CA1313245C - High frequency jet ventilation technique and apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Respiration therapy is accomplished by generating pulses of inhalation gas for delivery to a patient. The inhalation gas pulses may be generated by entraining humidified low pressure gas with pulses of high pressure entrainment gas. The pulses of entrainment gas may be of variable frequency, duration and duty cycle and are produced by modulating the flow of a highly pressured gas with a solenoid operated valve.

Description

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This application is a division of Appli-cation Ser. No. 528,258, filed January 27, 1987.
This invention relates generally to ventilators for supplying gas to facilita-te and support human respiration and particularly to ventilators which employ a high frequency jet of gas for respiratory therapy. More specifically, the present invention is directed to enhancing ventilation at supraphysiologic rates and especially to maximizing the tidal volume of gas delivered to a patient during respiration therapy while simul-taneously minimizing patient discomfort and the possibility of causing or aggravating trauma.
Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
While not limited thereto in its utility, the present invention is particularly well suited to high frequency jet ventilation. The use of high frequency jet ventilation has proven to be quite beneficial in the treament of certain respiratory conditions. In high frequency ventilation, rather than moving gas in bulk quantity into the gas exchanging areas of the lungs, ventilation is achieved by enhancing the mass transfer processes in the lungs through high frequency oscillation of the supplied gas. However, as the pulsation frequency of the gas delivered by a ]et ventilator increases, supplying the necessary tidal volume of inhalation gas becomes more difficult and is limited by the response time of mechanisms employed for generating 13~ 3~

the gas pulses. In addition, the requirements of reliability, ease of maintenance and susceptibility to sterilization are important design considerations for a ventilator. Portability is a further desirable characteristic. Accordingly, the principal objectives of the present invention are to provide a new and improved ventilation technique and a multi-frequency jet ventilator which operates in accordance with this technique and is compact, relatively easy to maintain, capable of being easily sterilized and supplies a maximized tidal volume of ventilation gas flow over a wide range of frequencies and duty cycles.
The novel method of the present invention includes the steps of creating a humidified bias flow of gas and entraining gas from that bias flow to create a highly humidiried inhalation gas. The entrainment consists of subjecting the bias flow to the effect of high velocity pulses of gas derived from a high pressure source of entrainment gas. The invention further contemplates the exercise of -; control over the entrainment gas to vary the frequency, duration and width of the gas pulses to satisfy the requirements of the treatment being performed.
In accordance with a particular embodiment of the invention there is provided a method for generating gas pulses for use in respiration therapy comprising the steps of:
providing a flow of primary gas;
periodically interrupting the primary gas flow by means of a normally closed solenoid actuated valve;

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generating solenoid control voltage pulses having first and second voltage magnitude levels, said voltage levels including an initial magnitude in excess of the rated voltage of the solenoid and a second contiguous magnitude less than the said rated voltage and sufficient to hold the solenoid in the actuated sta-te; and applying said voltage pulses to the valve solenoid to cause the generation of gas pulses.
lQ Figure 1 is a functional block diagram of a multi-frequency jet ventilator in accordance with the present invention;
Figure 2 is an enlarged fragmentary sectional view of a preferred embodiment of -the entrainment module of the multi-frequency jet ventilator of Figure l;
Figure 3 is a functional block diagram of the control module of the multi-frequency jet ventilator of Figure l;
Figures 4a, 4b and 4c are graphical illus-trations of gas pulse trai.ns provided to the entrainment module of Figure 2 in response -to the control signals generated by the control module of Figure 3;
and Figure 5 is a waveform diagram of a control voltage generated by the control module of Figure 3.

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DETAILE:D DE:SCRIPTION OF T~ 7ENTION
With reference to the drawing, wherein like numerals represent like parts throughout the several figures, a ventilator in accordance with one embodiment of the present invention is generally designated in Figure 1 by the numeral 10. ~entilator 10 may be selectively employed at conventional ventilation frequencies or may be utilized as a high frequency jet ventilator.
Ventilator 10 preferrably has a range of operational frequencies of from 4 breaths~minute (1/15 Hz.) to 3000 breaths/minute (50 la Hz~ and an inspiratory time, i.e., a duty cycle, in the range of 5% to 95% as will be more fully described below. The ventilator 10 will supply respiratory gas to a patient via either a cuffed or uncuffed endotracheal tube (not illustrated~ and is adaptable for ventilating with air, air/oxygen, helium/oxygen or any other suitable gas or combination of gases. Ventilator 10 has a compact, lightweight construction and may be either battery or line current powered.
Ventilator 10 is an integrated modular system ~hich generally comprises a control unit 12, a high pressure gas supply unit 1~, a 20 low pressure gas supply unit lS and an entrainment module 16. The control unit 12 comprises the electronic controls, safety system and the electrical power supply for the ventilator. The high pressure gas supply unit 14 comprises a source of high pressure gas, a gas pressure regulation system and a valve subassembly for producing controlled pulses of the gas derived from the high pressure source. The low pressure gas supply unit 15 comprises a source of low pressure gas and a humidification system for the gas. The entrainment module 16 produces, from the pulses of high ~ , ~
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p .sure gas and the humidified low pressure gas, the required output of the ventilator. The gas flow lines are designated by heavy lines and the electrical interconnections are designated by thin lines in the drawing. The control unit 1~ is connected to the supply unit 14 via conventional se~arable electrical connectors. The gas supply units 14 and 15 and entrainment module 16 are interconnected by stan~ard flexible hoses. The above-mentioned modular units and their sub-units may be easily connected and disconnected. The modular construction thus facilitates maintenance of the ventilator and also provides a ventilator which, to the extent required, may be easily disinfected and sterilized as will be more fully apparent from the discussion below.
With reference to Figure 1, the control unit 12 comprises an electronic control module 20 which generates control pulses for operating a solenoid actuated valve 22 in supply unit 14. The control module 20 also provides input signals to an electronic safety module 2~. The safety module 24, in the manner to be described below, controls an electrically operated shutoff valve 26 in the primary, i.e., high pressure, gas supply line and is also connected to an alarm system 28.
A source of pressurized gas 30, which is typically in the form of plural tanks containing compresse~ dry air, oxygen/nitrogen, or oxygen~helium, is coupled via shutoff valve 26 and an adjustable pressure regulator 32 to an accumulator 34.
The pressurized gas which appears at the output port of accumulator 34 has a regulated substantially constant pressure in the range of between 5 psi and 250 psi. The pr~ssurized gas flows 132~
fr~.~ the accumulator 34 via a flow sensor 36 and valve 22 to entrainment module 16. The flow sensor 36 provides an information bearing input signal to safety module 2~ whereby the nature of the gas flow to the entrainment module 16 derived from high pressure source 30 may be continuously monitored to provide a means for actuating the alarm ~8 in the event that the aforementioned gas flow is not within the selected and required operational limits of the ventilator. The alarm 28 is preferably both an audible and a visual alarm. The safety module 24, which is preferrably a microprocessor, is programmed to monitor the operation of the ventilator, especially the primary gas flow to the entrainment module and the pressure downstream of the entrainment module, in order to determine whether the operati.ng parameters are within pre-established ranges. Should a monitored parameter move into a range which is un5afe to the patient, module 24 will command the closing of shutoff valve 26.
A secondary pressurized gas source 42, within unit 15, is coupled via a shutoff valve 44 to a humidifier 46. The output of humidifier ~6 is a bias flow of heated humidified gas which is continuously supplied to the entrainment module 40 at A relatively low pressure such as 5 psi. The secondary gas source ~2 is typically in the form of one or more tanks containing the same gas as supplied by "high" pressure source 30. Humidifier ~6 is preferably a cascade bubble humidifier and causes the bias flow to have approximately 100% relative humidity. In addition, an ultrasonic nebuli~er 47 may be employed to introduce a vapor mist to the bias flow of humidified gas. The stream of humidified gas and vapor mist, which is flowing at low velocity, is entrained in , ~3~32~
dule 16 by high velocity gas pulses, pro~uced in the manner to be described below, to form an output gas stream. As ncted above, the output stream is supplied to the patient via an endotracheal tube (not fully illustrated). The entrainment module 16 also functions to receive gases exhaled by the patient.
Depending on the state of a t~Jo-way flow control valve ~8, the exhaled gas is either vented to the ambient atmosphere or delivered to a reclamation unit 49. A pressure sensor 50 may be interposed in the gas path which extends from the entrainment lo module to the endotracheal tube for sensing the pressure immediately upstream of the endotracheal tube and providing a corresponding input signal to the safety module 24 for insuring safe operation of the ventilator.
With reference to Figure 2~ the entrainment module 16 comprises a housing 51 which interiorly forms an entrainment chamber 52. ~ousing 51 is a generally T~shaped cylindrical member which has an open output end. A fitting 54 at the output end fluidically couples chamber 52 to a conduit 56 which leads to or comprises the end of the endotracheal tubeO Gas pulses, produced by modulating the gas exiting accumulator 3~ by means of valve 22, are injected into the entrainment chamber 52 through a nozzle 58. Nozzle 58 is a convergent or convergent-divergent nozzle and thus the velocity of the gas downstream of the nozzle throat is high. Nozzle 58 extends axially into chamber 52 through an end wall of the housing 51 along the central axis of the chamber. Nozzle 58 is aerodynamically shaped to enhance entrainment by directing the low pressure bias flow in the downstream direction in chamber 52. Nozzle 58 thus preferrably ~3:132~
a forwardly tapering convergen~ external profile. An inlet leg 62 and an outlet leg 64 protrude radially at diametrically opposite locations of housing 51. Legs 62 and 64 are substantially identical and are equidistant from the centrally disposed nozzle 58. Inlet leg 62 functions as a connector structure for coupling to a conduit for supplying the low velocity bias stream of humidified gas to the entrainment chamber 52 via port 66 as illustrated by the arrows in Figure 2. The humidified gas is continuously supplied to the entrainment chamber. During an inspiratory phase of the ventilation cycle, humidified gas is entrained by a high velocity gas pulse injected into chamber 52 via the nozzle 58 and propelled axially through the chamber to conduit 56 and thence to the patient via the endotrachial tube. The ventilating gas pulses delivered to the patient will be comprised primarily of humidified gas supplied via inlet leg 62~entrained by the pulses of dry gas supplied via nozzle 58. Accordingly, the patient will receive gas having the highest possible relative humidity.
During the expiratory phase of the ventilation cycle, the gas exhaled by the patient returns via conduit 56 to the entrainment chamber 52. The exhaled gas is entrained by the low velocity bias flow and is thus discharged through discharge port 68 which leads -to outlet leg 64. Leg 6~ is coupled to a conduit - for conducting the exhaled gas and excess humidified gas to valve 48. The expired carbon dioxide from the patient is discharged through port 68 in part due to the driving force of the bias flow of humidified gas which prevents the exhaled gases from entering port 66.

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- The entrainment of the humidified gas by the hi~h velocity pulses or slugs of primary gas is facilitated by the convergent exterior shape of nozzle 58 which, as mentioned above, functions as a flow control surface. The entrainment of the humidified gas is improved by the placement of the outlet 60 of nozzle 58 at an axial location of the chamber which is proximate the downstream axial terminus of the inlet port 66. Consequently, the high velocity pulse is injected into the chamber at a location slightly do~nstream from the entry of the humidified gas. As should now be obvious, the continuous supply of the low pressure humidified secondary gas functions to alternately supply humidified gas for entrainment and to remove the expired carbon dioxide from the ventilator unit without the use of any mechanical valves which would otherwise tend to deteriorate the entrainment effects and, thus, would result in lower tidal volumes.
A low compliant tube connects nozzle 58 to the solenoid actuated control valve 22. Valve 22 is a bi-state val~e having an open and closed position. The command signals generated by control module 20 and applied to the solenoid of valve 22 determine the frequency and duration of the gas pulses delivered to nozzle 58. Thus, valve 22 is cyclically opened and closed for selected time intervals to interrupt the flow of pressurized gas to nozzle 58 to thereby produce the desired gas pulse train characteristics to provide optimum treatment for the patient.
The characteristics of the train of pressurized gas pulses produced by valve 22 may best be appreciated by reference to Figures 4a, 4b and 4c. The horizontal axes represent the time in milliseconds and the vertical axes represent the flow rate of the ~3~ 32~~~
h lh velocity ventilation gas exiting nozzle 58. The letter T
represents the time of one ventilation cycle, i.e., the time of a inspiratory phase plus the time of a following expiratory phase. The symbol tl represents the time interval during which valve 22 is open. For each of the graphs of Figure ~, the time interval in which valve 22 is opened, i.e., the inspiratory time, is 30 percent of the ventilation cycle T. The graph of Figure 4a represents the pulse train characteristics when valve 22 is opened and closed at a 5 Hz. frequency. Graph 4b represents the pulse characteristics when valve 22 is opened and closed at a 10 Hz. frequency. Figure 4c represents the pulse charactistics when valve 22 is opened and closed at a 20 Hz. frequency.
The volume of gas supplied by t~e valve per breath is equal to the area under the flow rate-time curve of the graphs of Figure 4. The solid lines represent the flow characteristics for ventilator 10. The broken lines represent the flow characteristics for a ventilator which does not incorporate a feature for redocing the time required for the valve to change states in accordance ~ith the present invention. It will be appreciated that the depicted curves have a trapezoidal shape rather than a square wave shape due to the incremental time interval required for valve 22 to change from one state to another, i.e., from a fully closed state to a fully open state and vice versa. In the prior art, at high pulse frequencies there was insufficient time for the valve to open completely before receipt of a "closen command. ~ccordingly, the triangular flow pattern indicated by the broken lines of Figure ~c resulted.
flow pattern as represented by the broken line showing of Fi re 4c leads to a drastic reduction in the tidal ~olume, i.e., the volume of gas s~pplied to the patient, during the inspiratory phase. Consequently, in order that sufficient tidal volumes be supplied at high ventilation frequencies, the valve must be caused to react quickly to "open" commands and should remain open for a significant portion of the inspiratory phase.
With reference to Figure 3, valve 22 is opened and closed by means of solenoid 70 which is responsive to command signals generated by the control module 20. Control module 20 includes a lo square wave generator 72. A resistance capacitance network 73 is adjustable in the conventional manner to vary the time constant of and thus the output frequency of square wave generator 72.
The square wave output signal of generator 72 is applied to a timer circuit 74. Referring jointly to Figures 3 and 5, an adjustable voltage magnitude selection circuit and an adjustable duty cycle selection circuit are coupled to timer 74 to cause the timer to provide an output waveform having a selected amplitude (voltage Vl), pulse width ttime tl) and frequency f. Voltage v is selected to be the minimum solenoid holding voltage required to sustain valve 22 in the opened position. This voltage is typically lower than the voltage necessary to cause the solenoid to open the valve. ~se of a low voltage to maintain the valve open reduces the closing time for the valve. The closing time is further reduced ~y a short duration large negative voltage spike 2 which is generated at the end of the inspiratory phase of the cycle upon removal of the timer 74 output voltage from the valve solenoid. Time tl is selected to provide the optimum inspiratory time per ventilation cycle. Frequency f is selected to provide ~3~32'~`

th optimum ventilation frequency in accordance with the condition of the patient. The square wave from circuit 72 is also applied to an overdrive timer circuit 76. The overdrive timer circuit is also adjustable to generate a second waveform having a second amplitude (voltage V3-Vl) and second pulse width (time t2) with the same frequency f as and in phase with the waveform provided by timer 74. Voltage V3-V1 and tiTne t2 are selected to reduce the valve opening time as detailed hereinafter. The two waveforms are combined, as represented lo schematically by summing circuit 77, and applied to solenoid 70.
The waveform applied to solenoid 70 is illustrated in Figure 5.
The period of one opening and closing phase or cycle of valve 22, and hence the ventilation cycle, is given by time T. By applying the overdrive voltage V3-Vl to the solenoid, the overdrive voltage having an amplitude which is at le~st three times as great as the holding voltage V1, a greater electromagnetic force is generated, and the opening time of the valve is significantly reduced. Thus, the tidal volumes produced by the ventilator at high frequencies is not substantially reduced by the time required for the valve to change its state. As noted above, in the graphs o~ Figure 4, the broken lines illustrate generally the pulse characteristics without application of the overdrive voltage to the solenoid and the solid lines represent the pulse characteristics of the ventilator when the foregoing described overdrive voltage is applied from the control module.
It will be appreciated that the ventilator 10 is operated by selecting an optimum ~requency and duty cycle, i.e., the ratio of inspiration time to ventilation cycle time, for the condition of . :.

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th ?atient. The tidal volume oE ventilation gas supplied to the patient is a function of pulse frequency and duration as well as gas pressure. Pressure regulator 32 regulates the pressure by conventional means. The control module functions to electronically control valve 22 to provide the optimum ventilation characteristics. The latter characteristics may chang~ over the treatment period and the ventilator of the present invention is capable of manual or automatic readjustment in accordance with varying patient requirements. In actual practice, the control o and safety modules may be a single subassembly including a programmable microprocessor and the operational mode may be entered from a keyboard and/or selected from preprogrammed data.
Since this can be accomplished without disconnec~ing the patient from the ventilator, trauma is avoided that could otherwise occur. It should be appreciated that since the ventilator is of modular construction, sterilization and maintenance of the unit can be relatively easily achieved. The entrainment module 16 has no moving components and thus may be easily disconnected from the ventilator for sterilization and/or replacement.
The present invention has the flexibility, particularly operational parameters which are adjustable over broad ranges, which enables its use in a synchronous intermittent mandatory ~entilation (IMV) mode. The IMV mode will be selected, via the microprocessor based control module 20, when it is desired to attempt to wean a patient from the ventilator. In the IMV mode a pulse, at a frequency less than the normal breathing rate, will be provided by a clock in the microprocessor to trig~er the generation of command signals for the valve 22 solenoid. A

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s~~sor 80, which could be a pressure sensor in the endotracheal tube, will sense spontaneous breathing by the patient and provide signals commensurate therewith which are inputted to control module 20. The valve 22 will open at the selected frequency except each time spontaneous exhalation is sensed, in which case the opening of the valve ~ill be delayed until the end of exhalation and the clock will be reset to zero. The present invention may also, with the removal of the entrainment module 16 and low pressure gas supply unit 15, be employed in the case of a transcutaneous cricothyroidalostomy. In emergency situations, for example under battlefield conditions or in the case of medical technicians at the scene of an accident, a patient experiencins breathing difficulty cannot be provided with an endotracheal tube.
That is, the proper insertion of an endotracheal tube may require as long as one-half hour. requires good lighting and requires a highly trained medical professional. The present invention, wi-th the entrainment module removed but a nozzle similar to nozzle 58 retained, can be utilized by medical technicians in the following manner. A needle with associated catheter will be inserted into the trachea, the needle will then be withdrawn and the nozzle then inserted into the trachea via the catheter. Jet ventilation may then be started with e~halation being via the patient's mouth and/or nose.
While preferred embodiments of the invention have been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the invention disclosed herein. Accordingly, various modifications, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-1. A method for generating gas pulses for use in respiration therapy comprising the steps of:
providing a flow of primary gas;
periodically interrupting the primary gas flow by means of a normally closed solenoid actuated valve;
generating solenoid control voltage pulses having first and second voltage magnitude levels, said voltage levels including an initial magnitude in excess of the rated voltage of the solenoid and a second contiguous magnitude less than the said rated voltage and sufficient to hold the solenoid in the actuated state; and applying said voltage pulses to the valve solenoid to cause the generation of gas pulses.

2. The method of claim 1 wherein the step of generating the control pulses includes varying the frequency of the pulses as a function of the required therapy.

3. The method of claim 2 wherein the step of generating the control pulses further includes vary-ing the duty cycle of the pulses as a function of the required therapy.

4. The method of claim 2 wherein the step of generating the control pulses further includes vary-ing the width of the pulses as a function of the required therapy.

5. The method of claim 3 wherein the step of generating the control pulses further includes vary-ing the width of the pulses as a function of the required therapy.
6. The method of claim 1 further comprising:
providing a stream of humidified secondary gas; and entraining secondary gas from said stream with said pulses of primary gas.

7. The method of claim 6 wherein the step of providing the stream of secondary gas includes directing said stream along an axis across an entrainment chamber generally in a first direction and wherein the step of entrainment includes deliver-ing the pulses of primary gas into the chamber in a second direction which is generally transverse to said first direction and at a point downstream of said axis in said second direction.
8. The method of claim 2 further comprising:
providing a stream of humidified gas; and entraining secondary gas from said stream with said pulses of primary gas.

9. The method of claim 8 wherein the step of providing the stream of secondary gas includes directing said stream along an axis across an entrainment chamber generally in a first direction and wherein the step of entrainment includes deliver-ing the pulses of primary gas into the chamber in a second direction which is generally tranverse to said first direction and at a point downstream of said axis in said second direction.

10. The method of claim 9 wherein the step of generating the control pulses further includes vary-ing the duty cycle of the pulses as a function of the required therapy.

11. The method of claim 9 wherein the step of generating the control pulses further includes vary-ing the width of the pulses as a function of the required therapy.

12. The method of claim 10 wherein the step of generating the control pulses further includes vary-ing the width of the pulses as a function of the required therapy.