between the clamp member 60 and the blade 79. The transducer 50 is now energized by the drive signal 416 at the second drive frequency f 2 by actuating either the foot switch 434 or the switch 312a on the handle 20 assembly 68. It will be appreciated by those skilled in the art that the drive current (I) output also may be stepped as described with reference to FIGS. 6-8 based on the transducer impedance Z. According to one step function algorithm 402, the processor 400 initially sets a first drive frequency fi that is significantly off resonance to separate the inner muscle layer of the vessel from the adventitia layer. During this period of operation the processor 400 monitors the 25 transducer impedance Z to determine when the inner muscle layer is transected or separated from the adventitia layer. Because the transducer impedance Z is correlated to the load applied to the blade 79, for example, cutting more tissue decrease the load on the blade 79 and the transducer impedance Z. The transection of the inner muscle layer is detected when the transducer impedance Z drops below a predetermined threshold. When the change in transducer impedance 30 Z indicates that the vessel has been separated from the inner muscle layer, the processor 400 sets the drive frequency to the resonant frequency fo. The vessel is then grasped between the blade 79 and the clamp member 60 and the transducer 50 is activated by actuating either the foot switch or the switch on the handle assembly 68 to transect and seal the vessel. In one embodiment, the impedance Z change may range between about 1.5 to about 4 times a base impedance -16measurements from an initial point of contact with the tissue to a point just before the muscle layer is transected and sealed. FIG. 10 illustrates one embodiment of a surgical system 190 comprising an ultrasonic surgical instrument 120 and a generator 500 comprising a tissue impedance module 502. 5 Although in the presently disclosed embodiment, the generator 500 is shown separate from the surgical instrument 120, in one embodiment, the generator 500 may be formed integrally with the surgical instrument 120 to form a unitary surgical system 190. In one embodiment, the generator 500 may be configured to monitor the electrical impedance of the tissue Zt and to control the characteristics of time and power level based on the tissue impedance Zt. In one embodiment, the 10 tissue impedance Zt may be determined by applying a subtherapeutic radio frequency (RF) signal to the tissue and measuring the current through the tissue by way of a return electrode on the clamp member 60. In the embodiment illustrated in FIG. 10, an end effector 810 portion of the surgical system 190 comprises a clamp arm assembly 451 connected to the distal end of the outer sheath 72. The blade 79 forms a first (e.g., energizing) electrode and the clamp arm assembly 451 15 comprises an electrically conductive portion that forms a second (e.g., return) electrode. The tissue impedance module 502 is coupled to the blade 79 and the clamp arm assembly 451 through a suitable transmission medium such as a cable 504. The cable 504 comprises multiple electrical conductors for applying a voltage to the tissue and providing a return path for current flowing through the tissue back to the impedance module 502. In various embodiments, the tissue 20 impedance module 502 may be formed integrally with the generator 500 or may be provided as a separate circuit coupled to the generator 500 (shown in phantom to illustrate this option). The generator 500 is substantially similar to the generator 30 with the added feature of the tissue impedance module 502. FIG. 11 illustrates one embodiment of a drive system 321 of the generator 500 comprising 25 the tissue impedance module 502. The drive system 321 generates the ultrasonic electrical drive signal 416 to drive the ultrasonic transducer 50. In one embodiment, the tissue impedance module 502 may be configured to measure the impedance Zt of tissue grasped between the blade 79 and the clamp arm assembly 451. The tissue impedance module 502 comprises an RF oscillator 506, a voltage sensing circuit 508, and a current sensing circuit 510. The voltage and current sensing 30 circuits 508, 510 respond to the RF voltage vrf applied to the blade 79 electrode and the RF current if flowing through the blade 79 electrode, the tissue, and the conductive portion of the clamp arm assembly 451. The sensed voltage vif and current irf are converted to digital form by the ADC 432 via the analog multiplexer 430. The processor 400 receives the digitized output 433 of the ADC 432 and determines the tissue impedance Zt by calculating the ratio of the RF voltage -17vif to current if measured by the voltage sense circuit 508 and the current sense circuit 510. In one embodiment, the transection of the inner muscle layer and the tissue may be detected by sensing the tissue impedance Zt. Accordingly, detection of the tissue impedance Zt may be integrated with an automated process for separating the inner muscle layer from the outer 5 adventitia layer prior to transecting the tissue without causing a significant amount of heating, which normally occurs at resonance. FIG. 12 illustrates one embodiment of the clamp arm assembly 451 that may be employed with the surgical system 190 (FIG. 10). In the illustrated embodiment, the clamp arm assembly 451 comprises a conductive jacket 472 mounted to a base 449. The conductive jacket 472 is the 10 electrically conductive portion of the clamp arm assembly 451 that forms the second, e.g., return, electrode. In one implementation, the clamp arm 56 (FIG. 3) may form the base 449 on which the conductive jacket 472 is mounted. In various embodiments, the conductive jacket 472 may comprise a center portion 473 and at least one downwardly-extending sidewall 474 which can extend below the bottom surface 475 of the base 449. In the illustrated embodiment, the 15 conductive jacket 472 has two sidewalls 474 extending downwardly on opposite sides of the base 449. In other embodiments, the center portion 473 may comprise at least one aperture 476 which can be configured to receive a projection 477 extending from the base 449. In such embodiments, the projections 477 can be press-fit within the apertures 476 in order to secure the conductive jacket 472 to the base 449. In other embodiments, the projections 477 can be deformed after they 20 are inserted into the apertures 476. In various embodiments, fasteners can be used to secure the conductive jacket 472 to the base 449. In various embodiments, the clamp arm assembly 451 may comprise a non-electrically conductive or insulative material, such as plastic and/or rubber, for example, positioned intermediate the conductive jacket 472 and the base 449. The electrically insulative material can 25 prevent current from flowing, or shorting, between the conductive jacket 472 and the base 449. In various embodiments, the base 449 may comprise at least one aperture 478, which can be configured to receive a pivot pin (not illustrated). The pivot pin can be configured to pivotably mount the base 449 to the sheath 72 (FIG. 10), for example, such that the clamp arm assembly 451 can be rotated between open and closed positions relative to the sheath 72. In the illustrated 30 embodiment, the base 449 includes two apertures 478 positioned on opposite sides of the base 449. In one embodiment, a pivot pin may be formed of or may comprise a non-electrically conductive or insulative material, such as plastic and/or rubber, for example, which can be configured to prevent current from flowing into the sheath 72 even if the base 449 is in electrical contact with the conductive jacket 472, for example. Additional clamp arm assemblies -18comprising various embodiments of electrodes may be employed. Examples of such clamp arm assemblies are described in commonly-owned and contemporaneously-filed U.S. Patent Application Ser. Nos. 12,503,769, 12/503,770, and 12,503,766 (Docket Numbers END6493USNP/080592, END6493USNPl/080593, and END6493USNP2/080594) each of 5 which is incorporated herein by reference in its entirety. FIG. 13 is a schematic diagram of the tissue impedance module 502 coupled to the blade 79 and the clamp arm assembly 415 with tissue 514 located therebetween. With reference now to FIGS. 10-13, the generator 500 comprises the tissue impedance module 502 configured for monitoring the impedance of the tissue 514 (Zt) located between the blade 79 and the clamp arm 10 assembly 451 during the tissue transection process. The tissue impedance module 502 is coupled to the ultrasonic surgical instrument 120 by way of the cable 504. The cable 504 includes a first "energizing" conductor 504a connected to the blade 79 (e.g., positive [+] electrode) and a second "return" conductor 504b connected to the conductive jacket 472 (e.g., negative [-] electrode) of the clamp arm assembly 451. In one embodiment, RF voltage vf is applied to the blade 79 to 15 cause RF current if to flow through the tissue 514. The second conductor 504b provides the return path for the current if back to the tissue impedance module 502. The distal end of the return conductor 504b is connected to the conductive jacket 472 such that the current if can flow from the blade 79, through the tissue 514 positioned intermediate the conductive jacket 472 and the blade 79, and the conductive jacket 472 to the return conductor 504b. The impedance module 20 502 connects in circuit, by way of the first and second conductors 504a, b. In one embodiment, the RF energy may be applied to the blade 79 through the ultrasonic transducer 50 and the waveguide 80 (FIG. 2). It is worthwhile noting that the RF energy applied to the tissue 514 for purposes of measuring the tissue impedance Zt is a low level subtherapeutic signal that does not contribute in a significant manner, or at all, to the treatment of the tissue 514. 25 Having described operational details of various embodiments of the surgical system 190, operations for the above surgical system 190 may be further described with reference to FIGS. 10 13 in terms of a process for cutting and coagulating a blood vessel employing a surgical instrument comprising the input device 406 and the tissue impedance module 502. Although a particular process is described in connection with the operational details, it can be appreciated that 30 the process merely provides an example of how the general functionality described herein can be implemented by the surgical system 190. Further, the given process does not necessarily have to be executed in the order presented herein unless otherwise indicated. As previously discussed, the input device 406 may be employed to program the step function output (e.g., current, voltage, frequency) to the ultrasonic transducer 50/blade 79 assembly. -19- In one embodiment, the ultrasonic surgical instrument 120 may be operated in accordance with a programmed step function algorithm 402 responsive to the tissue impedance Zt. In one embodiment, a frequency step function output may be initiated based on a comparison of the tissue impedance Zt and predetermined thresholds that have been correlated with various tissue 5 states (e.g., desiccation, transection, sealing). When the tissue impedance Zt transitions above or below (e.g., crosses) a threshold, the processor 400 applies a digital frequency signal 418 to the DDS circuit 420 to change the frequency of an ultrasonic oscillator by a predetermined step in accordance with the step function algorithm 402 responsive to the tissue impedance Zt. In operation, the blade 79 is located at the tissue treatment site. The tissue 514 is grasped 10 between the blade 79 and the clamp arm assembly 451 such that the blade 79 and the conductive jacket 472 make electrical contact with the tissue 514. The processor 400 applies a first digital frequency signal 418 to set a first drive frequency fi that is off resonance (e.g., fo/2, 2fo or other structural resonant frequencies, where fo is the resonant frequency). The blade 79 is electrically energized by the low level subtherapeutic RF voltage vf supplied by the tissue impedance module 15 502. The drive signal 416 is applied to the transducer 50/blade 79 in response to actuation of the switch 312a on the handle assembly 68 or the foot switch 434 until the tissue impedance Zt changes by a predetermined amount. A force or load is then applied to the clamp arm assembly 451 and the blade 79. During this period the ultrasonic transducer 50 mechanically activates the blade 79 at the first drive frequency fi and as a result, the tissue 514 begins to desiccate from the 20 ultrasonic action applied between the blade 79 and the one or more clamp pads 58 of the clamp arm assembly 451 causing the tissue impedance Zt to increase. Eventually, as the tissue is transected by the ultrasonic action and applied clamp force, the tissue impedance Zt becomes very high or infinite. It will be appreciated by those skilled in the art that the drive current (I) output also may be stepped as described with reference to FIGS. 6-8 based on the tissue impedance Zt. 25 In one embodiment, the tissue impedance Zt may be monitored by the impedance module 502 in accordance with the following process. A measurable RF current ii is conveyed through the first energizing conductor 504a to the blade 79, through the tissue 514, and back to the impedance module 502 through the conductive jacket 472 and the second conductor 504b. As the tissue 514 is desiccated and cut by the ultrasonic action of the blade 79 acting against the one or 30 more clamp pads 58, the impedance of the tissue 514 increases and thus the current ii in the return path, i.e., the second conductor 504b, decreases. The impedance module 502 measures the tissue impedance Zt and conveys a representative signal to the ADC 432 whose digital output 433 is provided to the processor 400. The processor 400 calculates the tissue impedance Zt based on these measured values of vf and if. The processor 400 steps the frequency by any suitable -20increment or decrement in response to changes in tissue impedance Zt. The processor 400 controls the drive signals 416 and can make any necessary adjustments in amplitude and frequency in response to the tissue impedance Zt. In one embodiment, the processor 400 can cut off the drive signal 416 when the tissue impedance Zt reaches a predetermined threshold value. 5 Accordingly, by way of example, and not limitation, in one embodiment, the ultrasonic surgical instrument 120 may be operated in accordance with a programmed stepped output algorithm to separate the inner muscle layer of a vessel from the adventitia layer prior to transecting and sealing the vessel. As previously discussed, according to one step function algorithm, the processor 400 initially sets a first drive frequency fi that is significantly off 10 resonance. The transducer 50 is activated to separate the inner muscle layer of the vessel from the adventitia layer and the tissue impedance module 502 applies a subtherapeutic RF voltage vf signal to the blade 79. During this period T 1 of operation the processor 400 monitors the tissue impedance Zt to determine when the inner muscle layer is transected or separated from the adventitia layer. The tissue impedance Zt is correlated to the load applied to the blade 79, for 15 example, when the tissue becomes desiccated or when the tissue is transected the tissue impedance Zt becomes extremely high or infinite. The change in tissue impedance Zt indicates that the vessel has been separated or transected from the inner muscle layer and the generator 500 is deactivated for a second period of time T 2 . The processor 400 then sets the drive frequency to the resonant frequency fo. The vessel is then grasped between the blade 79 and the clamp arm 20 assembly 451 and the transducer 50 is reactivated to transect and seal the vessel. Continuous monitoring of the tissue impedance Zt provides an indication of when the vessel is transected and sealed. Also, the tissue impedance Zt may be monitored to provide an indication of the completeness of the tissue cutting and/or coagulating process or to stop the activation of the ultrasonic generator 500 when the tissue impedance Zt reaches a predetermined threshold value. 25 The threshold for the tissue impedance Zt may be selected, for example, to indicate that the vessel has been transected. In one embodiment, the tissue impedance Zt may range between about 10 Ohms to about 1000 Ohms from an initial point to a point just before the muscle layer is transected and sealed. The applicants have discovered that experiments that run varying current set points (both 30 increasing and decreasing) and dwell times indicate that the described embodiments can be used to separate the inner muscle layer from the outer adventitia layer prior to completing the transection resulting in improved hemostasis and potentially lower total energy (heat) at the transection site. Furthermore, although the surgical instruments 100, 120 have been described in regards to impedance threshold detection schemes to determine when the muscle layer is -21separated from the adventitia, other embodiments that do not employ any detection scheme are within the scope of the present disclosure. For example, embodiments of the surgical instruments 100, 120 may be employed in simplified surgical systems wherein non-resonant power is applied to separate the layers for a predetermined time of approximately 1 second or less, prior to 5 applying a resonant power to cut the tissue. The embodiments are not limited in this context. Having described operational details of various embodiments of the surgical systems 19 (FIG. 1) and 190 (FIG. 10), operations for the above surgical systems 19, 190 may be further described generally in terms of a process for cutting and coagulating tissue employing a surgical instrument comprising the input device 406 and the tissue impedance module 502. Although a 10 particular process is described in connection with the operational details, it can be appreciated that the process merely provides an example of how the general functionality described herein can be implemented by the surgical systems 19, 190. Further, the given process does not necessarily have to be executed in the order presented herein unless otherwise indicated. As previously discussed, the input device 406 may be employed to program the stepped output (e.g., current, 15 frequency) to the ultrasonic transducer 50/blade 79 assembly. FIG. 14 illustrates one embodiment of a method 600 for driving an end effector coupled to an ultrasonic drive system of a surgical instrument. With reference to FIGS. 1-3, and 6-14, by way of example, and not limitation, the ultrasonic surgical instruments 100, 120 may be operated in accordance with the method 600 to separate the inner muscle layer of a vessel from the 20 adventitia layer prior to transecting and sealing the vessel. Accordingly, in various embodiments, an end effector (e.g., end effector 81, 810) of a surgical instrument (e.g., surgical instrument 100, 120) may be driven in accordance with the method 600. A generator (e.g., generator 30, 500) is coupled to an ultrasonic drive system. The ultrasonic drive system comprises an ultrasonic transducer (e.g., ultrasonic transducer 50) coupled to a waveguide (e.g., waveguide 80) and the 25 end effector 81 is coupled to the waveguide 80. The ultrasonic drive system is configured to resonate at a resonant frequency (e.g., 55.5kHz). In one embodiment, at 602, the generator 30 generates a first ultrasonic drive signal. At 604, the ultrasonic transducer 50 is actuated with the first ultrasonic drive signal for a first period in response to activating a switch (e.g., switch 34) on a handle assembly (e.g., handle assembly 68) or a foot switch (e.g., foot switch 434) connected to 30 the generator 30. After the first period, at 606, the generator 30 generates a second ultrasonic drive signal. At 608, the ultrasonic transducer 50 is actuated with the second ultrasonic drive signal for a second period in response to activating the switch 34 on the handle assembly 68 or the foot switch 434 connected to the generator 30. The first drive signal is different from the second -22drive signal over the respective first and second periods. The first and second drive signals define a step function waveform over the first and second periods. In one embodiment, the generator 30 generates a third ultrasonic drive signal. The ultrasonic transducer 50 is actuated with the third ultrasonic drive signal for a third period. The 5 third drive signal is different from the first second drive signals over the first, second, and third periods. The first, second, and third drive signals define a step function waveform over the first, second, and third periods. In one embodiment, generating the first, second, and third ultrasonic drive signals comprises generating a corresponding first, second, and third drive current and actuating the ultrasonic transducer 50 with the first drive current for the first period, actuating the 10 ultrasonic transducer 50 with the second drive current for the second period, and actuating the ultrasonic transducer 50 with the third drive current for the third period. In one embodiment, the generator 30 generates the first ultrasonic drive signal at a first frequency, which is different from the resonant frequency. The ultrasonic transducer 50 is then actuated with the first ultrasonic drive signal at the first frequency for the first period. Actuation 15 at the first frequency provides a first level of mechanical vibration to the end effector 81 suitable for separating a first tissue from a second tissue, for example, to separate the inner muscle layer of a vessel from the adventitia layer. The generator 30 generates the second ultrasonic drive signal at the resonant frequency, e.g., 55.5kHz, and the actuates the ultrasonic transducer 50 with the second ultrasonic drive signal at the resonant frequency for the second period subsequent to the 20 first period. Actuation at the second, resonant frequency, provides a second level of mechanical vibration to the end effector 81 suitable for transecting and sealing the first tissue, such as the vessel, once it separated from the inner muscle layer. In one embodiment, the second ultrasonic drive signal at the resonant frequency is generated automatically by the generator 30 after the first period. In one embodiment, the first frequency is substantially different from the resonant 25 frequency and the first period is less than about one second. For example, in one embodiment, the first frequency is defined by the following equation: fi = 2*fo, wherein fi is the first frequency and fo is the resonant frequency. In another embodiment, the first frequency is defined by the following equation: fi = fo/2, wherein fi is the first frequency and fo is the resonant frequency. The first, second, and third ultrasonic drive signals are also envisioned to excite be vibratory 30 modes of the ultrasonic transducer 50 in longitudinal, flexural, and torsional modes and harmonics thereof In one embodiment, the generator 30 monitors a measurable characteristic of the ultrasonic drive system and generates any one of the first and second drive signals based on the measured characteristic. For example, the generator 30 monitors the impedance Z of the ultrasonic -23transducer 50. The generator 30 comprises electronic circuitry suitable for measuring the impedance of the transducer 50. For example, a current sense circuit (e.g., current sense circuit 426) senses the current flowing through the transducer 50 and a voltage sense circuit (e.g., voltage sense circuit 428) senses the output voltage applied to the transducer 50. A multiplexer (e.g., 5 multiplexer 430) routes the appropriate analog signal to an analog-to-digital converter (e.g., ADC 432), whose digital output is provided to a processor (e.g., processor 400). The processor 400 calculates the transducer impedance Z based on the measured values of current and voltage. In one embodiment, the generator 500 comprises an impedance module (e.g., tissue impedance module 502) to measure the impedance of a tissue portion contacting an end effector 10 (e.g., end effector 810). The impedance module 502 includes an RF oscillator (e.g., RF oscillator 506) to generate a subtherapeutic RF signal. The subtherapeutic RF signal is applied to a blade (e.g., blade 79) portion of the end effector 810, which forms an energizing electrode. The tissue portion is grasped between the end effector 810 and a return electrode of a clamp arm assembly (e.g., clamp arm assembly 451) and the impedance of the tissue (e.g., tissue 514). The tissue 15 impedance is then measured by a voltage sense circuit (e.g., voltage sense circuit 508) and current sense circuit (e.g., current sense circuit 510) and of the impedance module 502. These signals are applied to the ADC 432 via the multiplexer 430. The digital output of the ADC 432 is provided to the processor 400, which calculates the tissue impedance Zt based on the measured values of current through the tissue and the voltage applied to the blade 79 portion of the end effector 810. 20 FIGS. 15A-C illustrate various embodiments of logic flow diagrams of 700, 800, 900 of operations for determining a change of state of tissue being manipulated by an ultrasonic surgical instrument and providing feedback to the user to indicate that the tissue has undergone such change of state or that there is a high likelihood that the tissue has undergone such change of state. As used herein, the tissue may undergo a change of state when the tissue is separated from other 25 layers of tissue or bone, when the tissue is cut or transected, when the tissue is coagulated, and so forth while being manipulated with an end effector of an ultrasonic surgical instrument, such as, for example, the end effector 81, 810 of the ultrasonic surgical instrument 100, 120 shown in FIGS. 1 and 10. A change in tissue state may be determined based on the likelihood of an occurrence of a tissue separation event. 30 In various embodiments, the feedback is provided by the output indicator 412 shown in FIGS. 9 and 11. The output indicator 412 is particularly useful in applications where the tissue being manipulated by the end effector 81, 810 is out of the user's field of view and the user cannot see when a change of state occurs in the tissue. The output indicator 412 communicates to the user that a change in tissue state has occurred as determined in accordance with the operations -24described with respect to the logic flow diagrams 700, 800, 900. As previously discussed, the output indicator 412 may be configured to provide various types of feedback to the user including, without limitation, visual, audible, and/or tactile feedback to indicate to the user (e.g., surgeon, clinician) that the tissue has undergone a change of state of the tissue. By way of example, and 5 not limitation, as previously discussed, visual feedback comprises any type of visual indication device including incandescent lamps or LEDs, graphical user interface, display, analog indicator, digital indicator, bar graph display, digital alphanumeric display. By way of example, and not limitation, audible feedback comprises any type of buzzer, computer generated tone, computerized speech, VUI to interact with computers through a voice/speech platform. By way 10 of example, and not limitation, tactile feedback comprises any type of vibratory feedback provided through the instrument housing handle assembly 68. The change of state of the tissue may be determined based on transducer and tissue impedance measurements as previously described, or based on voltage, current, and frequency measurements in accordance with the operations described with respect to the logic flow diagrams 700, 800, 900 described below with 15 respect to FIGS. 15A-C. In one embodiment, the logic flow diagrams 700, 800, 900 may be implemented as executable modules (e.g., algorithms) comprising computer readable instructions to be executed by the processor 400 (FIGS. 9, 11, 14) portion of the generator 30, 500. In various embodiments, the operations described with respect to the logic flow diagrams 700, 800, 900 may be 20 implemented as one or more software components, e.g., programs, subroutines, logic; one or more hardware components, e.g., processors, DSPs, PLDs, ASICs, circuits, registers; and/or combinations of software and hardware. In one embodiment, the executable instructions to perform the operations described by the logic flow diagrams 700, 800, 900 may be stored in memory. When executed, the instructions cause the processor 400 to determine a change in tissue 25 state in accordance with the operations described in the logic flow diagrams 800 and 900 and provide feedback to the user by way of the output indicator 412. In accordance with such executable instructions, the processor 400 monitors and evaluates the voltage, current, and/or frequency signal samples available from the generator 30, 500 and according to the evaluation of such signal samples determines whether a change in tissue state has occurred. As further 30 described below, a change in tissue state may be determined based on the type of ultrasonic instrument and the power level that the instrument is energized at. In response to the feedback, the operational mode of the ultrasonic surgical instrument 100, 120 may be controlled by the user or may be automatically or semi-automatically controlled. -25- FIG. 15A illustrates a logic flow diagram 700 of one embodiment of determining a change in tissue state and activating the output indicator 412 accordingly. With reference now to the logic flow diagram 700 shown in FIG. 15A and the drive system 32 of the generator 30 shown in FIG. 9, at 702, the processor 400 portion of the drive system 32 samples the voltage (v), current 5 (i), and frequency (f) signals of the generator 30. In the illustrated embodiment, at 704, the frequency and voltage signal samples are analyzed separately to determine the corresponding frequency inflection and/or voltage drop points. In other embodiments, the current signal samples may be separately analyzed in addition to the voltage and frequency signal samples or in place of the voltage signal samples. At 706, the present frequency signal sample is provided to a 10 frequency inflection point analysis module for determining a change in tissue state as illustrated in the logic flow diagram 800 in FIG. 15B. At 708, the present voltage signal sample is provided to a voltage drop point analysis module for determining a change in tissue state as illustrated in the logic flow diagram 900 in FIG. 15C. The frequency inflection point analysis module and the voltage drop point analysis module 15 determine when a change in tissue state has occurred based on correlated empirical data associated with a particular ultrasonic instrument type and the energy level at which the instrument is driven. At 714, the results 710 from the frequency inflection point analysis module and/or the results 712 from the voltage drop point analysis module are read by the processor 400. The processor 400 determines 716 whether the frequency inflection point result 710 and/or the 20 voltage drop point result 712 indicates a change in tissue state. If the results 710, 714 do not indicate a change in tissue state, the processor 400 continues along the "No" branch to 702 and reads an additional voltage and frequency signal sample from the generator 30. In embodiments that utilize the generator current in the analysis, the processor 400 would now also read an additional current signal sample from the generator 30. If the results 710, 714 indicate a sufficient 25 change in tissue state, the processor 400 continues along the "Yes" branch to 718 and activates the output indicator 412. As previously discussed, the output indicator 412 may provide visual, audible, and/or tactile feedback to alert the user of the ultrasonic surgical instrument 100, 120 that a change in tissue state has occurred. In various embodiments, in response to the feedback from the output 30 indicator 412, the operational mode of the generator 30, 500 and/or the ultrasonic instrument 100, 120 may be controlled manually, automatically, or semi-automatically. The operational modes include, without limitation, disconnecting or shutting down the output power of the generator 30, 500, reducing the output power of the generator 30, 500, cycling the output power of the generator 30, 500, pulsing the output power of the generator 30, 500, and/or outputting a high-power -26momentary surge from the generator 30, 500. The operational modes of the ultrasonic instrument in response to the change in tissue state can be selected, for example, to minimize heating effects of the end effector 81, 810, e.g., of the clamp pad 58 (FIGS. 1-3), to prevent or minimize possible damage to the surgical instrument 100, 120 and/or surrounding tissue. This is advantageous 5 because heat is generated exponentially when the transducer 50 is activated with nothing between the jaws of the end effector 81, 810 as is the case when a change in tissue state occurs. FIG. 15B is a logic flow diagram 800 illustrating one embodiment of the operation of the frequency inflection point analysis module. At 802, a frequency sample is received by the processor 400 from 706 of the logic flow diagram 700. At 804, the processor 400 calculates an 10 exponentially weighted moving average (EWMA) for the frequency inflection analysis. The EWMA is calculated to filter out noise from the generator from the frequency samples. The EWMA is calculated in accordance with a frequency moving average equation 806 and an alpha value (a) 808: Sy= aYty+(1-a)Sy1 (2) 15 Where: Stf= the current moving average of the sampled frequency signal; St/1 = the previous moving average of the sampled frequency signal; a = the smoothing factor; and Yy 1 = current data point of the sampled frequency signal. 20 The a value 808 may vary from about 0 to about 1 in accordance with a desired filtering or smoothing factor, wherein small a values 808 approaching about 0 provide a large amount of filtering or smoothing and large a values 808 approaching about 1 provide a small amount of filtering or smoothing. The a value 808 may be selected based on the ultrasonic instrument type and power level. In one embodiment, blocks 804, 806, and 808 may be implemented as a variable 25 digital low pass filter 810 with the a value 808 determining the cutoff point of the filter 810. Once the frequency samples are filtered, the slope of the frequency samples is calculated at 812 as: Frequency Slope = Af/At (3) The calculated Frequency Slope data points are provided to a "slow response" moving 30 average filter 814 to calculate the EWMA moving average for the Frequency Slope to further reduce system noise. In one embodiment, the "slow response" moving average filter 814 may be implemented by calculating the EWMA for the Frequency Slope at 818 in accordance with the frequency slope moving average equation 820 and alpha value (a') 822: S'y= a'Y'y+(1-')S' 1 (4) -27- Where: S'y = the current moving average of the frequency slope of the sampled frequency signal; S'yi = the previous moving average of the frequency slope of the sampled frequency signal; a'= the smoothing factor; and 5 Y'y 1 = current slope data point of the sampled frequency signal. The a value 822 varies from about 0 to about 1, as previously discussed with respect to digital filter block 810 in accordance with a desired filtering or smoothing factor, wherein small a' value 822 approaching 0 provide a large amount of filtering or smoothing and large a' value 822 approaching 1 provide a small amount of filtering or smoothing. The a' value 822 may be 10 selected based on the ultrasonic instrument type and power level. The calculated Frequency Slope data points are provided to a "fast response" filter 816 to calculate the moving average for the Frequency Slope. At 824, the "fast response" filter 816 calculates the moving average for the Frequency Slope based on a number of data points 826. In the illustrated embodiment, the output of the "slow response" moving average filter 814 15 "Slope EWMA" is applied to a (+) input of an adder 828 and the output of the "fast response" filter 816 "Slope Avg" is applied to (-) input of the adder 828. The adder 828 computes the difference between the outputs of the "slow response" moving average filter 814 and the "fast response" filter 816. The difference between these outputs is compared at 830 to a predetermined limit 832. The limit 832 is determined based on the type of ultrasonic instrument and the power 20 level at which the particular type of ultrasonic instrument is energized at. The limit 832 value may be predetermined and stored in memory in the form of a look-up table or the like. If the difference between the "Slope EWMA" and the "Slope Avg" is not greater than the limit 832, the processor 400 continues along the "No" branch and returns a value 834 to the results 710 block that indicates that no inflection point was found in the sampled frequency signal and, therefore, no 25 change in tissue state was detected. However, if the difference between the "Slope EWMA" and the "Slope Avg" is greater than the limit 832, the processor 400 continues along the "Yes" branch and determines that a frequency inflection point 836 was found and returns point index 838 to the results 710 block indicating that an inflection point was found in the sampled frequency data and, therefore, a change in tissue state was detected. As previously discussed with reference to FIG. 30 15A, if a frequency inflection point 836 is found, then, at 718 (FIG. 15A) the processor 400 activates the change in tissue state indicator 718. FIG. 15C is a logic flow diagram 900 illustrating one embodiment of the operation of the voltage drop analysis module. At 902, a voltage sample is received by the processor 400 from 708 of the logic flow diagram 700. At 904, the processor 400 calculates an exponentially -28weighted moving average (EWMA) for the frequency inflection analysis. The EWMA is calculated to filter out noise from the generator from the frequency samples. The EWMA is calculated in accordance with a voltage moving average equation 906 and an alpha value (a) 908: St, = aYt,+(1-a)Siv_1 (5) 5 Where: St, = the current moving average of the sampled voltage signal; Sti_1 = the previous moving average of the sampled voltage signal; a = the smoothing factor; and Yt, = current data point of the sampled voltage signal. 10 As previously discussed, the a value 908 may vary from 0 to 1 in accordance with a desired filtering or smoothing factor and may be selected based on the ultrasonic instrument type and power level. In one embodiment, blocks 904, 906, and 908 may be implemented as a variable digital low pass filter 910 with the a value 908 determining the cutoff point of the filter 910. Once the voltage samples are filtered, the slope of the voltage samples is calculated at 912 as: 15 Voltage Slope = Av/At (6) The calculated Voltage Slope data points are provided to a "slow response" moving average filter 914 to calculate the EWMA moving average for the Voltage Slope to further reduce system noise. In one embodiment, the "slow response" moving average filter 914 may be implemented by calculating the EWMA for the Voltage Slope at 918 in accordance with the 20 voltage slope moving average equation 920 and alpha value (a') 822: S't, = a'Y'ti+(1-a')S'ti_1 (7) Where: S't, = the current moving average of the voltage slope of the sampled voltage signal; S'tv-1 = the previous moving average of the voltage slope of the sampled voltage signal; 25 a'= the smoothing factor; and Y'i = current slope data point of the sampled voltage signal. The a' value 922 varies from about 0 to about 1, as previously discussed with respect to digital filter block 910 in accordance with a desired filtering or smoothing factor, wherein small a' value 922 approaching about 0 provide a large amount of filtering or smoothing and large a' value 30 922 approaching about 1 provide a small amount of filtering or smoothing. The a' value 922 may be selected based on the ultrasonic instrument type and power level. The calculated Voltage Slope data points are provided to a "fast response" filter 916 to calculate the moving average for the Voltage Slope. At 924, the "fast response" filter 916 calculates the moving average for the Voltage Slope based on a number of data points 926. -29- In the illustrated embodiment, the output of the "slow response" moving average filter 914 "Slope EWMA" is applied to a (+) input of an adder 928 and the output of the "fast response" filter 916 "Slope Avg" is applied to (-) input of the adder 928. The adder 928 computes the difference between the outputs of the "slow response" moving average filter 914 and the "fast 5 response" filter 916. The difference between these outputs is compared at 930 to a predetermined limit 932. The limit 932 is determined based on the type of ultrasonic instrument and the power level at which the particular type of ultrasonic instrument is energized at. The limit 932 value may be predetermined and stored in memory in the form of a look-up table or the like. If the difference between the "Slope EWMA" and the "Slope Avg" is not greater than the limit 932, the 10 processor 400 continues along the "No" branch and resets a counter to zero at 940, then returns a value 934 to the results 710 block that indicates that no voltage drop point was found in the sampled voltage signals and, therefore, no change in tissue state was detected. However, if the difference between the "Slope EWMA" and the "Slope Avg" is greater than the limit 932, the processor 400 continues along the "Yes" branch and increments a counter at 942. At 944, the 15 processor 400 decides whether the counter is greater than 1, or some other predetermined threshold value for example. In other words, the processor 400 takes at least two data points in regards to the voltage drop point. If the counter is not greater than the threshold (e.g., 1 in the illustrated embodiment) the processor 400 continues along the "No" branch and returns a value 934 to the results 710 block that indicates that no voltage drop point was found in the sampled 20 voltage signals and, therefore, no change in tissue state was detected. If the counter is greater than the threshold (e.g., 1 in the illustrated embodiment) the processor 400 continues along the "Yes" branch and determines that a voltage drop point 936 was found and returns a point index 938 to the results 712 block indicating that a voltage drop point was found in the sampled voltage signals and, therefore, a change in tissue state was detected. As previously discussed with 25 reference to FIG. 15A, if a voltage point 836 is found, then, at 718 (FIG. 15A) the processor 400 activates the change in tissue state indicator 718. While several embodiments have been illustrated and described, it is not the intention of the applicant to restrict or limit the scope of the appended claims to such detail. Numerous variations, changes, and substitutions will occur to those skilled in the art without departing from 30 the scope of the invention. Moreover, the structure of each element associated with the described embodiments can be alternatively described as a means for providing the function performed by the element. Accordingly, it is intended that the described embodiments be limited only by the scope of the appended claims. -30- Reference throughout the specification to "various embodiments," "some embodiments," "one embodiment," or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases "in various embodiments," "in some 5 embodiments," "in one embodiment," or "in an embodiment" in places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Thus, the particular features, structures, or characteristics illustrated or described in connection with one embodiment may be combined, in whole or in part, with the features 10 structures, or characteristics of one or more other embodiments without limitation. -31-

Claims (31)

1. A method for driving an end effector coupled to an ultrasonic drive system of a surgical instrument, the method comprising: generating a first ultrasonic drive signal by a generator coupled to an ultrasonic drive system, wherein the ultrasonic drive system comprises an ultrasonic transducer coupled to a waveguide and an end effector coupled to the waveguide, and wherein the ultrasonic drive system is configured to resonate at a resonant frequency; actuating the ultrasonic transducer with the first ultrasonic drive signal for a first period; generating a second ultrasonic drive signal by the generator; and actuating the ultrasonic transducer with the second ultrasonic drive signal for a second period, subsequent to the first period; wherein the first drive signal is different from the second drive signal over the respective first and second periods; and wherein the first and second drive signals define a step function waveform over the first and second periods.

2. The method of claim 1, comprising: generating a third ultrasonic drive signal by the generator; and actuating the ultrasonic transducer with the third ultrasonic drive signal for a third period; wherein the third drive signal is different from the first second drive signals over the first, second, and third periods; and wherein the first, second, and third drive signals define a step function waveform over the first, second, and third periods.

3. The method of claim 2, wherein: generating the first, second, and third ultrasonic drive signals comprises generating a corresponding first, second, and third drive current; and actuating the ultrasonic transducer comprises: actuating the ultrasonic transducer with the first drive current for the first period; actuating the ultrasonic transducer with the second drive current for the second period; and actuating the ultrasonic transducer with the third drive current for the third period. -32-

4. The method of claim 1, comprising: generating the first ultrasonic drive signal at a first frequency different from the resonant frequency; actuating the ultrasonic transducer with the first ultrasonic drive signal at the first frequency for the first period to provide a first level of mechanical vibration to the end effector suitable for separating a first tissue from a second tissue; generating the second ultrasonic drive signal at the resonant frequency; and actuating the ultrasonic transducer with the second ultrasonic drive signal at the resonant frequency for the second period subsequent to the first period to provide a second level of mechanical vibration to the end effector suitable for transecting and sealing the first tissue.

5. The method of claim 4, wherein first frequency is substantially different from the resonant frequency, and wherein the first period is less than one second.

6. The method of claim 5, wherein the first frequency is defined by the following equation: fi = 2*fo; wherein fi is the first frequency; and wherein fo is the resonant frequency.

7. The method of claim 5, wherein the first frequency is defined by the following equation: fi = fo/2; wherein fi is the first frequency; and wherein fo is the resonant frequency.

8. The method of claim 4, comprising automatically generating the second ultrasonic drive signal at the resonant frequency after the first period.

9. The method of claim 1, comprising: monitoring a measurable characteristic of the ultrasonic drive system; and generating any one of the first and second drive signals based on a measured characteristic.

10. The method of claim 9, wherein the measurable characteristic is impedance of the ultrasonic transducer. -33-

11. The method of claim 9, wherein the measurable characteristic is impedance of a tissue portion contacting the end effector.

12. The method of claim 11, comprising: generating a subtherapeutic radio frequency (RF) signal; applying the RF signal to the tissue portion; and measuring the impedance of the tissue portion.

13. An apparatus for driving an end effector coupled to an ultrasonic drive system of a surgical instrument, the method comprising: a generator configured to couple to an ultrasonic drive system of an ultrasonic instrument, the ultrasonic drive system comprising an ultrasonic transducer coupled to a waveguide and an end effector coupled to the waveguide; wherein the generator is configured to: generate a first ultrasonic drive signal; actuate the ultrasonic transducer with the first ultrasonic drive signal for a first period; generate a second ultrasonic drive signal by the generator; and actuate the ultrasonic transducer with the second ultrasonic drive signal for a second period, subsequent to the first period; wherein the first drive signal is different from the second drive signal over the respective first and second periods; and wherein the first and second drive signals define a step function waveform over the first and second periods.

14. The apparatus of claim 13, comprising a measurement module to monitor a measurable characteristic of the ultrasonic drive system, wherein the generator is configured to generate any one of the first and second drive signals based on a measured characteristic.

15. The apparatus of claim 14, wherein the measurement module comprises: a radio frequency (RF) generator coupled to the end effector, the radio frequency generator configured to generate a subtherapeutic RF signal; and a clamp arm assembly coupled to a distal end of the ultrasonic drive system, the clamp arm assembly comprising an electrically conductive portion forming a return electrode coupled to -34- the RF generator, the clamp arm assembly being operatively coupled to the end effector to grasp tissue therebetween.

16. The apparatus of claim 14, wherein the measurement module comprises: a voltage sense module; a current sense module; an analog-to-digital converter (ADC) coupled to the voltage sense module and the current sense module; and a processor coupled to the ADC, the processor being configured to measure impedance.

17. A system for driving an end effector coupled to an ultrasonic system of a surgical instrument, the method comprising: a generator; and an ultrasonic instrument comprising an ultrasonic drive system coupled to the generator, the ultrasonic drive system comprising an ultrasonic transducer coupled to a waveguide and an end effector coupled to the waveguide; wherein the generator is configured to: generate a first ultrasonic drive signal; actuate the ultrasonic transducer with the first ultrasonic drive signal for a first period; generate a second ultrasonic drive signal by the generator; and actuate the ultrasonic transducer with the second ultrasonic drive signal for a second period, subsequent to the first period; wherein the first drive signal is different from the second drive signal over the respective first and second periods; and wherein the first and second drive signals define a step function waveform over the first and second periods.

18. The system of claim 17, wherein the generator comprises a measurement module to monitor a measurable characteristic of the ultrasonic drive system, wherein the generator is configured to generate any one of the first and second drive signals based on a measured characteristic. -35-

19. The system of claim 18, wherein the measurement module comprises: a radio frequency (RF) generator coupled to the end effector, the radio frequency generator configured to generate a subtherapeutic RF signal; and a clamp arm assembly coupled to a distal end of the ultrasonic drive system, the clamp arm assembly comprising an electrically conductive portion forming a return electrode coupled to the RF generator, the clamp arm assembly being operatively coupled to the end effector to grasp tissue therebetween.

20. The system of claim 18, wherein the measurement module comprises: a voltage sense module; a current sense module; an analog-to-digital converter (ADC) coupled to the voltage sense module and the current sense module; and a processor coupled to the ADC, the processor being configured to measure impedance.

21. A method of determining a change in tissue state in an ultrasonic surgical instrument, the method comprising: sampling, by a processor, one or more drive signals of a generator for driving an ultrasonic transducer; calculating, by the processor, a first weighted moving average of at least one of the sampled drive signals; calculating, by the processor, a slope of the calculated first moving average of at least one of the sampled drive signals; calculating, by the processor, a second weighted moving average for the slope of the sampled drive signal; calculating, by the processor, a moving average for the slope; comparing, by the processor, the second weighted moving average for the slope and the moving average for the slope; and determining, by the processor, the occurrence of a change in tissue state based on the comparison.

22. The method of claim 21, comprising activating by the processor an output indicator when the change in tissue state occurs. -36-

23. The method of claim 22, wherein activating the output indicator comprises activating any one of a visual, audible, and tactile feedback to notify a user of the ultrasonic surgical instrument of the occurrence of a change in tissue state.

24. The method of claim 22 comprising: in response to the output indicator, disconnecting, by the processor, an output power of the generator.

25. The method of claim 22, comprising: in response to the output indicator, reducing, by the processor, an output power of the generator.

26. The method of claim 22, comprising: in response to the output indicator cycling, by the processor, an output power of the generator

27. The method of claim 22, comprising: in response to the output indicator, pulsing, by the processor, the output power of the generator outputting a high-power momentary surge from the generator.

28. The method of claim 21, wherein calculating, by the processor, the first weighted moving average of at least one of the sampled drive signals comprises calculating, by the processor, an exponentially weighted moving average for the sampled drive signal in accordance with the following formula: St = aYt+(1-a)St_1 wherein, St is a current moving average of the sampled drive signal; St_1 is a previous moving average of the sampled drive signal; a is a smoothing factor; and Yt is a current data point of the sampled drive signal.

29. The method of claim 28, wherein calculating, by the processor, the second weighted moving average for the slope of the sampled drive signal comprises calculating, by the processor, -37- the second weighted moving average for the slope of the sampled drive signal in accordance with the following formula: S't = a',Y't+(1-a',)S' t_1 wherein, S't is a current moving average of a slope of the sampled drive signal; S't_1 is a previous moving average of the slope of the sampled drive signal; a is a smoothing factor; and Y't is a current slope data point of the slope of the sampled drive signal.

30. The method of claim 21, wherein the sampled drive signal is a frequency signal and determining, by the processor, the occurrence of a change in tissue state based on the comparison comprises determining, by the processor, a frequency inflection point.

31. The method of claim 21, wherein the sampled drive signal is a voltage signal and determining, by the processor, the occurrence of a change in tissue state based on the comparison comprises determining, by the processor, a voltage drop point over at least two consecutive samples. -38-