EP0512987A1 - Determination amelioree de la saturation arterielle en oxygene et controle de la tension sanguine arterielle - Google Patents

Determination amelioree de la saturation arterielle en oxygene et controle de la tension sanguine arterielle

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Publication number
EP0512987A1
EP0512987A1 EP90908025A EP90908025A EP0512987A1 EP 0512987 A1 EP0512987 A1 EP 0512987A1 EP 90908025 A EP90908025 A EP 90908025A EP 90908025 A EP90908025 A EP 90908025A EP 0512987 A1 EP0512987 A1 EP 0512987A1
Authority
EP
European Patent Office
Prior art keywords
pressure
arterial
body part
light
patient
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP90908025A
Other languages
German (de)
English (en)
Other versions
EP0512987A4 (en
Inventor
Justin S. Clark
William Dean Wallace
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
MEDICAL PHYSICS
Original Assignee
MEDICAL PHYSICS
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by MEDICAL PHYSICS filed Critical MEDICAL PHYSICS
Priority claimed from CA002074956A external-priority patent/CA2074956A1/fr
Publication of EP0512987A1 publication Critical patent/EP0512987A1/fr
Publication of EP0512987A4 publication Critical patent/EP0512987A4/en
Withdrawn legal-status Critical Current

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Classifications

    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
    • A61B5/6801—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be attached to or worn on the body surface
    • A61B5/683—Means for maintaining contact with the body
    • A61B5/6838—Clamps or clips
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/02—Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
    • A61B5/021—Measuring pressure in heart or blood vessels
    • A61B5/022—Measuring pressure in heart or blood vessels by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers
    • A61B5/02225—Measuring pressure in heart or blood vessels by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers using the oscillometric method
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/02—Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
    • A61B5/021—Measuring pressure in heart or blood vessels
    • A61B5/022—Measuring pressure in heart or blood vessels by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers
    • A61B5/0225—Measuring pressure in heart or blood vessels by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers the pressure being controlled by electric signals, e.g. derived from Korotkoff sounds
    • A61B5/02255—Measuring pressure in heart or blood vessels by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers the pressure being controlled by electric signals, e.g. derived from Korotkoff sounds the pressure being controlled by plethysmographic signals, e.g. derived from optical sensors
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
    • A61B5/1455—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using optical sensors, e.g. spectral photometrical oximeters
    • A61B5/14551—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using optical sensors, e.g. spectral photometrical oximeters for measuring blood gases
    • A61B5/14552—Details of sensors specially adapted therefor
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
    • A61B5/6801—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be attached to or worn on the body surface
    • A61B5/6813—Specially adapted to be attached to a specific body part
    • A61B5/6814—Head
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
    • A61B5/6801—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be attached to or worn on the body surface
    • A61B5/6813—Specially adapted to be attached to a specific body part
    • A61B5/6814—Head
    • A61B5/6815—Ear
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
    • A61B5/6801—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be attached to or worn on the body surface
    • A61B5/6813—Specially adapted to be attached to a specific body part
    • A61B5/6825—Hand
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
    • A61B5/6801—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be attached to or worn on the body surface
    • A61B5/6813—Specially adapted to be attached to a specific body part
    • A61B5/6825—Hand
    • A61B5/6826—Finger
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
    • A61B5/6801—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be attached to or worn on the body surface
    • A61B5/6813—Specially adapted to be attached to a specific body part
    • A61B5/6829—Foot or ankle

Definitions

  • the present invention is related to noninvasive systems and methods which are used to monitor the physiological condition of a patient's circulatory system. More particularly, the present invention is related to an enhanced noninvasive system and method for monitoring a patient's arterial oxygen saturation, and which also provides continuous measurement of blood pressure.
  • anesthetized patient undergoes surgery, it is generally the anesthesiologist's role to monitor the general condition of the patient while the surgeon proceeds with his tasks. If the anesthesiologist has knowledge of the patient's arterial oxygen saturation, heart rate, and blood pressure, the general condition of the patient's circulatory system can be assessed.
  • Arterial oxygen saturation (abbreviated herein as S a O 2 ) is expressed as a percentage of the total hemoglobin in the patient's blood which is bound to oxygen. The hemoglobin which is bound to oxygen is referred to as oxyhemoglobin.
  • the S a O 2 value is above 95% since blood traveling through the arteries has just passed through the lungs and has been oxygenated. As blood courses through the capillaries, oxygen is off-loaded into the tissues and carbon dioxide is on-loaded into the hemoglobin. Thus, the oxygen saturation levels in the capillaries (abbreviated herein as S c O 2 ) is lower than in the arteries. Furthermore, the blood oxygen saturation levels in the veins is even lower, being about 75% in healthy patients.
  • the physician may take action such as reducing or increasing the amount of oxygen being administered to the patient.
  • Proper management of S a O 2 is particularly important in neonates where S a O 2 must be maintained high enough to support cell metabolism but low enough to avoid damaging oxygen-sensitive cells in the eye and causing impairment or complete loss of vision.
  • Blood pressure monitoring includes at least three values which are of interest to a physician.
  • the systolic pressure is the high pressure generated in the arteries during contraction (or systole) of the left ventricle of the heart.
  • the diastolic pressure is the pressure maintained in the arteries during relaxation (or diastole) of the left ventricle. Due to the elastic nature of the walls of the arteries, the diastolic pressure is above zero but less than the systolic pressure.
  • a third value of interest to a physician is the mean arterial pressure.
  • the mean arterial pressure may be simply described as the arithmetic average of all the blood pressure values between, and including, the systolic and diastolic pressures.
  • a physician is also interested in obtaining the blood pressure waveform.
  • patients having identical systolic and diastolic values may have very different mean arterial pressures and their blood pressure waveforms may be dramatically different. Having the blood pressure waveform at hand allows the physician to more accurately assess the patient's condition.
  • Blood pressure is generally measured quantitatively in millimeters of mercury (mmHg) referenced against atmospheric pressure (about 760 mmHg).
  • mmHg millimeters of mercury
  • the blood pressure may be 120 mmHg above atmospheric pressure during systole and 70 mmHg above atmospheric pressure during diastole. Such values are commonly recorded as "120 over 70" (120/70).
  • One method of determining S a O 2 is to withdraw blood from an artery and analyze the same to determine the amount of oxyhemoglobin present. While in vitro analysis provides the most accurate blood gas determinations, the disadvantages of drawing a blood sample each time an S a O 2 determination is desired by the physician is readily apparent. Significantly, even in the operating room in vitro S a O 2 determinations may take up to several minutes. Since nerve cells deprived of sufficient oxygen begin to die in a matter of minutes, the time taken to obtain the results of an in vitro S a O 2 analysis may seriously compromise patient safety.
  • oximetry has been adopted in the art to refer to noninvasive apparatus and methods for determining blood oxygen saturation levels.
  • Previously available oximetry systems make use of the fact that the absorption characteristics of different blood components, namely, HbO 2 and Hb and also referred to as the coefficient of extinction, differ depending upon which wavelength of light (e.g., infrared or visible portions of the spectrum) is being used.
  • the previously available systems and methods of monitoring blood pressure also all have a variety of disadvantages.
  • the most commonly performed method the auscultatory sphygmomanometer method (utilizing a pressure cuff, mercury manometer, and a stethoscope), often provides reasonable estimates of systolic and diastolic blood pressure. But the method does not provide any information concerning the mean blood pressure or the pressure waveform. Moreover, a trained professional must take one or more minutes to carry out the method and even then may be unsuccessful.
  • Arterial catheterization provides very accurate blood pressure measurements and waveforms in critical care situations.
  • the extreme invasiveness and the risks of catheterization, including infection, thrombus formation, hemorrhage, and cerebral embolization precludes the method from being routinely used on patients.
  • Another object of the present invention is to implement a noninvasive system and method for carrying out arterial blood oximetry which is more accurate than previously available apparatus and methods and which is also capable of being used on more than one body part of the patient.
  • Yet another object of the present invention is to provide a noninvasive system and method for both blood oximetry and accurately determining a patient's systolic, diastolic, and mean arterial blood pressure and displaying the patient's blood pressure waveform.
  • the present invention provides a noninvasive system and method for enhanced monitoring of arterial oxygen saturation (S a O 2 ) which may be used alone or in combination with a method for continuously and noninvasively monitoring blood pressure.
  • the monitoring of blood pressure provides determinations of systolic pressure, diastolic pressure, mean arterial pressure, and perhaps most significantly, producing an accurate arterial pressure waveform.
  • the present invention allows the same hardware to be used for both monitoring of arterial oxygen saturation and monitoring of arterial blood pressure.
  • the apparatus of the presently preferred embodiment of the present invention includes a light means comprising two or more light emitting devices which are positioned to direct at least two light beams into a body part of the patient.
  • the two light beams are comprised of two different wavelengths, preferably a reference light beam, which is absorbed substantially equally by both oxyhemoglobin and reduced hemoglobin, preferably having a wavelength in the infrared portion of the spectrum and a measurement light beam, which is absorbed unequally by oxyhemoglobin and reduced hemoglobin, preferably having a wavelength in the visible red portion of the spectrum.
  • other portions of the spectrum may also be used within the scope of the claimed invention.
  • a detection means which detects the amount of the light beams which are absorbed by the blood.
  • the detection means and equivalent devices may be positioned to detect either the light transmitted through, or reflected by, the body part.
  • the visible red light beam which will be transmitted or reflected will vary according to the ratio of oxyhemoglobin (HbO 2 ), to reduced hemoglobin (Hb) in the blood.
  • Oxyhemoglobin is the component of blood responsible for carrying almost all of the oxygen to the body tissues.
  • the intensity of the detected infrared light beam will not vary significantly with the ratio of HbO 2 to
  • Hb This is due to the fact that the amount of infrared light absorbed by the body part is affected relatively little by the changing proportions of HbO 2 and Hb.
  • an enhancement means is provided to increase the arterial contribution of the pulsatile component of the light beams which are detected by the phototransducer means.
  • the enhancement means comprises a pressure means for imposing an increased pressure on the body part.
  • the pulsatile component may also be referred to as the "AC component” of the light beam “signal.”
  • the pulsatile component is impressed upon a relatively steady light beam “signal” referred to as the “DC” “signal.”
  • the importance of the pulsatile component is known to those skilled in the art and will be further explained later in this disclosure.
  • the enhancement means operates by applying an increased enhancement pressure onto the body part into which the light beams are directed.
  • an enhancement pressure By applying an enhancement pressure to the body part, the enhancement pressure being approximately equal to the mean arterial pressure of the major artery or arteries located in the body part, the arterial pulsatile component of the light beam detected by the phototransducer means will be maximized due to unloading of the transluminal pressure which results in maximizing arterial compliance.
  • the increase in the pulsatile component will be about an order of magnitude greater than the pulsatile component of the detected light beams without application of the enhancement pressure.
  • the enhancement pressure decreases the relative contribution of the capillary blood oxygen saturation (S c O 2 ) to the intensity of the detected light beams.
  • the increased enhancement pressure both increases the modulation of the light beam due to the increase in amplitude of the arterial pulses and by reducing the amount of capillary blood in the body part.
  • the imposition of the enhancement pressure on the body part may be considered a "physiological calibration.” Having carried out such a “physiological calibration” by enhancing the contribution of the pulsatile arterial oxygen saturation level to the light detected by the phototransducer means, a processor means, for example a microprocessor or other computing device, may derive a calibration factor representing the contribution of the capillary oxygen saturation to the total light detected by the phototransducer means.
  • a processor means for example a microprocessor or other computing device
  • the processor means controls the operation of the system to carry out the method of the present invention to completion and thus continually updates and displays the arterial oxygen saturation level of the patient on a display means such as a video monitor.
  • the enhancement pressure may be imposed by a device such as an inflatable pressure cuff, accompanied by a controllable pressure pump, adapted for placement on a finger, forehead, or some other body part.
  • the enhancement pressure is only applied during a first interval of the calibration period.
  • the enhancement pressure is released and a calibration factor is obtained which reflects the ratio of S a O 2 to S c O 2.
  • the monitoring period is begun and the calibration information is used to determine the proportion of the pulsatile signal detected by the phototransducer means which is caused by the arterial oxygen saturation level rather than the capillary oxygen saturation level.
  • the present invention also includes utilizing the above described hardware for continual blood pressure monitoring and waveform display.
  • the pressure monitoring function is carried out by determining the mean arterial pressure and the systolic blood pressure using the oscillometric method.
  • the mean arterial pressure is determined by adjusting the inflation of a pressure cuff placed around a body part until the pulsatile signal is maximized, once the amplitude of the pulsatile signal is maximized, the pressure within the cuff is approximately equal to the mean arterial pressure.
  • the oscillometric method determines the systolic pressure by increasing the pressure applied to a body part to above the systolic pressure, i.e., completely occluding the artery so that no pulsatile signal is present, and then gradually reducing the pressure within the cuff until a pulsatile signal appears, providing a data point which can be used to calculate the patient's systolic pressure using a procedure described herein.
  • the present invention also provides for calculation of a complete pressure waveform and diastolic pressure.
  • the present invention allows the change in volume of the artery, which is proportional to the pressure within the artery, to be detected by the phototransducer means as a modulation of the intensity of the measurement (red) light beam directed into the body part.
  • the pressure-volume relationship of an artery is not linear or the same from patient to patient or from hour to hour.
  • the pressure-volume relationship of the patient's artery may be described and predicted using a model known as the "Hardy model compliance curve.”
  • the information needed to determine the pressure-volume relationship, including the systolic pressure and the mean arterial pressure, are obtained using the oscillometric method during the calibration period when the pressure cuff is inflated in the below-described manner.
  • the pressure within the cuff is released and the volume change in the artery is detected by the phototransducer means.
  • the present invention uses a recursive procedure wherein an estimated diastolic pressure and the Hardy model compliance curve is used to derive a calculated mean arterial pressure. If the difference between the calculated mean arterial pressure and the measured mean arterial pressure is within a predetermined standard, then the estimated diastolic pressure is displayed on the display means as the patient's diastolic pressure. If the calculated mean arterial pressure and the measured mean arterial pressure do not agree within predetermined limits, a new estimated diastolic pressure is chosen and the calculations repeated until the estimated diastolic pressure produces a calculated mean arterial pressure substantially the same as the measured mean arterial pressure.
  • V m maximum volume of the arterial blood vessels in the patient's body part
  • V o volume of the arterial blood vessels in the patient's body part at zero pressure
  • the value of any point on a blood pressure waveform between the systolic and diastolic pressures may be calculated.
  • a continuous and complete blood pressure waveform may be generated using the method.
  • the ability to calculate a complete and accurate representation of the patient's arterial blood pressure waveform is a great advance over previously available systems using photoplethysmography.
  • the blood oximetry functions of the present invention may be carried out alone or a system can be designed to carry out the oximetry function as well as the blood pressure monitoring function without requiring any hardware in addition to that used to carry out the oximetry function of the present invention.
  • Figure 1 is a perspective view of the presently preferred embodiment of the present invention which is configured to provide both blood pressure monitoring and arterial oxygen saturation monitoring functions.
  • Figure 2 is a block diagram of the system of the presently preferred embodiment of the present invention.
  • Figure 2A is a cross sectional view of another preferred embodiment of the pressure cuff represented in Figure 2.
  • Figures 3A and 3B are flow charts representing the steps of one presently preferred method of the present invention for determining arterial blood oxygen saturation levels.
  • Figure 4 is a waveform diagram showing the application and release of pressure on the patient's body by the pressure cuff of the described embodiment and its effect on the detected light beams.
  • Figures 5A and 5B are flow charts representing the steps of another presently preferred method of the present invention for determining arterial blood oxygen saturation levels.
  • both hemoglobin and oxyhemoglobin have approximately the same absorption coefficient for light in the infrared portion of the spectrum.
  • the absorption coefficients of the two compounds is very different for red light in the visible portion of the spectrum.
  • the difference in absorption coefficients allow S a O 2 to be measured noninvasively using two light beams of two appropriate and differing wavelengths.
  • the phrase "light beam” as used herein is intended to include any electromagnetic radiation having an appropriate wavelength which is directed toward, or impinged upon, the patient's body regardless of whether the light beam is collimated or uncollimated, coherent or incoherent.
  • Figure 1 provides a perspective view of the major components of the described embodiment including a micro computer 10, a visual display 12, a pump 28 (incorporating a pump driver), a finger cuff 34 (incorporating a pressure cuff, light emitting diodes, and a phototransducer), as well as cables 26 and 30, and tubing 32 interconnecting the components. It will be appreciated that components which are equivalent to many of the functional blocks represented in Figure 2 are contained within the structures illustrated in Figure 1 and thus are not separately represented in Figure 1.
  • FIG. 1 Shown in Figure 1 is a patient's finger 36 and the presently preferred embodiment of the present invention being used to determine the patient's S a O 2 level at the numerical display represented generally at 12.
  • the patient's blood pressure is also being monitored with the systolic, mean, and diastolic blood pressure values being provided at numerical displays represented generally at 20, 18, and 16, respectively.
  • the patient's blood pressure waveform is also being shown on the visual display indicated at 22.
  • the sensing elements of the embodiment including the pressure cuff 34 which surrounds the light emitting diodes, the photodetector, and the pressure transducer, are located between the first and second knuckle of the patient's index finger. While this position is illustrated for purposes of describing the presently preferred embodiment, other positions on the body may be used in specific circumstances as will be discussed later. Also, the specific arrangement of the sensing elements in relation to the body part will be described as appropriate in the description of the preferred embodiment.
  • FIG 2 illustrates the major functional blocks of the embodiment illustrated in Figure 1 and described herein. It is to be understood that the hardware represented by the functional blocks illustrated in Figure 2 may be implemented in many different ways.
  • the microcomputer may be a general purpose microcomputer 40 such as an IBM Personal Computer or an equivalent device.
  • a more powerful microcomputer or to devise a microprocessor-based apparatus specifically designed to carry out the data processing functions incidental to this invention.
  • the hardware which embodies the processor means of the present invention must function to perform the operations essential to the invention and any device capable of performing the necessary operations should be considered an equivalent of the processor means.
  • advances in the art of modern electronic devices may allow the processor means to carry out internally many of the functions carried out by hardware illustrated in Figure 2 as being independent of the processor means. The practical considerations of cost and performance of the system will generally determine the delegation of functions between the processor means and the remaining dedicated hardware.
  • microcomputer 40 is interconnected with the remaining apparatus hardware by way of I/O ports 44 and a plurality of analog to digital converters 46. Also, a visual display 42 is connected to the microcomputer 40.
  • Visual display 40 performs the function of a display means.
  • the display means may be any device which enables the operating personnel to observe the values and waveforms calculated by the microcomputer.
  • the display means may be a device such as a cathode ray tube, an LCD display, a chart recorder, or any other device performing a similar function.
  • the method of the present invention is carried out under the control of a program resident in the microcomputer.
  • a program resident in the microcomputer Those skilled in the art, using the information given herein, will readily be able to assemble the necessary hardware, either by purchasing it off-the-shelf or by fabricating it and properly program the microcomputer in either a low level or a high level programming language. While it is desirable to utilize clock rates that are as high as possible and as many bits as possible in the A/D converters 46, the application of the embodiment and economic considerations will allow one skilled in the art to choose appropriate hardware for interfacing the microcomputer with the remainder of the embodiment. Also, it should be understood that for reasons of simplifying the diagrams, power supply connections, as well as other necessary structures, are not explicitly shown in the figures, but are provided in actuality using conventional techniques and apparatus.
  • an LED current driver 48 is provided.
  • the LED current driver 48 controls the amount of current directed to the infrared LED and the red LED. Since LEDs are current controlled devices, the amount of current passed through the devices determines, within device limits, the intensity of the light bean emitted thereby.
  • FIG. 2 Schematically shown in Figure 2 is a side view of a patient's finger 36 with the pressure cuff 34 shown in cross section, also referred to as the enhancement means, which surrounds the finger. Disposed on the interior of the pressure cuff are the infrared LED 56, the red LED 54, and a photodiode 64.
  • Both the infrared LED 56 and the red LED 54 may be devices which are commonly available in the semiconductor industry. They provide high power outputs and relatively stable operation at a reasonable cost per device.
  • the red LED 54 preferably emits a light beam having a wavelength of 660 nanometers and the infrared LED 56 preferably emits a light beam having a wavelength of 930 nanometers.
  • Light emitting devices other than those mentioned above could be used and are intended to be within the scope of the inventive concepts claimed herein.
  • the light emitting devices may be placed outside of the pressure cuff 34 with a fiber optic pathway provided to the interior of the pressure cuff.
  • other wavelengths of light may be used as suitable devices for generating such wavelengths become available.
  • the phrase light means is intended to include the above-mentioned LEDs as well as any devices which perform functions equivalent to those performed by the LEDS.
  • any source or sources of light capable of emitting light having two differing and appropriate wavelengths may function as the light means.
  • unitary light emitting devices capable of emitting two or more wavelengths of light, or devices emitting wavelengths of light other than those specified above are within the intended scope of the phrase structure defined by light means.
  • the photodiode 64 disposed within the pressure cuff 34 is preferably one having a spectral response which is substantially equal at the wavelengths emitted by the infrared LED 56 and the red LED 54 and which, like the LEDS, is capable of stable operation over a long period of time. It may be desirable to include a temperature sensing device (not shown) adjacent the LEDS and the photodiode to provide the microcomputer 40 data on the temperature dependent variations in the operations of LEDs 54 and 56 and the photodiode 64. It is preferable that the LEDS and the photodiode be readily replaceable so that any drift which occurs in the operating parameters of the devices (possibly due to the effects of aging) may be remedied by replacing old components with new ones.
  • photodiode 64 may be best labeled by the phrases detection means, light detection means, and transducer means.
  • any device which performs the function of detecting the amount of light transmitted through, or reflected from, a body part and creating an electrical signal of some kind which contains information on the intensity of the light striking the device may function as the detection means, light detection means, or transducer means.
  • phototransducers such as phototransistors and many other devices now available, or available in the future, have application within the scope of the present invention.
  • Methods for determining arterial blood oxygen levels using either light beams passed through, or reflected from, a body part will be described later in this disclosure.
  • the LEDs 54 and 56 be positioned about the finger so that the light beams pass through the digital arteries on each side of the phalanx bone.
  • the arterial blood's contribution to the modulation of the light beams is maximized rather than the light beams being absorbed by tissue and bone.
  • a pair of LEDS each pair including a red LED and an infrared LED, may be positioned immediately adjacent each other.
  • Each pair of LEDs is positioned on the interior of the pressure cuff so that the respective light beams pass through one of the arteries located on each side of the phalanx bone of the finger. This provides that both an infrared and a red light beam will be equally modulated by the same artery.
  • the pressure transducer 58 is used when determining the patient's blood pressure but is not necessary to the blood oximetry function of the present invention.
  • Pressure transducer 58 acts as a pressure detection means or a pressure transducer means and functions to generate an electrical signal which is proportional to the pressure being imposed upon the body part by the pressure cuff.
  • any device performing the same, or an equivalent function should be considered a pressure detection means or pressure transducer means.
  • the sensing elements may be located on body parts such as on a toe, ear, the web of the hand, or over the temporal artery on the patient's forehead, of course, each of these locations will require a different arrangement for the pressure cuff or other structure for imposing the enhancement pressure.
  • the sensing structures over the temporal artery on the forehead requires that the LEDs and photodiode be positioned so that the photodiode senses the light beams which are reflected from, rather than transmitted through, the body part.
  • a structure other than a pressure cuff must be used to apply pressure to the temporal artery and to hold the pressure imposing device in place.
  • the temporal artery may be the most preferred location for the sensing structures in many cases due to the fact that perfusion at the temporal artery is affected less by vascular disease and drugs than the arteries found in the extremities.
  • use of the temporal artery may provide more accurate S a O 2 determinations than a location on a patient's extremities, in some cases.
  • an LED multiplexer 52 driven by a clock 50, alternately connects the current driver 48 to either the infrared LED 56 or the red LED 54.
  • the operation of the clock 50 and the LED multiplexer 52 ensures that only one of either the red LED 54 or the infrared LED 56 will operate at one time.
  • the output of clock 50 is also input to channel multiplexer 74 to provide synchronized operation.
  • the pressure cuff 34 should be opaque so that the photodiode 64 is shielded from any stray ambient light.
  • the pressure cuff 34 is inflated and deflated by a pump 68 which operates under the control of the pump driver which is in turn controlled by the microcomputer 40.
  • the pump 68 need only be capable of inflating the pressure cuff 34 to a pressure equal to the mean arterial pressure. If the embodiment is to be used to also determine blood pressure, the pump 68 should be capable of inflating the pressure cuff 34 to a pressure well above the patient's systolic pressure so that the arteries may be completely occluded and the systolic pressure determined as explained earlier.
  • the pressure cuff 34, pump 68, and pump driver 70 comprise the enhancement means or pressure means of the present invention.
  • any structure which functions to partially or fully occlude a patient's artery should be considered the equivalent of the enhancement means or pressure means.
  • the body part which is used as a sensing location will often dictate the best devices and structures used as the enhancement or pressure means.
  • a preamplifier 66 receives the output of the photodiode 64.
  • the preamplifier 66 boosts the photodiode output to a level usable by the automatic gain control (AGC) 72.
  • AGC automatic gain control
  • the automatic gain control 72 functions to limit the dynamic range of the voltage signal output from the preamplifier 66 to that which is appropriate for the circuits which follow.
  • the gain-controlled output from the AGC 72 is applied to a channel multiplexer 74 which is also driven by the clock 50.
  • a channel multiplexer 74 which is also driven by the clock 50.
  • the LED multiplexer 52 causes the red LED 54 to operate
  • the output of the AGC 72 is directed to Channel 1 (red) as represented at 76 in Figure 2.
  • the LED multiplexer 52 causes the infrared LED 56 to operate
  • the output of the AGC 72 is directed to Channel 2 (infrared) as represented at 78 in Figure 2.
  • Each channel 76 and 78 includes a low pass filter 80 and 82 to reduce high frequency (e.g., ⁇ 40 Hz) noise.
  • the signal output from each of the low pass filters 80 and 82 is applied to pulsatile signal amplifiers 84 and 86, respectively, which include high-pass filters to prevent passage of direct current and very low frequencies (e.g., ⁇ 1 Hz).
  • the pulsatile signal amplifiers 84 and 86 can be thought of as AC amplifiers.
  • the output of the pulsatile signal amplifiers provide ⁇ IR signal and, ⁇ V R signal to the microprocessor by way of the A/D converters 46.
  • the ⁇ 1R and ⁇ V R signals reflect only the AC, i.e., pulsatile, component of the light beams passed through the patient's body part.
  • the total signal amplifiers 88 and 90 are not frequency limited and thus pass to their outputs an amplified waveform containing both the DC and AC components of the V IR and V R signals which were output from the low pass filters 80 and 82, respectively.
  • the microcomputer 40 controls the intensity of the LEDs 54 and 56, the inflation of the pressure cuff 34, and the gain of the output from the photodiode 64.
  • the microcomputer receives as input data, the ⁇ V IR and ⁇ V R signals (pulsatile component of the signals) and the V IR and V R signals (the total signals including both the AC and DC components).
  • the flow chart of Figure 3 is divided into three principal periods: the initialization period, the calibration period, and the monitoring period. Furthermore, the calibration period is divided into an enhancement pressure-on interval when the enhancement pressure is applied to the patient's body part and an enhancement pressure-off interval when the enhancement pressure is not applied.
  • the steps carried out during the initialization period include those pertaining to determining certain set up parameters, and implementing any software routines which must be running while data is being acquired.
  • the steps carried out during the calibration period include imposing an increased enhancement pressure on the body part, acquiring data, determining the S a O 2 with the enhancement pressure on, and then with the enhancement pressure off, continuing to acquire data which can be used to determine a "physiological calibration factor" which is used during the monitoring period.
  • no pressure is applied to the body part and further data is obtained to determine the patient's SO 2 level.
  • the data previously acquired and the resulting calculated values are used according to the method described herein to determine the S a O 2 level during the monitoring period.
  • the method of the present invention begins during the initialization period with the initialization of the hardware and software of the system as represented at step 100.
  • the various software routines which should be run after power is applied, but before data is acquired.
  • such a noise discrimination routine may be one known to those skilled in the art which includes an algorithm to distinguish information associated with each pulse and heart beat from noise, which in the present system, may be due to ambient light temporarily striking the photodiode or artifacts in the signals caused by notion of the patient.
  • the patient's heart rate will be determined and may be displayed for the information of the attending medical professional.
  • the calibration period includes an "enhancement pressure-on interval” and an “enhanced pressure-off interval” which is followed by a monitoring period.
  • the length of each of these periods (T EP , T NP , and T MON , respectively) are determined at step 104 according to the criteria discussed below. While not represented in the flow chart of Figure 3A, in some embodiments it may be desirable to include a software routine which will vary T EP , T NP , and T MON according to the physiological condition of the patient.
  • T EP will be less than or equal to about 0.2 to about 0.5 of the sum of T NP and T MON resulting in a pressure imposed duty cycle of less than about 20% to about 50%.
  • T EP + T NP the calibration period
  • the calibration period must be long enough to allow accurate data to be collected.
  • physiological parameters change over time, and may change rapidly due to stress, injury to the patient, drugs, or other treatment administered to the patient, the steps of the calibration period must be carried out regularly.
  • the enhancement pressure is applied to the body part as represented at step 106.
  • the enhancement pressure may be applied to one of several body parts containing a significant artery.
  • the imposition of the enhancement pressure accomplishes two primary results: Increasing the amplitude of the AC (or pulsatile) component of the arterial pulse component of the transmitted (or reflected in the case of the method represented in Figures 5A and 5B) light beams; and Decreasing the absorption of the light beams by blood in the capillaries increasing the amplitude of the AC (or pulsatile) component of the arterial pulse of the artery.
  • the result of increasing the amplitude of the pulse of the artery is brought about by the well known effect that the amplitude of the blood pressure pulses is maximized as the pressure imposed upon the artery equals the mean arterial pressure.
  • the increase in artery pulses i.e., the pulsatile signal detected by the system, allows more accurate S a O 2 determinations even under conditions of low perfusion. Because the difference between S a O 2 and S c O 2 may vary dramatically from patient to patient and from hour to hour, the "physiological calibration" carried out by the present invention is essential to improving the accuracy of S a O 2 determinations.
  • the blood oximetry system In practice, it is not necessary for the blood oximetry system to hold the enhancement pressure at exactly the mean arterial pressure for the entire enhancement pressure-on interval.
  • the enhancement pressure when the enhancement pressure is increased to, for example, 100 mmHg (assuming the mean arterial pressure is 100 mmHg) the pulsatile signals ⁇ V R and ⁇ V IR (waveforms B and D, respectively) increase by about an order of magnitude.
  • the enhancement pressure need only be about equal to the mean arterial pressure to cause the desired increase in the pulsatile signals ( ⁇ V R and ⁇ V IR ).
  • the enhancement pressure e.g., 5 mmHg/sec
  • it may be useful to slowly ramp the enhancement pressure e.g., 5 mmHg/sec
  • a ramping pressure must be imposed to accurately determine the mean arterial pressure for use in blood pressure.
  • the and ⁇ are input to the microcomputer by
  • the waveforms are not continuous but are time division multiplexed with Channel 1 (the red channel) and Channel 2 (the infrared channel) each having a voltage from the photodiode gated to the channel amplifiers an equal amount of time.
  • the gating of the output of the photodiode is not represented in waveforms B, C, D, and E in order to increase the clarity of the waveforms.
  • the operation of the clock represented in Figure 2 desirably may be synchronized with the operation of the analog-to-digital converters and also should be fast enough that a very accurate representation of the waveforms may be preserved.
  • each of these waveforms is represented in Figure 4.
  • the ⁇ V R and ⁇ V IR waveforms include only the C or pulsatile component of the photodiode signal as processed by, and output from, the pulsatile signal amplifiers of each channel.
  • the V R and the V IR represented by waveforms C and E, respectively, of Figure 4 are an average, or more specifically a mean, of the total signal output from the photodiode.
  • the dedicated hardware such as the amplifiers 84, 86, 88, and 90, which is illustrated and described is preferably included in the system.
  • the average (mean) of multiple determinations of , and V are each calculated and stored until the elapsed time of the enhancement pressure on interval (t EP ) is equal to or greater than the preset enhancement pressure interval T EP , as represented at step 114.
  • T EP the elapsed time of the enhancement pressure on interval
  • T EP the preset enhancement pressure interval
  • Step 116 a value for RLOG EP using equation (1) is determined using the stored average values:
  • Equation (1) is applied to a data obtained by transmitting the light beams through a body part since the transmission of light through whole blood only somewhat follows the LambertBeers law. Equation (1) requires that the log of the pertinent values be calculated. This equation is familiar to those skilled in the art and may be easily carried out by the microcomputer.
  • the RLOG EP look up table is derived from empirical data gathered during use of the system described herein. For example, once a red LED, infrared LED, photodiode, and other hardware items have been configured to provide the system described herein, the values obtained for RLOG EP may be correlated with the S a O 2 value obtained using another
  • the subject's S a O 2 may be altered by altering the composition of the inspired gases and monitoring the composition of the expired gases.
  • the look-up table Once the look-up table has been completed, it can be used in the case of any number of patients if the performance of the apparatus hardware is maintained within appropriate parameters considering the effects of age, temperature, and variability of mass produced components.
  • the S a O 2 which was determined from the RLOG EP look-up table at step 118 is displayed as represented at step 120 in Figure 3 on the display means 42 represented in Figure 2. It should be appreciated that the S a O 2 value displayed at step 120 during the enhancement pressure on interval is more accurate and reliable than S a O 2 values provided by previously available pulse oximetry systems due to the enhancement of the arterial pulsatile signal output from the photodiode and the decrease of the capillary oxygen saturation contribution to the same signal.
  • the interval during which the enhancement pressure is imposed must be limited due to several considerations including avoiding pain for the patient and affecting the physiology of the patient so that the measurements obtained are altered in any significant fashion.
  • the enhancement pressure is released from the body part for the remainder of the calibration period and monitoring period as represented at step 122 as shown in Figure 3B.
  • the enhancement pressure-off interval of the calibration period begins when the enhancement pressure is released and the pressure on the body part returns to the ambient pressure.
  • the enhancement pressure-off interval includes steps to determine four variables as shown at Step 126.
  • the average of multiple determinations of the enhancement pressure-off interval variables is calculated until the length of the enhancement pressure-off interval (t NP ) is equal to or greater than the time previously set for the enhancement pressure-off interval (T NP ) as represented at step 130 in Figure 4.
  • RLOG NP (2 )
  • R may be calculated according to equation (3) below:
  • F(SO 2 ) NP the inverse of the look-up table function for functional oxygen saturation without the enhancement pressure imposed
  • F(SO 2 ) EP the inverse of the look-up table function for functional oxygen saturation with the enhancement pressure imposed
  • C in equation (4) represents a calibration factor which must be introduced to maintain accuracy of the system because of the differences, which may be very small, between the look-up tables for RLOG EP and RLOG MON .
  • the first step in the monitoring period (t MON ) shown at 136 in Figure 3B requires that the values for the following variables be determined: the pulsatile signal output from the photodiode when the red LED is operating during the monitoring period the pulsatile signal output from the photodiode when the infrared LED is operating during the monitoring period the average of the total signal output from the photodiode when the red LED is operating during the monitoring period the average of the total signal output from the photodiode when the infrared LED is operating during the monitoring period
  • a running average of the four variables is calculated. It may be desirable to allow the physician using the system of the present invention to determine how heavily past values for the four variables will be weighted in subsequent calculations.
  • ⁇ Va IR ⁇ V IR (1-a) (6) where a equals the capillary pulse volume fraction.
  • RLOG a (7)
  • the S a O 2 level may be determined by obtaining a value from the look-up table as represented at step 144.
  • the look-up table is
  • step 146 The steps of the monitoring period are repeated until t MON ⁇ T MON as shown at step 148.
  • inventive concepts taught herein may also be carried out by configuring the light emitting means and the photo detection means to operate in a reflective mode.
  • a structure adapted for operating in a reflective mode is represented in Figure 2A which is a cross sectional view showing LED 54A and LED 56A positioned within a pressure cuff 34A adjacent the photodiode 64A. Positioning the LEDs 354 and 56A adjacent to the photodiode 64A, or in another similar position, allows the photodiode 64A to receive that portion of the light beams reflected from the blood, tissue, and bone of the patient's finger 36A. It will be appreciated that it is necessary to operate the embodiment in such a reflective mode to best utilize body parts such as the patient's forehead as a sensing location.
  • the S a O 2 corresponding to the calculated value of Y EP is found by reference to a Y EP look-up table as indicated at step 218A.
  • the Y EP look-up table is derived from empirical data gathered during use of the system described herein. For example, once a red LED, infrared LED, photodiode, and other hardware items have been configured to provide the system described herein, the values obtained for Y EP may be correlated with the S a O 2 value obtained using another S a O 2 determination method, for example, an in vitro method. Alternatively, the subject's S a O 2 may be altered by altering the composition of the inspired gases and monitoring the composition of the expired gases.
  • the Y EP look-up table Once the Y EP look-up table has been completed, it can be used in the case of any number of patients if the performance of the apparatus hardware is maintained within appropriate parameters considering the effects of age, temperature, and variability of mass produced components.
  • the S a O 2 which was determined from the Y EP look-up table at step 118A is displayed as represented at step 120A in Figure 5A on the display means 42 represented in Figure 2. It should be appreciated that the S a O 2 value displayed at step 120A during the enhancement pressure on interval is more accurate and reliable than S a O 2 values provided by previously available pulse oximetry systems due to the enhancement of the arterial pulsatile signal output from the photodiode and the decrease of the capillary oxygen saturation contribution to the same signal.
  • the interval during which the enhancement pressure is imposed must be limited due to several considerations including avoiding pain for the patient and affecting the physiology of the patient so that the measurements obtained are altered in any significant fashion.
  • the enhancement pressure is released from the body part for the remainder of the calibration period and monitoring period as represented at step 122A as shown in Figure 5B.
  • the enhancement pressure-off interval of the calibration period begins when the enhancement pressure is released and the pressure on the body part returns to the ambient pressure. Again, as represented at step 124A, it is necessary to wait at least two heartbeats before measuring any variables.
  • the enhancement pressure-off interval includes steps to determine four variables as shown at step 126A.
  • the same variables previously defined shown at step 126 in Figure 3B have the same definition in the flow chart of Figures 5A and 5B when the embodiment operates in a reflective mode.
  • the average of multiple determinations of the enhancement pressure-off interval variables is calculated until the length of the enhancement pressure-off interval (t NP ) is equal to or greater than the time previously set for the enhancement pressure-off interval (T NP ) as represented at step 130A in
  • corrections may be made to subsequent S a O 2 measurements to account for the effect of S c O 2 and to reduce or eliminate the contribution of S c O 2 on the S a O 2 level of the patient to be displayed. Having carried out these steps, the calibration period is complete.
  • a running average of Y MON is calculated. Having calculated an average value of Y MON , the S a O 2 level may be determined by obtaining a value from the Y MON look-up table as represented at step 144A.
  • the Y MON lookup table is derived in an empirical fashion similar to the fashion described for the Y EP look-up table.
  • the value obtained from the Y MON look-up table represents the S a O 2 value since the S c O 2 contribution has already been "calibrated out" in previous steps.
  • the value obtained from the Y MON look-up table is displayed as represented at step 146A. As shown at step 148A, the steps of the monitoring period are repeated until t MON ⁇ T MON .
  • two of the three parameters may be measured using the widely known oscillometric method and the third parameter (diastolic arterial pressure) may be calculated using a recursive procedure wherein an estimate of the diastolic pressure is made and the estimated diastolic pressure, and the other parameters set forth earlier, are used in Hardy model calculations. If the estimate was correct, the calculated mean arterial pressure will agree with the measured arterial pressure. Once all three parameters have been determined, the Hardy model compliance curve can be used to continuously calculate a blood pressure waveform using the
  • V R signal It will be appreciated that the signal produced by the red LED will most accurately reflect volume changes in the arteries being examined. With the relative changes in volume being available by examining the V R signal, the pressure-volume relationship of the artery described by the Hardy model allows the pressure waveform to be calculated.
  • the period during which the oscillometric determination is carried out is referred to as a "super calibration period.” It should be understood that the oscillometric method requires that the artery be completely occluded and thus whatever means which is used to impose the enhancement pressure on the body part should be capable of imposing such a pressure. Also, because the pressure imposed is greater than the systolic pressure, it may require that an appropriate waiting period be provided before S a O 2 determinations can be reliably made.
  • the enhancement pressure which equals the mean arterial pressure, is applied during every calibration period for S a O 2 determinations. This allows the measured mean arterial pressure to be compared to the mean arterial pressure being used in the Hardy model calculations and, if a significant discrepancy between the two is found, a super calibration period may be begun.
  • the present invention provides a great advantage in allowing both arterial oxygen and blood pressure determinations to be made using little more hardware than that which is required for determining arterial oxygen levels. Also, the present invention is able to distinguish arterial oxygen saturation levels from capillary oxygen saturation levels and to provide arterial oxygen saturation level determinations which are more accurate and reliable than those available from previously known oximetry systems.

Abstract

Un procédé et un système non invasif permettent de contrôler les niveaux de saturation artérielle en oxygène et la tension sanguine. L'appareil comprend une diode électroluminescente de lecture (54) et une DEL aux infrarouges (56) qui dirigent leurs faisceaux de lumière respectifs sur une partie du corps d'un patient de manière qu'ils y pénètrent ou soient réfléchis. Un phototransducteur (64) est positionné de manière à recevoir les faisceaux de lumière (608, 62) transmis à travers la partie du corps. Un manchon (34) de mesure de la tension entoure la partie du corps (36) et les DELs (54, 56). Pendant des périodes de calibrage, on applique une pression sur la partie du corps (36) et on détermine les tension systolique et moyenne du sang, ainsi que le niveau de saturation artérielle en oxygène. La pression sur la partie du corps (36) est ensuite relâchée et un autre niveau de saturation artérielle en oxygène est déterminé. La différence entre les deux niveaux de saturation en oxygène est utilisée comme facteur de calibrage lors des périodes ultérieures de contrôle afin d'éliminer l'effet des niveaux de saturation non artérielle en oxygène sur les valeurs obtenues pendant les périodes ultérieures de contrôle.
EP19900908025 1990-01-30 1990-01-30 Enhanced arterial oxygen saturation determination and arterial blood pressure monitoring Withdrawn EP0512987A4 (en)

Applications Claiming Priority (2)

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CA002074956A CA2074956A1 (fr) 1990-01-30 1990-01-30 Amelioration de la determination de la saturation en oxygene arteriel et de la surveillance de la pression sanguine arterielle
PCT/US1990/000518 WO1991011137A1 (fr) 1990-01-30 1990-01-30 Determination amelioree de la saturation arterielle en oxygene et controle de la tension sanguine arterielle

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EP0512987A1 true EP0512987A1 (fr) 1992-11-19
EP0512987A4 EP0512987A4 (en) 1993-02-24

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