Noncontactive arterial blood pressure monitor and measuring method

An automatic, continuous, non-occlusive blood pressure monitor includes a pressure applying cuff for temporarily applying pressure to a person's finger, a light source for illuminating the finger and an artery in the finger, a photosensor for detecting light transmitted through or reflected from the finger and for producing a signal representing relative volume of a unit length of the artery, a pressure sensor for producing a pressure signal representing the pressure applied by the cuff to the finger, and a microprocessor for developing from the relative volume signal and the pressure signal a formulation of arterial pressure P(t) as a function of relative arterial volume, and for calculating the arterial pressure P(t) from the relative volume signals.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
This invention relates to arterial blood pressure monitoring and more 
particularly to a method and apparatus for continuous and nonn-contactive 
blood pressure measurement. 
(b) Description of the Prior Art 
Continuous blood pressure monitoring techniques suffer from the problems of 
either being invasive or occluding the flow of blood. Invasive 
measurement, by way of arterial catheterization has been commonly used in 
intensive care units and operating rooms for a number of years. However, 
the risks of infection, thrombus formation, hemorrhage, etc., have given 
rise to a search for non-invasive approaches which would provide the 
desired continuous and accurate measurement. 
The typical non-invasive approaches taken to date include devices using the 
so-called tracking cuff principle, e.g., see U.S. Pat. No. 4,524,777. The 
device disclosed in this patent utilizes a hydraulic servocontrol system 
to maintain a finger arterial volume constant (in an unloaded state) so 
that the counter cuff pressure follows the intraarterial blood pressure 
thus giving an instantaneous arterial blood pressure measurement from the 
counter cuff pressure. Although this method does provide continuous and 
non-invasive measurement of arterial blood pressure, long-term maintenance 
of cuff pressure restrains microcirculation in the finger capillaries and 
this causes pain. Also, a downward drift occurs for some period of time 
before stability is reached. 
One other non-invasive approach, but which is not continuous, uses a 
sphygmomanometric technique based on Riva-Rocci's principle in which 
auscultatory (sound) measurements of blood flow are made to determine the 
arterial blood pressure (systolic and diastolic only), again using cuff 
pressure. These measurements are less desirable not only because they are 
not continuous but also because they are not as accurate. 
Another non-invasive approach, known as the oscillometric method, utilizes 
volume measurements, rather than auscultatory measurements, and cuff 
pressure to more accurately determine systolic blood pressure. This method 
also measures mean blood pressure, but not diastolic pressure. 
An accurate, non-invasive, continuous and non-occlusive (does not occlude 
the flow of blood) apparatus and method would be valuable for use in 
intensive care units and operating rooms to avoid complications which can 
arise with the above-described prior art devices and methods. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a method and apparatus for 
measuring arterial blood pressure in a continuous, non-occlusive fashion. 
The above and other objects of the invention are realized in a specific 
illustrative embodiment thereof which includes a continuous, indirect and 
non-occlusive blood pressure monitor operable under control of a 
microprocessor. The monitor icludes an annular-inflatable cuff for 
placement about a patient's finger. Positioned in the cuff are a pressure 
transducer for producing a signal indicating the cuff pressure, a light 
emitting diode positioned on one interior side of the cuff, and a 
photoelectric transducer positioned on the opposite interior side of the 
cuff. The light emitting diode produces light which is partially 
transmitted through a patient's finger to the photoelectric transducer 
which detects the light level or intensity and produces a signal 
indicating arterial volumetric changes in the finger. The 
volume-indicating and pressure-indicating signals are amplified and then 
supplied to anolog-to-digital converters, with the resultant digital 
output being supplied to the microprocessor. The microprocessor controls a 
ramp pressure generator which is coupled to the cuff to alternately 
inflate the cuff (causing the pressure to increase linearly) and deflate 
the cuff. 
The above-described monitor is first used in a calibration cycle in a 
temporarily occlusive manner to determine certain parameters which will 
then be used by the monitor to continuously and non-occlusively measure 
the arterial pressure waveform which includes the mean arterial blood 
pressure P.sub.m, systolic blood pressure P.sub.s and diastolic blood 
pressure P.sub.d, which are the measurements of interest. At first, the 
microprocessor determines the mean blood pressure P.sub.m and systolic 
blood pressure P.sub.s in a conventional way (using the well-known 
oscillometric method) from the measured volume and pressure signals. With 
the relative volume signal collected during cuff pressure application, the 
cuff pressure is then relieved. The microprocessor then makes a first 
estimate of diastolic blood pressure P.sub.d (using a formula for 
estimating P.sub.d from P.sub.m and P.sub.s), and uses that estimated 
value, along with the measured mean blood pressure and systolic blood 
pressure, in a recursive procedure based on a pressure-volume relationship 
known as the Hardy model, to derive a calculated mean blood pressure 
designated as P.sub.M. The calculated mean blood pressure P.sub.M is 
compared with the measured mean blood pressure P.sub.m and if the 
difference between the two is greater than some predetermined standard, 
then a new estimated diastolic blood pressure P.sub.d is used in the 
recursive procedure to obtain a new mean calculated blood pressure 
P.sub.M. This recursive procedure continues until the calculated mean 
blood pressure P.sub.M is within a certain range of the measured mean 
blood pressure P.sub.m. 
In the course of determining the calculated mean blood pressure P.sub.M, 
three parameters are also developed to define a Hardy model compliance 
curve for the particular patient in question. These parameters include k 
which represents the compliance index for the blood vessels of the patient 
being rreated and is unique to that patient, V.sub.m which represents the 
maximum volume of the vessels in the patient's finger (being examined), 
and V.sub.0 which represents the volume of the patient's finger vessels at 
zero pressure. With these parameters determined for the patient, the Hardy 
model compliance curve may then be used to relate the relative blood 
vessel volume the patient's arterial blood pressure and vice versa. With 
the Hardy model parameters and compliance curve for the patient in 
question being determined, the blood pressure monitor may now be utilized 
to continuously measure the relative volume V as a function of time from 
which the continuous arterial pressure waveform as a function of time can 
be determined using the Hardy model. 
Periodically, recalibration will be carried out, i.e., new Hardy model 
parameters will be developed for use in making the continuous blood 
pressure measurements. 
As can be seen, except for the calibration cycle, continous and 
non-occlusive blood pressure measurement may be carried out. This avoids 
the pain and trauma of both the invasive blood pressure measuring 
techniques and the occluding blood pressure measuring techniques.

DETAILED DESCRIPTION 
The present invention, except for occasional calibration requirements, 
provides for continuous, noncontactive or nonpressure imposed arterial 
blood pressure monitoring. The blood pressure measurements, in the form of 
a pressure waveform, are obtained by calculation from a measured arterial 
volume signal. The arterial volume signal, which is a relative measurement 
of the arterial volume over time, is developed using optical sensing 
techniques which are generally known. The ultimate determination of the 
pressure waveform is made using the so-called compliance model of Hardy 
and Collins (hereinafter referred to as the Hardy model) discussed in 
Hardy, H. H. and Collins, R. E., "On the Pressure-Volume Relationship in 
Circulatory Elements", Med. and Biol., Eng. and Comput., September, 1982, 
pages 565-570. 
The Hardy model shows that under static conditions, the pressure-volume 
(P-V) relationship of blood vessels may be described by 
EQU k(V.sub.m -V)=dV/dP, P.gtoreq.0 (1) 
where P represents the transmural pressure (the difference in pressure 
inside and outside the artery), V.sub.m is the limiting or maximum volume 
of the blood vessel in question and k is a physiological constant which 
characterizes the elasticity of the vascular wall. The constant k is 
sometimes referred to as the vascular compliance index. Solving the 
differential equation (1) yields the following pressure/volume 
relationship 
EQU V=V.sub.m -(V.sub.m -V.sub.0)e.sup.-kP, P.gtoreq.0 (2) 
where V.sub.0 is the vessel volume under zero transmural pressure. Equation 
2 is referred to as the transformation of the P-V relationship. Combining 
equations 1 and 2 gives the following equation 
EQU dV/dP=k(V.sub.m -V.sub.0)e.sup.-kP, P.gtoreq.0 (3) 
which defines the vascular compliance of a vessel (unique to each 
individual). 
With the above described Hardy model, the relationship between pressure P 
and absolute volume V is established. However, it would be advantageous to 
define the pressure waveform P(t) in terms of relative volume V(t) which 
is a parameter measureable by a photoelectric plethysmogram. The 
relationship between absolute volume V(t) and relative volume V(t) is 
given by 
EQU V(t)=V.sub.x +B V(t) (4) 
where B is the mapping coefficient from the analog signal V(t) to the 
absolute arterial volume as illustrated in FIG. 2A, and V.sub.x is a 
parameter defining the contribution of tissue absorption of transmitted 
light emitted by the photoelectric plethysmogram. In other words, there is 
a linear relationship between the relative volume V(t) and absolute volume 
V(t), in which V.sub.x is the intercept, and B is the slope. 
Now define systolic volume V.sub.s and diastolic volume V.sub.d in 
accordance with equations 2 and 4 to get 
##EQU1## 
where V.sub.s and V.sub.d are analogue systolic and diastolic volumes 
respectively. Substracting equation 6 from equation 5 gives the mapping 
coefficient 
##EQU2## 
The addition of equation 6 and equation 5 gives the mapping intercept 
V.sub.x. 
##EQU3## 
Taking the log of both sides of equation 2, substituting equation 4, and 
rearranging terms, gives the pressure waveform 
##EQU4## 
which may be called the inverse transformation of the P-V relationship 
with respect to equation 2. Combining equations 7, 8, and 9, the 
calibrated blood pressure as a function of the analogue arterial volume 
signal is obtained, i.e., the inverse transformation of P(t)-V(t) 
relationship, given as 
##EQU5## 
where V(t) is the continuous relative volume as a function of time, 
P.sub.s is the systolic pressure, P.sub.d is the diastolic pressure, 
V.sub.s the systolic relative volume, and V.sub.d is the diastolic 
relative volume all of which can be noninvasively measured as will be 
discussed hereafter. Thus, if these last mentioned parameters and the 
arterial compliance index k are noninvasively determined in advance, the 
arterial blood pressure (P(t) when cuff pressure is zero) can be developed 
by measuring the relative volume V(t) and using equation (10). 
The relative systolic volume V.sub.s and relative diastolic volume V.sub.d 
may be non-invasively determined in a conventional fashion as shown in 
FIG. 2A. The systolic pressure P.sub.s may be non-invasively determined 
using conventional oscillometric methods, as mentioned earlier. 
Specifically, the systolic pressure may be determined by the cuff pressure 
at which the pulsatile plethysmograph developed using the oscillometric 
method disappears. This is well known in the art. The diastolic pressure 
P.sub.d cannot be determined directly from pulsatile information of a 
plethysmograph, but can be determined using an iterative procedure to be 
descibed momentarily. Calculation of the compliance index k is dependent 
on parameters which include the diastolic pressure P.sub.d. The iterative 
procedure to be described will yield both P.sub.d and k to thus provide 
the five parameters V.sub.s, V.sub.d,P.sub.s,P.sub.d and k needed in 
equation 10. 
A least square approach may be used to determine the compliance index k 
which will now be described. First, refer to FIG. 2B which shows the 
photoelectric plethysmograph volume .DELTA.V under the corresponding 
transmural pressure P=P.sub.b -P.sub.c, where P.sub.b and P.sub.c are 
intra-arterial and cuff (measured) pressures respectively. Note the 
established fact that the amplitude of the pulsatile volume .DELTA.V is 
maximum when P.sub.c equals the mean blood pressure P.sub.m (see FIG. 2B). 
Equation 3 can be used to relate mean blood pressure P.sub.m cuff 
pressure P.sub.c, the amplitude of pulsatile photoelectric plethysmograph 
.DELTA.V, and pulsatile blood pressure .DELTA.P as follows: 
EQU .DELTA.V/.DELTA.P.perspectiveto.k(V.sub.m -V.sub.0)e.sup.-k (P.sub.m 
-P.sub.c), (11) 
where .DELTA.P=(P.sub.s -P.sub.c)-(P.sub.d -P.sub.c)=P.sub.s -P.sub.d, 
P.sub.s and P.sub.d being systolic and diastolic blood pressure 
respectively. Assume that intra-arterial blood pressure remains the same 
in the time of calibration, i.e., ramp cuff pressure application. A set of 
n equations representing the same relationship of equation 11, under 
different ramp cuff pressures can then be acquired as follows 
##EQU6## 
Equation set 12 is linearized by taking the natural logarithm with respect 
to equation 11, giving 
##EQU7## 
which has the linear form of 
EQU y=ax+b (14) 
where 
##EQU8## 
which transforms the set of n nonlinear equations 12 into the set of n 
linear equations, given as 
##EQU9## 
where y.sub.i =ln(.DELTA.V.sub.i /.DELTA.P),x.sub.i =P.sub.ci,(i=1,2, . . 
. ,n) The coefficients and their uncertainties are obtained from equation 
set 16 applying standard least mean square error analysis, thus providing 
the estimate of the arterial compliance index k of the Hardy model, in 
terms of x and y, i.e., 
##EQU10## 
in which 
##EQU11## 
Had an accurate estimate of P.sub.d been used in equations 15-17, the Hardy 
model parameters would be correctly determined. However, to obtain an 
accurate value for P.sub.d, the following iterative procedure is used: 
1. Select an initial value of P.sub.d to be P.sub.dj =3/2 P.sub.m -1/2 
P.sub.s where P.sub.m and P.sub.s are measured using known oscillometric 
methods. 
2. Calculate the compliance index k.sub.j based on P.sub.dj using equation 
17. 
3. Calculate P(t).sub.j in accordance with equation 10 using k.sub.j and 
the measured relative volume V(t). 
4. Calculate a mean pressure in accordance with the formula 
##EQU12## 
5. Compare the calculated mean pressure P.sub.Mj with the measured mean 
presure P.sub.m and if the difference is less than one mmHg the procedure 
is stopped, otherwise another "estimated" diastolic pressure P.sub.d(j+1) 
is determined by the gradient method from the formula 
EQU P.sub.d(j+1) =P.sub.dj -(P.sub.m -P.sub.mj).differential.(P.sub.m 
-P.sub.mj)/.differential.P.sub.dj (20) 
The procedure then returns to step 2 above. 
When the procedure yields a calculated mean pressure which is within the 
predetermined range of the measured mean pressure, needed parameters for 
the Hardy model will have been obtained, namely k, V.sub.m and V.sub.0. In 
effect, the parameters necessary to develop a Hardy model compliance curve 
for a patient are determined by the above iterative procedure. With this 
information, the blood pressure waveform P(t) can be produced from the 
measured relative volume V(t) on a continuous, nonocclusive basis. 
Apparatus for carrying out the desired measurements, both for obtaining the 
parameters for the Hardy model and for producing the waveform P(t), is 
shown in FIG. 1. The apparatus includes an inflatable annular finger cuff 
4 having a rigid outer wall 8 and a resilient inflatable annular bag 12 
held in place by inwardly extending end walls 16 and 20. The bag 12 is 
filled with air or other fluid for producing a pressure on a finger 24 
when the finger is inserted into the cuff. Disposed on the interior wall 
of the bag 12 between the bag and the finger 24 and on one side of the 
cuff 4 is a light emitting diode 28. Disposed on the interior wall of the 
bag 12 on the other side of the cuff 4 is a photoelectric transducer 32 
for detecting light transmitted from the light emitting diode 28 through 
the finger 24 to the photoelectric transducer. The amount of light 
reaching the transducer 32 is proportional to the volume of the blood 
vessel or vessels positioned between the diode 28 and transducer 32. The 
output signal of the transducer therefore represents the relative volum 
V(t) of the blood vessel or vessels in question. 
Disposed inside the bag 12 is a pressure transducer 36 for producing a 
signal representing the cuff pressure P.sub.c being applied to the finger 
24 by the bag 12 when it is inflated. A ramp pressure generator 40, of 
conventional design, supplies air to the cuff 4 in response to analog 
signals from a digital-to-analog converter 44 which receives the digital 
counterparts of the analog signals form a microprocessor or microcomputer 
48. The microcomputer produces signals for causing the ramp pressure 
generator 40 to alternately inflate the bag 12 with a linearly increasing 
ramp pressure, and then deflate the bag. 
The relative volume signal V(t) produced by the photoelectric transducer 32 
is supplied to an A.C. volume amplifier 52 which amplifies the A.C. 
component of the relative volume signal, and to a D.C. volume amplifier 56 
which amplifies the total relative volume signal. The outputs of the two 
amplifiers 52 and 56 shown graphically at 60 and 64 respectively are 
supplied to analog-to-digital converters 68 where the analog signals are 
converted to digital form for application to the microcomputer 48. The 
output P.sub.c of the pressure transducer 36 is also supplied to an 
amplifier 68 which amplifies the signal shown at 72 for application to the 
analog-to-digital converters 68 for conversion to digital form and 
ultimate transfer to the microcomputer 48. 
The signals received from the analog-to-digital converters 68 are used by 
the microcomputer to both calculate the parameters needed for the Hardy 
model (calibration for a particular patient), and produce the arterial 
blood pressure waveform P(t), shown at 76, from which the systolic, 
diastolic and mean blood pressures can be obtained. This information is 
displayed in real time on a display unit 80 which might illustratively 
include a CRT and digital displays. A power supply unit 84 provides power 
for operation of the monitor. 
Use of the blood pressure monitor shown in FIG. 1 will now be described. A 
patient whose blood pressure is to be determined inserts his finger into 
the annular cuff 4, and the bag 12 is inflated to apply pressure to the 
finger. As this is being done, relative volume signals V(t) and cuff 
pressure signals P.sub.c are developed by the photoelectric transducer 32 
and pressure transducer 36 respectively and supplied to corresponding 
amplifiers. The signals are amplified and supplied to the 
analog-to-digital converters 68, with the digital versions being supplied 
to the microcomputer 48. From these signals, the microcomputer 48 develops 
the mean pressure P.sub.m and systolic pressure P.sub.s using conventional 
oscillometric methods. The cuff pressure is then released so that there is 
no occlusion of the finger blood vessels. The microcomputer 48 then begins 
the iterative procedure described earlier by calculating a first estimated 
diastolic pressure P.sub.d which, along with the measured mean pressure 
and systolic pressure are used to obtain Hardy model parameters for 
defining a Hardy model compliance curve. From this compliance curve, a 
blood pressure waveform P(t) is generated and then from that a mean 
pressure P.sub.M is calculated. The calculated mean pressure P.sub.M is 
then compared with the measured mean pressure P.sub.m and if the two are 
not within a certain range of each other, another estimate for diastolic 
pressure P.sub.d is made and the procedure for calculating a mean pressure 
is repeated. When the calculated mean pressure is within the desired range 
of the measured mean pressure, the iterative procedure is stopped and the 
Hardy model parameters which had led to the last calculated mean pressure 
are stored since they define a Hardy model compliance curve which will be 
used to continuously monitor blood pressure. Thereafter, the photoelectric 
transducer 32 produces a relative volume signal V(t) from which the 
microcomputer 48 calculates the arterial pressure waveform P(t) as 
desired. 
Recalculation of the Hardy model compliance curve for the patient in 
question may be carried out periodically since it has been found that the 
parameters tend to change over time. 
In the manner described, continuous and noncontactive arterial blood 
pressure measurements may be made with the blood pressure monitor of FIG. 
1. 
It is to be understood that the above-described arrangements are only 
illustrative of the aplication of the principles of the present invention. 
Numerous modifications and alternative arrangements may be devised by 
those skilled in the art without departing from the spirit and scope of 
the present invention and the appended claims are intended to cover such 
modifications and arrangements. For example, sensing light reflected from 
rather than transmitted through an artery could be utilized to determine 
relative volume of the artery.