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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and method for measuring the
variability of cardiovascular parameters.
The heart rate fluctuates spontaneously about its average value in several
frequencies. These fluctuations are attributed to the activity of the two
branches of the Autonomic Nervous System (ANS): the sympathetic and the
parasympathetic nervous systems. Power spectrum analysis revealed
fluctuations in three main frequency ranges: the high frequency
fluctuations in the respiration rate, which are related to the
parasympathetic nervous system; the mid-frequency fluctuations, which are
usually related to both branches of the ANS and the low frequency
fluctuations, at the rate of 0.03-0.08 Hz, which are attributed to the
activity of the sympathetic nervous system.
The Heart Rate Variability (HRV) measurement provides information on the
ANS function: in several pathologies the HRV pattern is abnormal. The
measurement itself is simple and noninvasive and has potential to be used
as a clinical tool for the assessment of pathological ANS. At present,
such measurements are not used routinely because the difference between
pathological and normal HRV is not well defined. The pattern of the HRV
differs greatly even between normal subjects, and masks the different
pattern of the pathological HRV.
During systole (heart contraction), blood is ejected from the left
ventricle into the peripheral organs, thereby increasing their blood
volume. The measurement of this Systolic Blood Volume Increase (SBVI) is
called plethysmography. The simplest plethysmographic method is
Photoplethysmography (PPG), in which light is incident on some site of the
skin, so that part of it enters the tissue. That light is partly scattered
and partly absorbed by the red blood cells. The light which emerges out of
the skin is measured by a photodetector. The output signal shows
pulsations in the heart rate, due to the variations in tissue blood
volume, which occur by the heart beats.
The PPG method is not suitable for absolute evaluation of the SBVI, because
the absolute value of the signal depends on the skin color, on the
pressure of the probe on the skin and because the signal varies
spontaneously as a function of time even during the same examination. At
present, the method is used for the measurement of the heart rate (where
the absolute PPG is not important) and for pulse oximetry--measurement of
oxygen saturation in the arterial blood, by measuring the ratio of the PPG
signal for two or three different wavelenths (where only the ratio between
two wavelengths is required).
U.S. Pat. No. 4,834,107 discloses a system which, in some respects, is
similar to the present invention. In that patent, the PPG signal of one,
single pulse, is digitally analyzed, in order to determine the systolic,
diastolic and mean blood pressure and the pulse pressure.
SUMMARY OF THE INVENTION
In accordance with the present invention, however, no blood pressure value
is derived from the PPG signal. Furthermore, the parameter which is
derived, the degree of the variability of the PPG parameter or the maximal
correlation coefficient between two PPG parameters or between the values
of the same PPG parameter in two sites of the body and the lag between
them, cannot be derived from a single pulse, but from a series of more
than e.g., 30 pulses, in order to detect the low frequency sympathetic
nervous system regulated fluctuations.
It is therefore a broad object of the present invention to overcome the
above-mentioned and other drawbacks of the known methods for SBVI analysis
and to provide a system and a method facilitating improved evaluation of
the fluctuations in SBVI and other cardiovascular parameters.
In accordance with the present invention there is provided an apparatus for
measuring the variability of cardiovascular parameters, comprising at
least one photoplethysmographic (PPG) probe, each probe having a modulated
first light source and a photodetector, a demodulator connected to said
photodetector for demodulating PPG signals detected by said photodetector,
an analog to digital converter for digitizing the demodulated signals, and
a processor for repeatedly analysing said PPG signals for a predetermined
number of times.
The invention further provides a method for measuring variability of
cardiovascular parameters, comprising performing a series of PPG
measurement on a patient over a predetermined period of time, selecting
parameters to be analyzed for a group of parameters including the blood
volume (BV) of the measured tissue; the amplitude (AM) of the systolic
increase in the blood volume of the tissue and the time duration (P)
between the maxima of two adjacent PPG pulses and the maximal rate of
increase (Vmax) of the blood volume, and measuring the standard deviation
of each parameter.
The invention will now be described in connection with certain preferred
embodiments with reference to the following illustrative figures so that
it may be more fully understood.
With specific reference now to the figures in detail, it is stressed that
the particulars shown are by way of example and for purposes of
illustrative discussion of the preferred embodiments of the present
invention only and are presented in the cause of providing what is
believed to be the most useful and readily understood description of the
principles and conceptual aspects of the invention. In this regard, no
attempt is made to show structural details of the invention in more detail
than is necessary for a fundamental understanding of the invention, the
description taken with the drawings making apparent to those skilled in
the art how the several forms of the invention may be embodied in practice
.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a known technique for effecting PPG measurements;
FIG. 2 illustrates a PPG signal obtained by the measurements effected by
the technique of FIG. 1.
FIG. 3 illustrates a signal curve showing the dependency of the parameters
on the pulse number;
FIG. 4 is a power spectrum of the dependency of the parameters of FIG. 3,
on time;
FIG. 5 is a block diagram of the system according to the present invention,
and
FIG. 6 is a block diagram of a system for performing measurements at two
sites on a body.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
There is illustrated in FIG. 1 the known PPG measurement effected on a
patient's finger 2 by the application thereto of a light source 4 and a
light detector 6. The PPG can be measured either by transmission of light
through the tissue or by reflection from the skin.
The PPG signal obtained by the above-described measurement is shown in FIG.
2. Several parameters which can be derived from the pulse are also
depicted on the curve:
1. The baseline (BL), which is inversely related to the tissue blood
volume: higher blood volume results in higher absorption of the light and
lower output signal. The parameter, BV defined by BV=Const.-BL is
therefore a parameter which is directly related to the blood volume in the
tissue under investigation.
2. The amplitude (AM) of the PPG signal, which is related to the SBVI. SBVI
depends on the compliance to pressure of the blood vessels: higher
rigidity of the vessels reduces their volume change during systole, while
higher elasticity enables higher dilation during the systolic period.
3. The PPG pulse period P is actually the cardiac beating period, and its
variability is therefore the classical HRV.
The PPG signal illustrated in FIG. 2 was measured for several hundred
pulses and the three parameters BV, AM and P were derived for each pulse.
Shown in FIG. 3 is the dependence of each parameter on the pulse number.
The power spectrum of the dependence on time of the three parameters BV,
AM and P was then computed and the three spectra obtained for one of the
subjects are shown in FIG. 4. It can be seen that all three parameters
fluctuate in the three frequencies mentioned above, but the intensities of
the fluctuations in the different frequencies differ between BV, AM and P:
the high frequency fluctuations dominate the P curve, while the low
frequency fluctuations are more prominent in the BV and AM curves.
It was found that neurologic pathologies result in difference in the BV and
AM variability pattern, in accordance with the results obtained for HRV
examinations. The changes in the low frequency are better seen and
measured in the BV and AM curves, probably due to the direct effect of the
smypathetic nervous system on the diameter of the blood vessels, changing
thereby their volume and compliance. It is therefore expected that
pathologies in the sympathetic nervous system, which dominate the low
frequency fluctuations, will be better diagnosed by the PPG measurement
through the analysis of the dependence of the PPG parameters on time.
In the preferred embodiment shown in FIG. 5, there is seen the PPG system
arranged to reliably measure the variability of several parameters of the
PPG signal, such as, the baseline BL, (or BV which is equal to a
Const.-BL), the amplitude AM, the period P, the maximal rate of increase
Vmax of the tissue blood volume, and the time duration T from the maximum
to the minimum. As explained above, BV is related to the tissue blood
volume, AM is related to the blood vessels' elasticity and P is the
cardiac cycle period. Vmax is related to the maximal contraction rate of
the left ventricle, and is influenced by several cardiac and peripheral
vascular parameters.
The system comprises one or more PPG probes 8, each includes either one
light source 10 or two light sources of different wavelengths, and a
photodetector 12. The light intensity is modulated by modulator 14 and the
detected PPG signal is amplified, and filtered in order to avoid
background light, and then demodulated in circuit 16. The demodulated
signal then digitized in the A/D converter 18 and the digitized signal is
analyzed either by a microprocessor 20 or by computer, in order to obtain,
for each pulse, the desired parameters, BV, AM, P, Vmax and T. A display
22 is used for displaying either the PPG signal or the curves of the
different parameters as a function of the pulse number. When the
measurement is performed in two sites, the correlation function can also
be shown on the display.
In order to reliably measure the above-mentioned parameters the electronic
components should be accurately designed. RC filtration of the high DC
component of the PPG signal is not allowed, since it may significantly
modify the other parameters. The discrimination between the DC and the AC
components of the PPG signal (BL and AM, respectively) should therefore,
be done digitially, using an A/D convertor of high resolution.
The preferred site of measurement is the fingertip, since the fingertip
blood vessels are highly inervated by the symphathetic nervous system.
Other sites on the body can also be used for the examination, such as the
forearm, toe, leg or earlobe.
The examination of the PPG parameters variability can also be performed on
different sites of the body, such as different fingertips of the same hand
or on different hands, on fingers and toes, or on fingers and forearm. The
different curves from the different sites can then be compared in order to
detect pathological or physiological changes between the different sites.
The analysis of the PPG parameters may include:
1. Automatic derivation of the above parameters for each pulse signal as
described with reference to FIG. 2. For that task the minimum and the
maximum for each pulse is determined in order to derive BL, AM, P and T,
and the derivative of the PPG curve is calcualated in order to obtain the
maximal rate of increase of the blood volume;
2. Plottings of each parameter vs. time curve in order to check that the
automatic analysis was properly performed;
3. Power spectrum analysis of each parameter;
4. Cross correlation (CC) analysis for each desired pair of these
parameters, e.g., P.sub.1 and P.sub.2, according to the well known formula
for CC:
##EQU1##
CC provides information on the degree of correlation between the two
parameters, P.sub.1 and P.sub.2. CC is function of the parameter .tau.,
the lag between P.sub.1 and P.sub.2. The CC curve is displayed and two
parameters are derived therefrom: the maximal correlation coefficient and
the lag .tau. required for obtaining that maximal correlation.
The examination of the PPG parameters' variability can also be performed on
different sites of the body, such as different fingertips of the same hand
or on different hands, on fingers and toes, or on fingers and forearm. As
illustrated in FIG. 6, for each PPG parameter, the curves describing the
value of the parameter as a function of the pulse number can be obtained
for each site of measurement and the parameter dependence on time for the
different sites can be compared. A correlation processor 24 can
advantageously be connected between the processors 20 of each of the
probes.
One of the methods for detecting pathological changes between the different
sites is the use of the correlation function
##EQU2##
where P.sub.a (t) and P.sub.b (t) are the corresponding values of the same
parameter in the two different sites, a and b. As an example, the
correlation coefficient CC(.tau.=0) between two fingers in different hands
for normal subjects is above 0.90. Lower values indicate lower
coordination between the two sites.
It will be evident to those skilled in the art that the invention is not
limited to the details of the foregoing illustrated embodiments and that
the present invention may be embodied in other specific forms without
departing from the spirit or essential attributes thereof. The present
embodiments are therefore to be considered in all respects as illustrative
and not restrictive, the scope of the invention being indicated by the
appended claims rather than by the foregoing description, and all changes
which come within the meaning and range of equivalency of the claims are
therefore intended to be embraced therein.
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