Vital signs monitor

A vital signs monitor includes an ECG sensor, an oximetric finger probe, a respiration impedance sensor, a respiration strain gauge, and a blood pressure cuff with a pressure sensor. Signals generated by these sensors are processed by a microcomputer which is programmed to cross-reference data from multiple channels in order to improve accuracy. The microcomputer is programmed to select the more regular pulses from the ECG sensor and the finger probe to arrive at a better measure of heart rate. In addition, the microcomputer is programmed to use the strain gauge signal to remove artifacts from the signal generated by the impedance sensor. In this way a more artifact-free measure of respiration rate is obtained. Blood pressure is measured by using timing information derived from pulses sensed by the oximetric finger probe in combination with signals generated by the blood pressure cuff pressure sensor to determine systolic and diastolic pressures of the subject. A calibrated blood pressure waveform is generated automatically from the oximetric finger probe and the blood pressure cuff.

BACKGROUND OF THE INVENTION 
This invention relates to a vital signs monitor which combines the signals 
from multiple sensors to generate more reliable measurements of vital 
signs such as heart rate, respiration rate, and vasomotor activity. 
Conventional vital signs monitors use single sensors to measure individual 
vital signs. For example, an ECG sensor can be used to measure heart rate, 
or a chest strap connected to a strain gauge can be used to measure 
respiration rate. Such single sensor techniques are prone to measurement 
errors when the sensor generates artifacts as a result of body motion, 
poor sensor placement, and the like. At least in part for this reason, 
vital signal monitors are often restricted to use by highly trained 
personnel, as for example in an intensive care unit. 
There is a need for a vital signs monitor which is less dependent on the 
error-free operation of individual sensors. Such a monitor could be used 
effectively by less highly trained personnel, and would be better suited 
for use by a subject at home, without the assistance of a health care 
professional. 
SUMMARY OF THE INVENTION 
The vital signs monitor of this invention combines information from 
multiple sensors to improve the reliability and the accuracy with which 
vital signs are measured. It has been found that the signals generated by 
sensors commonly used in vital sign monitors can be cross-referenced to 
reduce measuring errors and increase accuracy, without significantly 
increasing the cost or the electronic complexity of the monitor. 
For example, an ECG sensor can be used to generate a first series of pulses 
indicative of heart rate, but chest motion of the subject may interfere 
with the ECG sensor. Similarly, an oximetric finger probe can be used to 
generate a second series of pulses indicative of heart rate, but hand 
motion of the subject may interfere with the finger probe signal. The 
monitor described below responds to both the first and second pulses by 
identifying those pulses with more regularly repeating pulse intervals and 
by using the identified pulses to determine heart rate of the subject. In 
this way, proper operation continues, even when the ECG signal or the 
finger probe signal is interrupted. 
As another example, a conventional electrical impedance sensor can be 
attached to the chest of a subject to generate a series of impedance 
pulses correlated with respiration rate. However, chest motion such as 
that associated with a cough will often create additional impedance pulses 
which can result in an over estimate of respiration rate. The monitor 
described below suppresses these additional impedance pulses by using a 
chest wall motion sensor (such as a chest strap coupled to a mechanical 
strain gauge) to detect chest motion artifacts. Impedance pulses 
associated with such chest motion artifacts are then suppressed. 
As a third example, vasomotor activity can be monitored by automatically 
comparing core blood pressure with the waveform generated by an oximetric 
finger probe. The ratio of pulse amplitude as measured with an occluding 
cuff over a major artery to pulse amplitude of the finger probe waveform 
is an excellent measure of the extent to which the vascular bed at the 
finger is dilated or constricted. As described below, this ratio can be 
automatically measured and checked for trends that may give advance 
warnings of impending changes in core blood pressure. 
In each of these three examples, two separate sensor signals are correlated 
to generate an accurate measure of the desired parameter. The heart rate 
measurement approach described above is relatively undemanding with 
respect to sensor placement, since either of the two sensors can supply 
information to the monitor if the other fails to provide a reliable 
signal. The respiration rate measuring approach is relatively immune to 
chest motion artifacts since the chest wall motion sensor provides an 
excellent indication of such artifacts. The vasomotor activity monitoring 
approach provides an accurate measure of dilation and constriction of a 
peripheral vascular bed. 
The invention itself, together with further objects and attendant 
advantages, will best be understood by reference to the following detailed 
description, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
Turning now to the drawings, FIG. 1 shows a block diagram of a vital signs 
monitor 10 that incorporates presently preferred embodiments of this 
invention. This monitor 10 includes a microcomputer 12 which controls a 
display 14. A number of sensors, including an ECG sensor 16, an oximetric 
finger probe 18, an impedance sensor 20, a strain gauge 22, and a blood 
pressure cuff pressure sensor 24, generate waveforms. These waveforms, 
after suitable signal processing, are interrelated as described below by 
the microcomputer 12 in order to generate reliable measures of heart rate, 
respiration rate, and blood pressure. 
The ECG sensor 16 is a conventional system used to monitor electrical 
voltages associated with cardiac activity. The waveforms generated by the 
ECG sensor 16 are processed in a pulse detector 26 to generate a train of 
ECG pulses which are applied as signal inputs to the microcomputer 12. 
The oximetric finger probe 18 is a conventional sensor which optically 
monitors the transmission or reflection of light through the finger of the 
subject to generate a periodic signal. The AC component of this periodic 
signal can be used as a measure of pulse rate of the subject. The periodic 
signal generated by the oximetric finger probe 18 is applied to a pulse 
detector 28, which generates a series of finger probe pulses applied as 
inputs to the microcomputer 12. 
Sensors such as the electrical impedance sensor 20 of FIG. 1 are 
conventionally attached to the chest of a subject to provide a measure of 
respiration. Rhythmic chest motion associated with breathing results in an 
AC component of the signal generated by the sensor 20. This signal is 
digitized in an A/D converter 30, and the digitized signal (including the 
periodic component) is applied as an input to the microcomputer 12. 
Similarly, chest straps are conventionally used to monitor respiration, and 
such chest straps typically contain strain gauges such as the strain 
gauges 22 of FIG. 1. The strain gauges 22 generate a waveform having a low 
amplitude periodic component correlated with the periodic chest motion of 
breathing and high amplitude peaks associated with artifacts such as 
coughing. This signal waveform is applied to a pulse detector 32 which is 
selectively responsive only to large peaks to generate pulses which are 
applied as an input to the microcomputer 12. Pulse detector 32 responds 
only to large pulses, and therefore the output of the pulse detector 32 is 
indicative of artifacts. 
As shown in FIG. 1 the monitor 10 also includes a blood pressure cuff 34. 
This cuff 34 is of the conventional type which is intended to be wrapped 
around the upper arm of the subject so as selectively to block blood flow 
through the arm. The cuff 34 is inflated by an inflation pump 36 
controlled by the microcomputer 12, and the cuff 34 is deflated by a 
deflation valve 38 also controlled by the microcomputer 12. The pressure 
sensor 24 monitors the air pressure within the blood pressure cuff 34, and 
produces a pressure signal which is digitized in an A/D converter 40 and 
applied as an input to the microcomputer 12. In addition, the pressure 
signal generated by the pressure sensor 24 is applied to a pulse detector 
42, which generates a series of pulses that are also applied as an input 
to the microcomputer 12. 
Those skilled in the art will recognize that a wide variety of conventional 
components can be used for each of the devices 12-42 described above, and 
the details of construction of these devices form no part of the present 
invention. Suitable ECG sensors, oximetric finger probes, impedance 
sensors, respiration strain gauges, blood pressure cuffs, and pressure 
sensors are all well known to those skilled in the art. Furthermore, a 
wide variety of pulse detectors can be used to perform the functions 
described above. Simply by way of example, the electronic circuit shown in 
FIG. 19 can be used for the pulse detector 26; the electronic circuit 
shown in FIG. 18 can be used for the pulse detector 28, and can be 
modified by tuning resistor values for the pulse detectors 32 and 42; the 
circuit shown in FIG. 17 can be used to implement the impedance sensor 20; 
and the circuit shown in FIG. 20 can be used to implement the respiration 
strain gauges 22. The circuits of FIGS. 17-20 have been provided merely by 
way of illustration, and they are in no way intended to limit the scope of 
this invention. It is anticipated that many applications of the inventions 
described herein will utilize circuitry which differs significantly from 
that of FIGS. 17-20. 
The microcomputer 12 is programmed to process the input signals described 
above to generate reliable measures of heart rate, respiration rate, blood 
pressure, and blood pressure waveform. The following detailed discussion 
will take up these four aspects of the monitor 10 in sequence. 
HEART RATE MONITORING 
FIGS. 2a-7b provide pairs of signal waveforms that will be used to explain 
the principle of operation used in heart rate monitoring. Within each pair 
of waveforms the time base is identical; thus, FIGS. 2a and 2b represent 
the same time interval, and vertically aligned portions of the two 
waveforms correspond to the same instant in time. The signals labeled "ECG 
Waveform" were taken from conductor 44 of FIG. 1 and the signals labeled 
"ECG Pulses" were taken from conductor 46. Similarly, the signals labeled, 
"Finger Probe Waveform" were taken from conductor 48 and the signals 
labeled "Finger Probe Pulses" were taken from conductor 50. 
FIGS. 2a and 2b show a typical ECG waveform and the ECG pulses derived 
therefrom. In the absence of artifacts such as those associated with chest 
motion, the ECG pulses are regular and periodic. However, as shown in 
FIGS. 3a and 3b, the ECG waveform can be distored by chest motion. Such 
distortions can cause the pulse detector 26 to fail to detect pulses in 
the ECG waveform associated with cardiac pulses of the subject. In the 
waveform of FIG. 3b, two chest motion artifacts are noted. 
FIGS. 4a and 5a show the waveform generated by the oximetric finger probe 
18. This waveform is normally regular and periodic, but finger motion can 
distort the waveform. FIGS. 4b and 5b show pulse trains generated by the 
pulse detector 28. These pulse trains can be interrupted by finger motion, 
and Figures 4b and 5b have been marked to show such finger motion 
artifacts. 
FIGS. 6a, 6b and 7a, 7b show the ECG pulses and the finger probe pulses for 
two time intervals. In the time interval of FIGS. 6a and 6b the finger 
probe pulses are regular and periodic throughout the time interval, while 
the ECG pulses are interrupted by a chest motion artifact. Conversely, 
during the time interval shown in FIGS. 7a and 7b, the ECG pulses are 
regular and periodic while the finger probe pulses are interrupted by 
finger motion artifacts. These waveforms show that the accuracy of the 
information in either the ECG channel or the finger probe channel may be 
compromised, by chest motion and coughing on the one hand and by finger 
motion on the other hand. However, in many cases the pulse information 
derived from the oximetric finger probe at the fingertip is not disturbed 
at the same time as is the pulse information derived from the ECG sensor. 
According to this invention the microprocessor monitors the pulse trains 
generated by both of the pulse detectors 26 and 28 and selects the pulse 
train with the more accurate information as the better indicator of heart 
rate. The general approach is to chose the channel having the signal with 
the more regularly repeating pulse intervals. In this way, the information 
provided by two separate sensors in interrelated and combined to provide 
an improved heart rate measurement. 
FIGS. 8a-8c provide a flow chart of a first preferred embodiment of a 
program for interrelating the ECG and finger probe pulses. 
The program of FIGS. 8a, 8b begins by initializing internal variables and 
setting the variables Skip Count and Half Count to zero. A pulse search 
routine is then executed which searches for pulses generated by either of 
the pulse detectors 26, 28. Once a pulse is found, the program branches at 
decision diamond 52, depending upon whether the pulse found is an ECG 
pulse or a finger pulse. 
Assuming the pulse is an ECG pulse, the program then searches in block 54 
for a next pulse. Once the next pulse is found the program branches at 
decision diamond 56, depending upon whether this next pulse is a finger 
pulse or an ECG pulse. 
During normal operation, ECG pulses will alternate with finger pulses. 
Assuming this to be the case the variable W.sub.EF (equal to the time 
window from an ECG pulse to a next adjacent finger pulse) is set equal to 
the measured interval and Skip Count is set equal to zero. Then the 
variable Half Count is incremented and checked at decision diamond 58. In 
the first pass through the program Half Count would be equal to 1 and 
control would then branch to a next pulse search as indicated at block 60. 
Once a pulse is found in block 60, control branches at decision diamond 
62, depending upon whether this pulse is an ECG pulse or a finger pulse. 
If both the ECG channel and the finger probe channel are functioning 
properly, and if the previous pulse was a finger pulse, this pulse should 
be an ECG pulse. If so, the variable W.sub.FE (equal to the time window 
from a finger pulse to the next adjacent ECG pulse) is set in block 64 and 
the variable Skip Count is set to zero. Then the variable Half Count is 
incremented and is compared with the constant 2. If Half Count equals 2, 
indicating that both W.sub.EF and W.sub.FE have been set, then the 
variable W.sub.T is set equal to W.sub.EF plus W.sub.FE in block 66. 
W.sub.T is indicative of the total time interval for a heartbeat, and in 
this branch of the program is equal to the sum of the time from an ECG 
pulse to the next adjacent finger pulse, and from that finger pulse to the 
next adjacent ECG pulse. 
Once W.sub.T has been set in block 66 the subroutine Update is called. As 
shown in FIG. 8c, this subroutine sets a new pulse rate equal to the 
inverse of W.sub.T, resets Half Count to zero, and then returns. Control 
is then returned to block 54. 
In the event that no finger pulse is detected between two adjacent ECG 
pulses, then the program branches at decision diamond 56 to block 68, at 
which the variable W.sub.EE is set equal to the time window or interval 
between two adjacent ECG pulses. The variable W.sub.EE is then compared in 
decision diamond 70 with a value equal to 75% of the most recent value of 
W.sub.T. If W.sub.EE is less than this value, indicating that the current 
ECG pulse has occurred too soon and is therefore probably an artifact, 
control branches at decision diamond 70 to block 72, which skips the 
current ECG pulse and increments Skip Count. Control is then returned to 
block 54. 
In the event that the comparison in the decision diamond 70 indicates that 
the most recent ECG pulse has not occurred earlier than the allowed 
window, then W.sub.EE is then compared for reasonableness in decision 
diamond 74. If W.sub.EE is less than 125% of the most recent value of 
W.sub.T, then Skip Count is checked in decision diamond 76. If Skip Count 
is equal to zero then W.sub.T is set equal to W.sub.EE and the subroutine 
Update is called. On the other hand, if Skip Count is not equal to zero 
(indicating one or more skipped pulses), then W.sub.T is reduced by 5% and 
control is returned to block 78. If W.sub.EE is greater than 125% of 
W.sub.T (and therefore outside of the expected range), then W.sub.T is 
either incremented or decremented by 5%, depending upon the state of the 
variable Skip Count, and control is returned to block 78. 
In general terms, the portion of the program associated with blocks 68 
through 76 is only executed when two adjacent pulses are ECG pulses. In 
this case W.sub.EE is set, compared with upper and lower values, and then 
used to update W.sub.T if Skip Count is zero. If W.sub.EE is either too 
small or too large, W.sub.T is modified without calling the routine Update 
in order to bring W.sub.T more nearly equal to W.sub.EE. 
Similarly, if two adjacent pulses are finger pulses, then control branches 
at decision diamond 62 to block 80, which sets the variable W.sub.FF (the 
time window or interval between two adjacent finger pulses), compares 
W.sub.FF with upper and lower limits as described above, checks the 
variable Skip Count, and then sets W.sub.T equal to W.sub.FF in the event 
W.sub.FF is reasonable and Skip Count is zero. The portion of the program 
associated with block 80 operates identically to that associated with 
block 68, except that block 80 relates to the use of two adjacent finger 
pulses to set the variable W.sub.FF. 
From the foregoing description it should be apparent that the program of 
FIGS. 8a, 8b automatically selects the pulse train with the more regular 
waveform for use in updating the variable W.sub.T and therefore the 
measured pulse rate. In the event both the ECG channel and the finger 
probe channel are functioning properly, the ECG and finger pulses 
alternate in time and the variables W.sub.EF and W.sub.FE are summed to 
obtain W.sub.T, the total time between pulses and the reciprocal of the 
new pulse rate. On the other hand, if either the ECG pulses or the finger 
pulses drop out (due to a motion artifact, for example), the program of 
FIGS. 8a, 8b selects the remaining pulse train and then measures the 
interval between adjacent pulses in the remaining pulse train to set to 
set W.sub.T and therefore the new pulse rate. 
The program of FIGS. 8a, 8b operates on a pulse by pulse basis to determine 
the more regular pulse train. This is not a requirement for all 
embodiments of this invention, and FIG. 9 shows an alternative program 
which operates with groups of pulses. 
The program of FIG. 9 first counts the number of ECG pulses and the number 
of finger pulses detected during a preset measurement time interval. The 
measured ECG pulse count is then checked to determine whether it is close 
to the measured finger pulse count in decision diamond 82. If not, the 
count is repeated until the ECG pulse count is within the desired 
tolerance of the finger pulse count. Once this is the case, the variable 
Previous Count is set equal to the average of the ECG and finger pulse 
counts in block 84. The program then counts in block 86 the number of ECG 
and finger pulses detected during a next measurement time interval, which 
is equal in duration to the previous measurement time interval. Control 
then branches depending upon whether the variable Previous Count is closer 
to the ECG count or to the finger count of block 86. If Previous Count is 
closer to the ECG count then Previous Count is set equal to the ECG count. 
Conversely, if Previous Count is closer to the finger count, then Previous 
Count is set equal to the finger count. In either case, the new pulse rate 
is set equal to the updated Previous Count divided by the measurement time 
interval, and control is returned to block 86. 
The program of FIG. 9 monitors the ECG pulses and the finger pulses 
occurring during the measurement time interval and automatically chooses 
the pulse train having a pulse count which more nearly corresponds to the 
previous pulse count. In this way, the more periodic and regular pulse 
train is selected for use in determining the new pulse rate. 
In both the programs of FIGS. 8a, 8b and the program of FIG. 9 the ECG and 
the finger probe pulse train are interrelated and the pulse train with the 
more regular pattern is selected to provide an improved, more reliable, 
more artifact-free measure of heart rate. Of course, this invention is not 
restricted to use with finger probes and ECG sensors. Rather, any two 
measures of pulse rate can be used, such as pulse sensors of the type 
which include an occluding cuff or which monitor ECG, chest impedance, or 
some other parameter from which pulse information may be derived. In 
addition the heart rate monitoring system described above can be used in 
monitors which do not measure respiration rate or blood pressure. 
RESPIRATION MONITORING 
The vital signs monitor 10 includes an impedance sensor 20 and a strain 
gauge 22, as described above. The outputs of these two sensors are 
combined to produce a particularly reliable measure of respiration rate. 
The impedance sensor 20 measures the fluctuating electrical impedance 
associated with the changing volume of the chest of the subject. The 
strain gauge 22 mechanically monitors the physical changes in chest 
dimension. Both of these measuring techniques are subject to artifacts 
associated with chest motion such as coughing. FIGS. 10a and 10b show the 
signal outputs of the impedance sensor 20 and the strain gauge 22, 
respectively, for a preset measuring interval. The impedance waveform of 
FIG. 10a includes regular peaks associated with respiration, as well as 
additional peaks associated with coughs. The peaks associated with coughs 
are only slightly greater in amplitude than the peaks associated with 
normal respiration, and it is difficult to distinguish reliably between 
these two types of peaks in the impedance waveform. However, as shown in 
FIG. 10b, the artifacts associated with chest motion stand out clearly in 
the strain gauge waveform. The microcomputer 12 is programmed to use the 
strain gauge waveform as a measure of chest motion artifacts in order to 
correct the breathing rate as determined by the impedance waveform. 
This program is flowcharted in FIG. 11a. As shown in FIG. 11a the first 
step is to count the number of respiration peaks in the impedance channel 
and the number of large peaks associated with chest motion artifacts in 
the strain gauge channel during a preset measuring interval. Then the 
variable Breath Count is set equal to the impedance channel count minus 
the strain gauge channel count, and finally the respiration rate is set 
equal to the Breath Count divided by the measuring interval. By combining 
the impedance waveform with the strain gauge waveform as described above, 
a more accurate measure of respiration rate is obtained. 
Another algorithm which can be used to obtain an accurate measure of 
respiration rate is flowcharted in FIG. 11b. With this algorithm, the 
variable BREATH COUNT is used to count peaks on the impedance channel 
waveform during a measuring period having a time duration equal to 
MEASUREMENT INTERVAL. In order to suppress artifacts associated with chest 
motion, BREATH COUNT is not incremented during periods of unusual chest 
motion, as indicated by the presence of a large peak on the strain gauge 
channel. Furthermore, the total duration of the periods of unusual chest 
motion is measured and stored in the variable MOTION INTERVAL. The 
respiration rate is then set equal to BREATH COUNT divided by the actual 
time period of measurement (MEASUREMENT INTERVAL--MOTION INTERVAL). 
As before, this invention is not restricted to use with the particular 
sensors described above. For example, piezoelectric films can be used to 
measure changes in chest dimension and electromyograms can be used to 
measure muscle potentials of the lower chest in place of the strain gauges 
described above. In addition, air flow sensors such as microphones on the 
trachea, flow meters on an airway, and thermistors; CO.sub.2 sensors in 
expired air; and other types of sensors can be substituted for the 
impedance channel described above. Furthermore, the respiration rate 
monitoring system described above can be used in monitors which do not 
measure heart rate or blood pressure. 
BLOOD PRESSURE MONITORING 
The vital signs monitor 10 also determines blood pressure of the subject, 
this time by combining pressure information from the blood pressure cuff 
34 with pulse information from the oximetric finger probe 18. 
In general terms, the blood pressure cuff 34 is inflated to a level higher 
than the systolic blood pressure, until arterial pulsations in the 
fingertip (as measured by the oximetric finger probe 18) cease. The 
occluding cuff 34 on the arm is then gradually deflated. Once the cuff 
pressure drops below the systolic blood pressure, blood flow resumes 
beneath the cuff. This immediately causes a pulsation detected by the 
oximetric finger probe 18 at the fingertip, and it is the appearance of 
this first pulsation that signals that the cuff has dropped to systolic 
pressure. The microcomputer 12 reads the systolic pressure from the 
pressure sensor 24. As the pressure in the cuff 34 continues to decrease 
below the systolic pressure, pulsations are received by the microcomputer 
12 from both the pressure sensor 24 associated with the cuff 34 and from 
the oximetric finger probe 18. The pulses arriving at the fingertip occur 
shortly after the pulses appearing at the cuff. There are two factors 
which contribute to this delay. First, it takes a period of time for the 
pulse to propagate from the upper arm to the fingertip. Second, the 
partial occlusion of the artery, due to the pressure in the cuff 34 having 
a value below systolic and above diastolic, causes a delay in the pulse 
below the cuff. The second factor is dependent upon the difference between 
cuff pressure and diastolic pressure. As the cuff pressure drops closer to 
the diastolic pressure, the second type of delay decreases, and when the 
cuff pressure falls below diastolic pressure, the second type of delay is 
no longer a factor. Therefore, once the cuff pressure falls below the 
diastolic blood pressure, the delay between the cuff pulse and the finger 
probe pulse becomes substantially constant. 
FIG. 13 is a graph showing the time delay between (1) the arm pulse as 
detected by the pressure sensor 24 and the pulse detector 42 and (2) the 
finger pulse as detected by the oximetric finger probe 18 and the pulse 
detector 28, as a function of the pressure of the blood pressure cuff 34. 
As shown in FIG. 13, when the cuff pressure is just below systolic 
pressure there is a delay of about 260 milliseconds between the cuff pulse 
and the finger pulse. As the cuff pressure decreases to the diastolic 
value, the delay decreases to a value of about 65 milliseconds. As the 
cuff pressure further decreases below diastolic, the delay is not 
decreased, but simply oscillates around the 65 millisecond value. As the 
cuff deflates, the transition from a decreasing delay to a fixed delay 
indicates the diastolic pressure value. 
The oscillations in the delay value (which are most easily seen when the 
cuff pressure is below diastolic) are synchronous with the subject's 
respiratory cycle. Previous work has shown that these oscillations are due 
to variations in cardiac stroke volume which occur as a result of 
fluctuations in intrathoracic pressure during the respiratory cycle. 
FIGS. 14a and 14b show waveforms associated with oscillations in pressure 
of the cuff 34 and oscillations in the signals generated by the finger 
probe 18, respectively. The waveform of FIG. 14a is similar to that of 
FIG. 12b, except that the time scale of FIG. 14a has been expanded to 
enable more careful analysis of pulse timing. The left hand portions of 
the waveforms of FIGS. 14a and 14b illustrate the suppression of pulses at 
the fingertip when the cuff pressure is higher than the systolic pressure. 
As the pressure drops to the systolic level, the trace of FIG. 14b begins 
to indicate arterial pulsations at the fingertip. The arrival of this 
first pulse is an indication that the instantaneous pressure indicated by 
the pressure sensor 24 corresponds to systolic blood pressure of the 
subject. In the center portion of FIGS. 14a and 14b both waveforms 
indicate the presence of pulsations as the cuff pressure drops from the 
systolic to the diastolic. Careful examination of the timing differences 
between pulses in FIGS. 14a and 14b show the progressive decrease in the 
delay time as described above in conjunction with FIG. 13. After the cuff 
pressure reaches diastolic, the right hand portion of the waveforms of 
FIGS. 14a and 14b illustrate the substantially constant delay between the 
respective pulses. Thus, the data shown in FIGS. 14a and 14b provide the 
necessary information for determining both the systolic and the diastolic 
blood pressure through the use of interrelationships between cuff 
pulsations detected by the pressure sensor 24 and fingertip pulsations as 
detected by the oximetric finger probe 18. 
FIG. 15 shows a flowchart of a suitable program for implementing the blood 
pressure monitoring technique described above. As shown in FIG. 15 the 
first step is to inflate the blood pressure cuff 34 until the amplitude of 
the pulses detected by the oximetric finger probe 18 is at very low level. 
This is used to determine that the pressure of the cuff 34 is above 
systolic. Then the cuff 34 is deflated at a rate such as 3 millimeters of 
mercury per second. 
The microcomputer 12 then monitors the pulse detector 42 for cuff pulses. 
If no cuff pulses are found within an allowed time, the routine aborts. 
Otherwise, the routine then monitors the pulse detector 28 for finger 
probe pulses. Once a finger probe pulse is found, the microcomputer sets 
the systolic pressure equal to the DC cuff pressure at the time when the 
previous cuff pulse occurred. The routine then monitors the pulse detector 
42 for additional cuff pulses and the pulse detector 28 for additional 
finger pulses. The delay time between each cuff pulse and the subsequent 
finger pulse is then calculated and compared with the previous delay time. 
As long as the newly calculated delay time is less than the previous delay 
time, the routine sets the previous delay time to the newly calculated 
delay time and returns to search for more cuff and finger pulses. However, 
in the event the newly calculated delay time is equal to or greater than 
the previous delay time, then the diastolic pressure is set equal to the 
currently prevailing DC value of the cuff pressure as measured by the 
pressure sensor 24 and the A/D converter 40. 
Once systolic and diastolic pressures have both been measured, the cuff is 
deflated completely and the routine terminates. 
It is important to note that the foregoing technique for blood pressure 
measurement does not increase the hardware complexity of the vital signs 
monitor 10 in any way. The oximetric finger probe 18 and the pulse 
detector 28 are present in the vital signs monitor 10 for other purposes, 
and therefore the present invention provides important improvements in 
accuracy over the prior art oscillometric technique without increasing the 
cost or hardware complexity of the vital signs monitor 10. 
Once again, this system is not restricted to use with the particular 
sensors described above. Other pulse detectors can be substituted for the 
oximetric finger probe to detect pulses distal to the cuff, and other 
pulse detectors can be used to detect pulses at or near the cuff. In 
addition, the blood pressure monitoring system described above can be used 
in monitors which do not measure heart rate or respiration rate. 
BLOOD PRESSURE WAVEFORM MONITOR AND VASOMOTOR ACTIVITY DETECTOR 
It is well known that oximetric sensors such as the oximetric finger probe 
18 provide excellent relative measures of the blood pressure waveform. 
However, standard oximetric waveforms are not calibrated as to core blood 
pressure, and this lack of calibration is a significant limitation in many 
settings. The monitor 10 overcomes this problem by automatically combining 
core blood pressure information derived from an indirect blood pressure 
measuring system such as that described in the preceding section with 
waveform information derived from a waveform detector such as the 
oximetric finger probe 18. By providing an electronic system such as the 
microcomputer 12 with signals from both sources, it is possible to 
generate a continuous waveform which is calibrated as to core blood 
pressure and which therefore is similar in many respects to the pressure 
signal generated by an invasive catheter blood pressure monitoring system. 
Pressure accuracy is ensured by updating the calibration of the waveform 
periodically. The period between updates is typically in the range of 15 
seconds to several minutes, but it is conceivable that both longer and 
shorter periods may be selected. During the period between recalibrations 
the waveform is continuously displayed using the calibrations obtained 
from the most recent blood pressure measurement. As described in greater 
detail below, the method of this invention can detect clinical conditions 
in which the waveform generated by the oximetric finger probe 18 decreases 
in amplitude without a corresponding change in core blood pressure. This 
condition can provide an early warning of circulatory failure. 
Turning now to FIGS. 16a-16e, the monitor 10 is programmed to implement the 
waveform display and vasomotor monitoring techniques described above. As 
described above, the monitor 10 includes an oximetric finger probe 18 
which produces on line 48 a waveform which is uncalibrated as to blood 
pressure but which has a shape that closely tracks the blood pressure 
waveform of the subject. The signal output of the oximetric finger probe 
18 is digitized by the A/D converter 30 and then applied as a digital 
input to the microcomputer 12. Similarly, the monitor 10 includes a blood 
pressure cuff 34, a pressure sensor 24, and related components which allow 
the microcomputer 12 to measure the systolic and diastolic core blood 
pressure of the subject as described in the preceding section. These 
components are used in conjunction with the program flow charted in FIGS. 
16a-16e to generate a calibrated waveform indicative of core blood 
pressure of the subject and to monitor vasomotor activity of the subject. 
Turning now to FIG. 16a, the main routine initializes, measures the core 
blood pressure of the subject and then sets the variables SYSTOLIC and 
DIASTOLIC to the measured systolic and diastolic pressures. In this 
embodiment systolic and diastolic pressures are measured as described in 
the preceding section. The routine PUlseamp (FIG. 16b) is called to 
measure the average peak to peak amplitude of the finger probe waveform 
over a 20 pulse period. Then the routine Rescale (FIG. 16c) is called in 
order to determine the calibration factors M and B which will be used to 
calibrate the displayed waveform. In general terms, the routine Rescale 
sets the calibration factor M equal to the ratio between (1) the 
difference between SYSTOLIC and DIASTOLIC and (2) the average amplitude of 
the finger probe waveform. The calibration factor B is set equal to the 
vertical offset required to place the calibrated waveform as desired on 
the display 14. 
Once the calibration factors M and B have been determined in the routine 
Rescale, the program of 16c then calls the routine Check (FIG. 16e). The 
routine Check monitors for various warning conditions, and displays 
appropriate warning messages if appropriate. In particular, Check checks 
for fluctuations in M, and classifies an increase or a decrease in M of 
25% or more (as compared with the average value of M over the previous ten 
minute period) as indicative of excessive vascular bed constriction or 
dilation. 
At this point a timer is reset and the routine of FIG. 16a takes a reading 
V.sub.IN of the pulse waveform via the A/D converter 30. Then the value 
V.sub.IN is scaled with the calibration factors M and B according to the 
formula 
EQU V.sub.OUT =M(V.sub.IN)+B. 
The scaled value V.sub.OUT is then output for display on the display 14. 
The Pulseamp routine is called to measure the amplitude of the finger probe 
waveform for a single pulse cycle and the routine Trend (FIG. 16d) is 
executed. Trend maintains the variable BASELINE equal to the running 
average of pulse amplitude AMP during the period since the last 
measurement of SYSTOLIC and DIASTOLIC, and the variable RUNAVG equal to 
the average of AMP of the last 20 pulse cycles. In the event RUNAVG 
deviates from BASELINE by more than .+-.25%, this is taken as an 
indication of possible vascular bed constriction or dilation and control 
is returned to the block in FIG. 16a that measures SYSTOLIC and DIASTOLIC. 
If RUNAVG is within 25% of BASELINE, Trend returns. 
The timer is then decremented and checked for a time out condition. If the 
timer has not yet timed out the routine then returns to read a next value 
of V.sub.IN. Once the timer times out the program automatically returns to 
measure the subject's blood pressure and to reset the variables SYSTOLIC 
and DIASTOLIC in accordance with the measured values. At any time, a new 
measurement of blood pressure, a new setting of SYSTOLIC and DIASTOLIC, 
and a new calculation of calibration factors M and B can be commanded with 
an appropriate key entry. 
FIG. 16b is a detailed flow chart of the routine Pulseamp. The heart pulse 
waveform characteristically includes two minima and two maxima within each 
cycle. The routine Pulseamp first reads a value of the oximetric finger 
probe waveform from the A/D converter 30 and stores it in the variable 
ADDAT. The routine then cycles in the loop 100 to set the variable MAX1 
equal to a first local maximum of ADDAT. Once a first local maximum is 
found in the loop 100, the routine then finds a first local minimum MIN1 
in the loop 102. Then a second local maximum MIN2 is found in the loop 104 
and a second local minimum MIN2 is found in the loop 106. Throughout 
operation of the loops 100-104, A/D measurements are output to the display 
14 using the previously established calibration factors. 
Once MAX1, MIN1, MAX2, and MIN2 have been set, the routine then sets the 
variable MIN equal to the smaller of MIN1 and MIN2 and sets the variable 
MAX equal to the larger of MAX1 and MAX2. The variable MINTOTAL is used to 
maintain a running total of each value of variable MIN, and the variable 
MAXTOTAL is used to maintain a running total of the variable MAX. Thus, in 
each pulse cycle of the waveform generated by the oximetric finger probe 
18, the routine of Figure 16b finds the maximum value and the minimum 
value of the waveform, and sums these maximum and minimum values in 
appropriate variables. For example, when the counter is initially set 
equal to 5, MAXTOTAL will include the sum of five consecutive maximum 
values of respective pulse cycles and MINTOTAL will contain the sum of the 
corresponding five minimum values of the pulse waveform. 
At this point the average amplitude AMP is calculated as 
(MAXTOTAL-MINTOTAL)/TOTAL, where TOTAL is the initial value of the 
counter. The routine Pulseamp then returns. 
FIG. 16c shows a flow chart of the routine Rescale, which updates the 
running ten minute average of M and then sets M and B according to the 
following formulas: 
##EQU1## 
The routine Check is then called and then the routine Rescale returns. 
FIG. 16e shows a flow chart of the routine Check. This routine first checks 
to determine whether the trend in the calibration factor M indicates a 
vascular bed constriction. M correlates core blood pressures as measured 
with the blood pressure cuff 34 with blood flow at an extremity as 
measured by the oximetric finger probe 18. In general, M will vary, both 
from subject to subject and over time for a given subject. The first type 
of variation is a result of differences in finger opacity, skin 
pigmentation, and the like from subject to subject. The second type of 
variation relates to variations in blood volume and blood flow 
characteristics at the finger monitored by the oximetric finger probe 18. 
For example, when the vascular bed of the finger being monitored by the 
finger probe 18 constricts, the value of M will rise. Thus, by monitoring 
the changes in M over time, excessive increases in M can be taken as an 
indication of vascular bed constriction and an appropriate warning message 
displayed. In the example of FIG. 16e, an increase in M by more than 25% 
as compared with the average value of M over the last ten minutes is taken 
as an indication of significant vascular bed constriction. 
Conversely, if the vascular bed of the finger dilates, the amplitude of the 
waveform generated by the oximetric finger probe 18 will increase, causing 
a reduction in the value of M. In the example of FIG. 16e a decrease in M 
by more than 25% as compared with the average value of M over the last ten 
minutes is taken as an indication of significant vascular bed dilation and 
an appropriate warning message displayed. 
In alternate embodiments the approach used to detect significant trends in 
M may be varied, as can the thresholds and time periods described above. 
The routine Check also compares the measured blood pressure with stored 
limits and displays appropriate warning messages if these limits are 
exceeded. Similarly, the routine Check monitors the regularity of the 
heart beat as indicated by the waveform generated by the oximetric finger 
probe 18 and displays an appropriate warning message if an irregular heart 
beat is detected. 
The system described above provides an early warning of excessive dilation 
or constriction of the vascular bed at the finger. As explained above, a 
decrease in pulse amplitude as measured at the fingertip which does not 
correspond to a decrease in core blood pressure as measured with the 
occluding cuff indicates a constriction in the vascular bed at the 
fingertip. This may provide an warning of a potential drop in core blood 
pressure such as circulatory failure or shock. Similarly, an increase in 
pulse amplitude from the finger probe which does not correspond to an 
increase in core blood pressure as measured with the blood pressure cuff 
indicates a dilation of the vascular bed at the fingertip. This indication 
may precede an actual increase in core blood pressure and so may warn of 
such a possibility. Furthermore, the calibrated waveform display of this 
invention provides a number of important advantages. Because the waveform 
is automatically calibrated on a periodic basis, the displayed waveform 
provides both waveform information regarding the shape of the pulse 
waveform and core blood pressure values for the waveform. Such a 
calibrated waveform is of considerable diagnostic value, and in the system 
described above is obtained without increasing the hardware of the monitor 
10 and without requiring any sort of invasive blood pressure measurements. 
The use of the oximetric finger probe 18 provides important advantages in 
that it does not increase the hardware requirements of the system. The 
oximeter may be positioned on any suitable part of the skin of the 
subject, such as the finger described above or the forehead. (Of course, 
such repositioning of the oximeter may affect the way in which the 
oximeter signal is processed to monitor vasomotor activity.) However, 
other sources of a pulse waveform can be used with this invention, 
including waveforms generated from occluding cuffs, from strain gauges, 
and from oscillometric or impedance measurements. Similarly, a variety of 
techniques can be used to measure the blood pressure of the subject, 
including oscillometric techniques and techniques based on measuring the 
Korotkoff sounds. In some applications where it is of key importance to 
provide advance warning of drops or rises in core blood pressure, invasive 
catheters with suitable pressure transducers can be used to measure the 
core blood pressure. 
CONCLUSION 
It should be apparent from the foregoing discussion that an improved vital 
signs monitor has been described which provides important advantages in 
terms of artifact rejection and improvements in accuracy with little or no 
increase in hardware complexity. This is accomplished in all of the 
monitoring techniques described above by cross-referencing data from 
multiple sensor channels in order to reduce artifacts and to improve 
measuring accuracy. 
Of course, it should be understood that a wide range of changes and 
modifications can be made to the preferred embodiments described above. It 
is therefore intended that the foregoing detailed description be regarded 
as illustrative rather than limiting, and that it be understood that it is 
the following claims, including all equivalents, which are intended to 
define the scope of this invention.