Abstract:
An oscillometric, noninvasive blood pressure monitor comprising an inflatable cuff adapted for placement around a body member, a pump for cuff inflation, a pressure transducer connected to the cuff, means for detecting oscillations in arterial pressure occurring during a transition in cuff pressure between a pressure greater than normal systolic pressure and a pressure less than normal diastolic pressure, and a blood pressure measurement circuit which is capable of determining the maximum amplitude (A m ) of the oscillations, identifying mean cuff pressure (P m ) as the coincident value of the cuff-pressure signal from the pressure transducer, and determining systolic pressure as a function of both A m  and P m . In accordance with one aspect of the invention, the blood pressure monitor has an optical sensor including a light source and photodetector optically coupled to the body member proximate to the cuff. Oscillations in the output signal of the photodetector are detected, and the blood pressure measurement circuit determines the oscillation amplitude corresponding to systolic pressure (A s ) as a function of both A m  and P m .

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the noninvasive measurement of blood pressure, and more particularly to the noninvasive measurement of blood pressure by the oscillometric method. 
     A number of noninvasive methods of measuring blood parameters are known. For example, blood pressure has been measured by the auscultatory method which uses a cuff and a stethoscope, and by the oscillometric method which only requires a cuff applied to a body member. The conventional oscillometric method relies on the small-amplitude pulsatile pressure oscillations communicated to the cuff by the underlying artery in the body member during cuff deflation from above systolic pressure to zero pressure. Such arterial pressure oscillations cause corresponding small oscillations in cuff pressure which can be amplified and used to identify systolic, mean and diastolic pressure. For example, it has been established by Posey et al. that the cuff pressure for maximal amplitude oscillations corresponds to mean arterial pressure. See Posey et al., “The Meaning of the Point of Maximum Oscillations in Cuff Pressure in the Direct Measurement of Blood Pressure,” Part 1,  Cardiovascular Res. Ctr. Bull.  8(1):15–25, 1969. See also Ramsey, “Noninvasive Automatic Determination of Mean Arterial Pressure,”  Med. Biol. Eng. Comput.  17:17–18, 1979; and Geddes et al., “Characterization of the Oscillometric Method for Measuring Indirect Blood Pressure,”  Annals of Biomedical Engineering, Vol.  10, pp. 271–280, 1982. All such references are incorporated herein by reference. 
     Commercially available oscillometric devices are useful for noninvasive blood pressure measurement, but a need remains for improvement in accuracy, particularly with respect to identification of systolic and diastolic pressure. 
     SUMMARY OF THE INVENTION 
     The present invention meets the above-stated need and others by providing an oscillometric, noninvasive blood pressure monitor comprising an inflatable cuff adapted for placement around a body member, a pump for cuff inflation, a pressure transducer connected to the cuff, means for detecting oscillations in arterial pressure occurring during a transition in cuff pressure between a pressure greater than normal systolic pressure and a pressure less than normal diastolic pressure, and a blood pressure measurement circuit which is capable of determining the maximum amplitude (A m ) of the oscillations, identifying mean cuff pressure (P m ) as the coincident value of the cuff-pressure signal from the pressure transducer, and determining systolic pressure as a function of both A m  and P m . An inflatable cuff as that term is used herein is an inflatable bladder, capsule or other member suitable for occluding a blood vessel, and may cover a small area on a subject&#39;s skin or may surround a finger, limb or other body part. 
     In accordance with one aspect of the invention, the blood pressure monitor has an optical sensor including a light source and photodetector optically coupled to the body member through at least one surface of the cuff. The oscillations in arterial pressure are detected as oscillations in the output signal of the photodetector, and the blood pressure measurement circuit determines the oscillation amplitude corresponding to systolic pressure (A s ) as a function of both A m  and P m . In a preferred embodiment, the amplitude A s  corresponding to systolic pressure is determined based on an equation of the form
 
 A   s   =A   m ( a−b P   m )
 
     The invention provides more accurate blood pressure measurement by determining systolic pressure according to an algorithm which includes mean cuff pressure as a factor. The principles of the invention are particularly suited for use with the optical oscillometric method but are equally applicable to blood pressure measurement by the conventional pneumatic oscillometric method. 
     The objects and advantages of the present invention will be more apparent upon reading the following detailed description in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a cylindrical embodiment of a transilluminating cuff for use in a blood pressure monitor according to the present invention. 
         FIG. 2  is a perspective view of a hinged embodiment of a transilluminating pressure cuff. 
         FIG. 3  is a transverse cross-section of the cuff of  FIG. 2 . 
         FIG. 4  is a block diagram of one embodiment of a blood pressure monitor according to the present invention. 
         FIG. 5  is a set of sample waveforms obtained with a blood pressure monitor according to the present invention, with a cuff on the little finger of a human subject. 
         FIG. 6  is another set of sample waveforms. 
         FIG. 7  is a graph of blood pressure measured using an algorithm according to the present invention against blood pressure measured directly. 
         FIG. 8  is an example of a calibration curve for use in oxygen saturation measurement according to another embodiment the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one of ordinary skill in the art to which the invention relates. 
       FIG. 1  illustrates one embodiment of a member-transilluminating, transparent pressurizable cuff  10  for use in a blood pressure monitor according to the present invention. A rigid tube  12  contains an elastic sleeve  14  which may be provided with an inlet  16  for connection to a pressure source, e.g., an air supply, and an outlet  18  for connection to, e.g., a manometer. Alternatively, inlet  16  may be the only pressure line, as in the embodiment of  FIG. 2  described below. Pressure applied between the elastic sleeve and the rigid tube causes the sleeve to compress a body member therein such as a finger  20  placed therein. This embodiment is also useful on a small animal tail or tongue, for example, among other applications. The rigid tube includes a light source  22  and a photodetector  24  which may be diametrically opposed as illustrated in the drawing. Alternatively, two light sources may be provided as described below. In another alternative embodiment, the light source and photodetector are mounted side-by-side on the cuff housing, and blood pressure and oxygen saturation are measured based on reflection of light by tissue in the body member. 
     Referring to  FIGS. 2 and 3 , another embodiment of a cuff  30  for use with a blood pressure monitor according to the present invention includes a hinged cuff housing  32  having first and second semicylindrical sections  34  and  36  and a hinge  38  parallel to the longitudinal axis of the semicylindrical housing sections. The axes of the two housing sections are parallel to each other and coincide to form a common axis when the hinged housing is closed. In order to facilitate use of the cuff on a bone-containing body member, i.e., to avoid bone shadow, two light sources  40  and  42  are circumferentially spaced on one housing section in opposition to a photodetector  44  mounted on the other housing section. This configuration increases the transmission of light through the tissue bed around the bone  45  in the member in which blood pressure is measured noninvasively. The angular spacing of the LEDs and the photodetector may be as shown in  FIG. 3 , or, alternatively, the LEDs and photodetector may be spaced approximately 120° apart. An optically transparent, inflatable cuff  46 , which may be provided as a disposable item with an inflation tube  56 , is adapted to fit within cuff housing  32  and around the body member, and is held in place by means of a plurality of clips  48  which are provided in the housing for this purpose. Cuff  30  is further described in U.S. Pat. No. 6,801,798, entitled Body-Member-Illuminating Pressure Cuff For Use In Optical Noninvasive Measurement Of Blood Parameters, issued Oct. 5, 2004 and hereby incorporated by reference. 
     Blood pressure, including systolic, mean and diastolic pressures, can be obtained with the optical sensor unit from the amplitude spectrum of the pulses obtained during deflation of the cuff from a suprasystolic pressure to zero pressure, as described below. Monochromatic LEDs are suitable for monitoring blood pressure. For example, the transducer may employ infrared LEDs such as PDI-E801 or PDI-E804 880 nm LEDs available from Photonic Detectors, Inc. The LEDs and photodetector are preferably matched to operate at a desired wavelength. One example of a suitable photodetector is a Fairchild Semiconductor QSD723 phototransistor, with a peak sensitivity at 880 nm. Another suitable operating wavelength for the LEDs and photodetector is 805 nm, at which wavelength the blood pressure pickup has no oxygen-saturation error, as will be appreciated from the discussion of pulse oximetry below. An advantage of either of the example wavelengths is that there are few environmental light sources in this infrared region. 
     Referring to  FIG. 4 , the cuff is connected by inflation tube  56  to a pump  58  which is controlled by a microprocessor  60 . Pressure in the line to the cuff is measured by means of a pressure transducer  62  having a signal output connected to the microprocessor. Suitable transducers are available from Cobe Labs, Littleton, Colo. In embodiments such as that of  FIG. 1  in which the cuff has an inlet and an outlet, the pressure transducer is connected to the outlet. A/D conversion may be provided in the microprocessor or in the transducer or with a separate A/D converter provided between the two. The microprocessor controls the LEDs and, during blood pressure measurement, energizes both LEDs continually. The photodetector produces an output signal which is supplied to the microprocessor through an amplifier  64 . The amplified photodetector output signal is converted to digital form in the microprocessor itself if the microprocessor has an internal A/D converter, or in a separate A/D converter provided between the amplifier and the microprocessor. 
     The microprocessor is suitably programmed to identify, based on the digitized output signal of the photodetector, the points in the cuff pressure signal which correspond to systolic, mean and diastolic pressure, and displays the corresponding values on a display  65  which may comprise separate indicators as shown in  FIG. 4 , or may provide an output for distant recording. 
     One suitable embodiment of amplifier  64  is a variable-gain amplifier. With such an amplifier, and with a feedback circuit  66  connected to its gain-control input, as shown in  FIG. 4 , the sensitivity of the measuring system may be adjusted automatically to a proper level for measurement of blood pressure. It has been found useful to set the sensitivity based on the amplitude of the photodetector output pulses before inflation of the cuff. Such pulses may have a peak-to-peak amplitude on the order of one-third to one-half the maximum peak-to-peak amplitude of the pulses obtained during blood pressure measurement. Such pulses are identified by the reference numeral  68  in  FIG. 6 , which shows a sample waveform for the photodetector output signal both prior to and during blood pressure measurement. Sensitivity adjustment is inhibited when blood pressure is measured. That is, the gain of amplifier  64  and thus the system sensitivity is fixed at that time. It should be noted that pulse rate can be determined from the optical pulses occurring before cuff inflation and after cuff deflation and may be displayed along with blood pressure values as indicated in  FIG. 4 . 
     Blood pressure is measured during a transition in cuff pressure between a suprasystolic pressure and zero pressure. The transition may be an upward or downward transition but is described below in terms of a gradual downward transition such as shown in  FIG. 5 , which shows a sample optical pulse waveform  69  obtained during a cuff pressure cycle represented by curve  70 , which is marked to indicate the points corresponding to systolic (S), mean (M), and diastolic (D) pressure. When cuff pressure is raised above systolic pressure, all oscillations are extremely small. As pressure in the cuff falls below systolic pressure, the pulses increase, and as the pressure is reduced further, the optical pulse amplitude increases and reaches a maximum, labeled A m  in  FIG. 5 , at which point the cuff pressure is equal to mean arterial pressure, labeled M in  FIG. 5 . With a continued decrease in cuff pressure, the oscillation amplitude decreases and returns to a uniform level. 
     The peak-to-peak amplitudes of the optical pulse waveform at the points coinciding with the occurrence of systolic and diastolic pressure are designated respectively as A s  and A d  in  FIG. 5 . In the system described in the above-referenced U.S. Pat. No. 6,801,798, those amplitudes are calculated as fixed percentages of A m , and the corresponding points in time are identified on the optical pulse waveform, by interpolation if necessary between adjacent pulses, after which the values of cuff pressure at those points in time are identified as systolic (S) and diastolic (D) pressure, respectively. Appropriate ratios have been determined experimentally. With a conventional cuff applied to the upper arm of a human subject, and with the cuff width (axial length) nominally equal to 40% of the member circumference, systolic pressure is typically identified as the value of cuff pressure at the point when the amplitude ratio A s /A m  is 0.5; diastolic pressure is typically identified as the value of cuff pressure at the point when the ratio of A d /A m  equals 0.8. With the optical oscillometric method, a ratio of 0.7 has been found more suitable for identifying diastolic pressure. 
     Systolic pressure, however, is preferably not identified on the basis of a fixed percentage of A m . The amplitude of the optical pulse waveform corresponding to systolic pressure has been found to depend on mean pressure (P m ), unlike the fixed-value systolic pressure algorithm. More accurate measurements can be obtained by calculating A s , the optical pulse amplitude corresponding in time with systolic pressure, according to an algorithm which includes mean cuff pressure as a factor. The following equation represents one form of such an algorithm:
 
 A   s   =A   m ( a−b P   m )
 
where a and b are experimentally determined constants.
 
     The improvement in predicting systolic pressure using this algorithm can be appreciated from  FIG. 7 , in which line A, corresponding to results using this algorithm with the values a and b set equal to 0.84 and 0.004, respectively, is virtually coincident with line B, the line of equal values in the graph. That is, systolic pressure predicted with the above algorithm is virtually the same as directly measured systolic pressure throughout the range of interest. 
       FIG. 6  illustrates sample waveforms for an embodiment of the invention in which cuff pressure is increased linearly and then decreased linearly, as illustrated respectively by segments  72  and  74  of the cuff pressure signal, and two sets of optical pulsatile data  76  and  78  are acquired. As shown in the drawings, the first set of pulses  76  includes indications of the points in time during the cuff pressure rise  72  at which diastolic, mean and systolic pressure occur, in that order. Conversely, the second set of pulses  78  includes indications of the points in time during the cuff pressure fall  74  at which systolic, mean and diastolic pressure occur, in that order. In this way, two values for each pressure may be acquired and averaged and the average value may be displayed. 
     The system may have LEDs which operate at different wavelengths for oxygen saturation measurement. Blood oxygen saturation is defined as the ratio of oxygenated hemoglobin (HbO 2 ) to the total hemoglobin (Hb+Hb0 2 ), and is typically expressed as a percentage. The oximeter determines oxygen saturation (SaO 2 ) by measuring the optical transmission at two wavelengths of light passing through a tissue bed. Although other wavelengths are contemplated, it is presently preferred to operate at wavelengths of approximately 650 nm and 805 nm for oxygen saturation measurement. As shown in the above-referenced U.S. Pat. No. 6,801,798, hemoglobin (Hb) has negligible transmission at 650 nm, and hemoglobin (Hb) and oxygenated hemoglobin (HbO 2 ) transmit equally well at 805 nm; the latter wavelength is known as the isobestic point. That is, the transmission at 805 nm is independent of oxygen saturation. The optical sensor may have separate narrowband LEDs, e.g., a red LED emitting at approximately 650 nm and an infrared LED preferably emitting at approximately 805 nm, and a broadband photodetector. As an alternative to separate narrowband LEDs, a red LED and infrared LED may be combined in one multi-wavelength LED such as type Epitex L660/805/975-40D00, available from Epitex, Kyoto, Japan, and each light source  40  and  42  may comprise such a multi-wavelength LED. 
     The red and infrared LEDs are preferably energized alternately in rapid succession, e.g., at a rate of 200 pulses per second. This technique permits the use of high-intensity short-duration pulses. Synchronous detection is used to achieve the highest signal-to-noise ratio. Two benefits result: 1) a low average power and minimum heating, and 2) the system is less sensitive to stray ambient illumination. The red and infrared signals are sampled and processed to obtain SaO 2 , which may then be displayed on display  65  of  FIG. 4 . The automatic sensitivity adjustment is disabled during measurement of oxygen saturation. 
     A base line for measurement may be established by first inflating the cuff to a high pressure sufficient to squeeze all of the blood out of the member in the cuff and thus out of the optical path. For example, the cuff pressure may be held at a maximum pressure as indicated by the plateau  73  in  FIG. 6  for a desired time period to obtain the bloodless transmission reading, which can be assigned a value of 100% transmission. When the cuff pressure is released, blood enters the optical path and the red and infrared transmissions are measured. The optical density is computed for each of the transmitted signals, and the ratio of red to infrared optical density is calculated and scaled to provide an output value corresponding to the percentage of oxygen saturation. 
     Beer&#39;s law relates the optical density (D) to the concentration of a dissolved substance. Optical density (D) is equal to log 1/T, where T is the transmittance. Therefore the oxygen saturation (SaO 2 ) is given by: 
         SaO   2     =       A   ⁢           ⁢       D   650       D   805         +   B         
 
where A and B are experimentally determined constants for a given application. This equation predicts a linear relationship based on Beer&#39;s law. However, Beer&#39;s law applies to solutions in which the absorbing substance is dissolved. Blood is a suspension, and, consequently, the relationship between SaO 2  and the ratio of the optical density for red and infrared radiation is nonlinear, as shown in  FIG. 8 . Between 30% and 60% saturation, the relationship is almost linear; above this range the relationship is nonlinear. The curve in  FIG. 8  is an example of a suitable calibration curve which may be programmed into the microprocessor, e.g., in the form of a lookup table, for calculation of SaO 2 . Further information regarding methods of measuring blood oxygen saturation may be found in the following references which are hereby incorporated by reference: Geddes, “Heritage of the Tissue-Bed Oximeter,”  IEEE Engineering in Medicine and Biology,  87–91, March/April 1997; Geddes and Baker,  Principles of Applied Biomedical Instrumentation,  3 rd  ed., Wiley, New York, 1989.
 
     Calibration of the oximeter also involves balancing the outputs for the red and infrared channels to obtain the same optical sensitivity for both, and ensuring that both channels have a linear response to the red and infrared radiation. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. For example, although the embodiment of  FIG. 2  is described above as having two emitters and one detector, it may instead be provided with one emitter and two detectors or with a combination of multiple emitters and multiple detectors. The emitter(s) and detector(s) may be mounted inside the cuff, on the inside surface of the cuff&#39;s inner wall, that is, the wall which contacts the subject&#39;s skin during use, and may be affixed thereto with an optically clear adhesive, e.g., Superglue or other adhesive suitable for the particular material used for the cuff. The emitter(s) and detector(s) may be affixed to the cuff wall before the cuff is completely formed or sealed, and the cuff may then be sealed so as to enclose the emitter(s) and detector(s). The system may also be provided with an alarm which is triggered when the transducer is off the body member desired to be monitored; an alarm circuit may be designed to respond, for example, to an optical sensor output signal level that is beyond a predetermined threshold, indicative of the absence of absorbing material in the optical path, or to such a condition combined with the additional condition of an absence of optical pulses.