 |
Claims  |
|
|
What is claimed is:
1. A system for non-invasively monitoring at least one hemodynamic vascular
parameter of an individual, the system comprising:
(a) at least two infrared detectors being positionable in a spaced apart
configuration against a region of a skin of the individual above at least
one blood vessel, each of said at least two infrared detectors being for
individually collecting infrared spectral data from said region of the
skin, said infrared spectral data corresponding to a volume of blood
present within said at least one blood vessel; and
(b) a processing unit being in communication with said at least two
infrared detectors, said processing unit being for independently
processing said infrared spectral data collected by each of said at least
two infrared detectors, said processing unit implementing an algorithm
serving to account for blood reflection waves resulting from reflection
sites in blood vessels, so as to yield information pertaining to the at
least one hemodynamic vascular parameter of the individual.
2. The system of claim 1, wherein each of said at least two infrared
detectors detects changes in infrared reflection from said region of said
skin.
3. The system of claim 1, wherein each of said at least two infrared
detectors includes an infrared source for irradiating said region of said
skin and an infrared sensor for sensing infrared reflection reflected from
said region of the skin.
4. The system of claim 3, wherein said infrared source irradiates said
region with infrared radiation of a wavelength within a range of 800 nm to
960 nm.
5. The system of claim 1, wherein each of said at least two infrared
detectors is an infrared photoplethysmograph.
6. The system of claim 1, wherein said at least two infrared detectors
include three detectors each independently being for collecting infra red
spectral emission data from said region, said three detectors being
positionable in a spaced apart configuration against said region of said
skin.
7. The system of claim 1, wherein the at least one hemodynamic vascular
parameter is selected from the group consisting of blood viscosity, blood
density, a radius of said blood vessel, an elasticity of said blood
vessel, systolic blood pressure, diastolic blood pressure and continuous
blood pressure.
8. The system of claim 1, wherein said infra red spectral data is collected
by each of said at least two infrared detectors over the course of at
least one heart beat cycle.
9. The system of claim 1, wherein said infra red spectral data is
continuously collected by each of said at least two infrared detectors,
thus enabling continuous monitoring of the at least one hemodynamic
vascular parameter.
10. The system of claim 1, further comprising a device being for
obstructing flow in said blood vessel down stream from said region of said
skin.
11. The system of claim 1, further comprising an interface communicating
with said processing unit, said interface being for providing information
pertaining to the at least one hemodynamic vascular parameter to an
operator of the system.
12. The system of claim 11, wherein said information pertaining to the at
least one hemodynamic vascular parameter is provided to said operator in
at least one format selected from the group consisting of a textual
format, a graphic format and an audio format.
13. The system of claim 1, wherein said algorithm calculates said
information pertaining to the at least one hemodynamic vascular parameter
of the individual by determining a wave propagation velocity, a reflection
coefficient and a distance to a reflection site.
14. The system of claim 1, wherein said algorithm calculates said
information pertaining to the at least one hemodynamic vascular parameter
of the individual by extracting values pertaining to displacement of a
vessel wall under an assumption that a reflection coefficient is constant
with respect to a frequency of a specific harmonic.
15. The system of claim 1, wherein said algorithm calculates said
information pertaining to the at least one hemodynamic vascular parameter
of the individual taking into account information pertaining to a foot to
foot speed and calculating a wall displacement in order to calculate a
forward propagating wave.
16. The system of claim 1, wherein said algorithm calculates said
information pertaining to the at least one hemodynamic vascular parameter
of the individual by extracting values pertaining to wall displacement and
blood flow.
17. A method of non-invasively monitoring at least one hemodynamic vascular
parameter of an individual, the method comprising:
(a) positioning at least two infrared detectors in a spaced apart
configuration against a region of a skin of the individual above at least
one blood vessel;
(b) individually collecting in each of said infrared detectors, infrared
spectral data from said region of the skin, said infrared spectral data
corresponding to a volume of blood present within said at least one blood
vessel; and
(b) independently processing said infrared spectral data collected by each
of said at least two infrared detectors, while accounting for blood
reflection waves resulting from reflection sites in blood vessels, so as
to yield information pertaining to the at least one hemodynamic vascular
parameter of the individual.
18. The method of claim 17, wherein each of said at least two infrared
detectors includes an infrared source for irradiating said region of said
skin and an infrared sensor for sensing infrared reflection reflected from
said region of the skin.
19. The method of claim 18, wherein said infrared source irradiates said
region with infrared radiation of a wavelength within a range of 800 nm to
960 nm.
20. The method of claim 17, wherein each of said at least two infrared
detectors is an infrared photoplethysmograph.
21. The method of claim 17, wherein said at least two infrared detectors
include three detectors each independently being for collecting infra red
spectral emission data from said region, said three detectors being
positionable in a spaced apart configuration against said region of said
skin.
22. The method of claim 17, wherein said at least one blood vessel includes
an artery underlying said region of said skin.
23. The method of claim 17, wherein the at least one hemodynamic vascular
parameter is selected from the group consisting of blood viscosity, blood
density, a radius of said blood vessel, an elasticity of said blood
vessel, systolic blood pressure, diastolic blood pressure and continuous
blood pressure.
24. The method of claim 17, wherein said step of individually collecting in
each of said infrared detectors, infrared spectral data from said region
of the skin, is effected over the course of at least one heart beat cycle.
25. The method of claim 17, wherein said step of individually collecting in
each of said infrared detectors, infrared spectral data from said region
of the skin, is effected continuously thus enabling continuous monitoring
of the at least one hemodynamic vascular parameter.
26. The method of claim 17, further comprising the step of obstructing flow
in said blood vessel down stream from said region of said skin prior to
said step of collecting in each of said infrared detectors, infrared
spectral data from said region of the skin.
27. The method of claim 17, wherein accounting for blood reflection waves
resulting from reflection sites in blood vessels is by determining a wave
propagation velocity, a reflection coefficient and a distance to a
reflection site.
28. The method of claim 17, wherein accounting for blood reflection waves
resulting from reflection sites in blood vessels is by extracting values
pertaining to motion of a vessel wall under an assumption that a
reflection coefficient is constant with respect to a frequency of a
specific harmonic.
29. The method of claim 17, wherein accounting for blood reflection waves
resulting from reflection sites in blood vessels is by calculations taking
into account information pertaining to a foot to foot speed and
calculating a wall displacement in order to calculate a forward
propagating wave.
30. The method of claim 17, wherein accounting for blood reflection waves
resulting from reflection sites in blood vessels is by extracting values
pertaining to wall displacement and blood flow. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to a system and method for monitoring
hemodynamic vascular parameters of a patient and, more particularly, to a
system and method utilizing photoplethysmography to monitor parameters
associated with, for example, blood pressure and blood flow of a patient.
Hemodynamic vascular parameters such as blood pressure, blood flow and the
like which are typically measured using non-invasive procedures are
routinely monitored by physicians in order to determine the physiological
state of the heart and circulatory system of a patient. Of these
hemodynamic vascular parameters, blood pressure is the most commonly
monitored.
Blood pressure is the force within the circulatory system of an individual
that ensures a flow of blood and delivery of oxygen and nutrients to the
tissue.
Abnormal blood pressure readings and/or blood pressure fluctuations over
time are oftentimes indicative of heart or circulatory disorders.
Hypertension is one of the most common diseases in the adult population,
often accompanied by secondary cardiovascular damage. In addition,
prolonged reduction or loss of pressure severely limits the amount of
tissue perfusion and could therefore result in damage to, or even death
of, the tissue. Although some tissues can tolerate hypoperfusion for
fairly long periods of time, the brain, heart and kidneys are very
sensitive to a reduction in blood flow. Thus, blood pressure is a
frequently monitored both routinely and also during surgical procedures
where ample supply of blood to tissues is crucial for tissue survival.
During and after surgery, blood pressure is affected by the type of surgery
and physiological factors such as the body's response to the surgery.
Moreover, during and after surgery, blood pressure is manipulated and
controlled using various medications. Often, these physiological factors
and the given medications result in a situation requiring immediate blood
pressure measurement, and corrective action.
In some clinical situations, dramatic changes in blood pressure can occur
instantaneously. For example, a sudden change in pressure may occur due to
a reaction to drug therapy. Also, patient reactions to the surgery, sudden
occlusion of an artery due to an embolism, or even sudden cardiac arrest
are a few of the possibilities. It is very important to detect these
sudden changes immediately, and to insure that the direction and amount of
the changes be accurate within certain limits. Conversely, it is equally
important that false indications of significant blood pressure changes do
not occur.
Due to the above described reasons, constant monitoring of blood pressure
of a patient is often necessary. The traditional method of measuring blood
pressure is with the use of a occlusive cuff, a stethoscope and a pressure
manometer. However, this technique is slow, subjective in nature, requires
the intervention of a skilled clinician and does not provide the timely
readings frequently required in critical situations.
For these reasons, two methods of measuring blood pressure have been
developed: invasive, continuous (beat-to-beat) measurements and
noninvasive, intermittent methods that use an automated occlusive cuff
device.
Invasive methods suffer from several inherent limitations including the
risk of embolization, nerve damage, infection, bleeding and vessel wall
damage. In addition due to their invasive nature such methods are more
suitable to blood pressure monitoring during surgical procedures.
The noninvasive cuff method does not have the inherent disadvantages of the
invasive technique, however it also does not provide the continuous
beat-to-beat pressure variations obtainable with the invasive method.
Further, the noninvasive cuff method typically requires 15 to 45 seconds
to obtain a measurement, and since it is an occlusive technique, the
method should allow a minimum of 15 seconds to ensure sufficient venous
recovery. Thus, at best there is typically 1/2 to 1 minute between updated
pressure measurements. When fast acting medications are administered, this
is an inordinately long amount of time to wait for an updated pressure
reading. Also, frequent cuff inflation over extended periods of time may
result in ecchymosis and/or nerve damage in the area underlying the cuff.
Several systems have been developed to address the need for continuous,
noninvasive blood pressure measurement.
European Patent Document 0048060 and U.S. Pat. Nos. 4,406,289, 4,510,940
and 4,539,997 to Wesseling et al., U.S. Pat. No. 4,475,554 to Hyndman,
U.S. Pat. No. 4,524,777 (1985) to Kisioka, U.S. Pat. No. 4,846,189 to Sun
and U.S. Pat. No. 4,869,261 to Penaz, all relate to methods and devices
utilizing a technique known as photoplethysmography which is commercially
implemented in a device known as the FINAPRES system (Omeda).
The FINAPRES system uses a small inflatable air cuff containing an infrared
photoplethysmograph. The cuff is applied to one of the subject's fingers
or thumb, and the photoplethysmograph measures the absorption at a
wavelength specific for hemoglobin. The device first measures the
individual's mean arterial pressure, and then varies the cuff pressure
around the finger to maintain the transmural pressure at zero as
determined by the photoplethysmograph. The device tracks the
intra-arterial pressure wave by adjusting the cuff pressure to maintain
the optical absorption constant at all times.
There are several major disadvantages to this technique. The signal
amplitude detected by the photoplethysmograph is a function of the changes
in the diameter of the artery within the finger, and is determined by the
compliance characteristics of the artery. The device maintains this
amplitude at a constant value. This value, or set point, must correspond
to the point of zero transmural stress in order to determine the correct
pressure. During surgery for example, the device cannot differentiate
between changes in photoplethysmograph amplitude due to intra-arterial
pressure changes and those due to arterial wall compliance changes.
Consequently, the FINAPRES system cannot accurately respond to pressure
changes caused by changes in vasomotor tone. In addition, maintaining
continuous cuff pressure causes restriction of the circulation in the
finger being used, which is uncomfortable when maintained for extended
periods of time such as during surgery or during a stay in an intensive
care unit.
U.S. Pat. Nos. 4,669,485, 4,718,426, 4,718,427 and 4,718,428 all to Russel,
describe a device using a conventional blood pressure cuff applied to a
person's upper arm to sense an oscillometric signal. The subject's blood
pressure is obtained initially by the oscillometric technique, and then
changes in the oscillometric signal indicate changes from this initial
reference pressure.
There are two inherent limitations to this device. First, the use of a
large air bag as the sensing device provides a means for detecting the
fundamental and lower harmonics of the blood pressure signal (up to a few
Hertz), but also acts to attenuate many higher order harmonics containing
key information relating to blood pressure variations. Second, the use of
a cuff to detect the oscillometric signal creates a signal that is very
sensitive to patient movement. Since patient movement is often encountered
during surgery or in critical care situations, the device requires
frequent recalibration to be accurate.
U.S. Pat. Nos. 4,269,193, 4,799,491 and 4,802,488 to Eckerle, U.S. Pat. No.
4,423,738 to Newgard, and U.S. Pat. No. 5,165,416 to Shinoda et al., all
describe methods and devices for detecting the pressure wave in the
underlying artery of an individual using a technique known as the
tonometric technique.
These device and methods utilize a multi-element piezoresistive detector to
noninvasively detect the blood pressure wave at the radial artery. This
signal is then processed and changes in its amplitude are used to
interpret changes to the pressure values obtained using the conventional
oscillometric technique.
A major drawback to this technique lies in the method of interpreting
changes to the waveform signal. Reliance solely on amplitude changes is
misleading since the signal amplitude may increase or decrease with an
increase in blood pressure, etc. Secondly, it is dependent on the artery
being exactly flat, and variations in artery flatness can introduce
errors. It also assumes that the selected sensing element is small with
respect to the artery, and that it does not move from its position
centered over the artery. Thus, any movement such as that often
encountered in surgery or critical care situations will reduce the
accuracy of this device.
European Patent Document 0 443 267 A 1 to Smith, describes a technique for
monitoring changes in pulse transit time to provide a continuous,
noninvasive measure of blood pressure. This technique was developed by
Sentinel Monitoring, Inc., of Indianapolis, Ind., and uses a duplicity of
photometric detectors similar to those used with oximeters. Typically, one
detector is applied to the subject's ear lobe, and the other to a finger.
The detectors are used for determining changes in the arrival time of the
pulse at each of these sites, and to determine changes in local blood
volume. Following an initial calibration pressure measurement obtained
with a conventional blood pressure cuff, the Smith device adjusts these
pressures by interpreting changes in the pulse transit time and in the
optical density of the photoplethysmograph signal.
There are two disadvantages to the Smith technique. First, changes in pulse
transit time are very small along major arteries. As a result, small
errors caused by patient movement or noise render questionable data.
Second, small variations in photoplethysmographic waveform morphology or
detector noise can generate measurement errors greater than the
sensitivity of the technique to changes in blood pressure.
U.S. Pat. No. 4,960,128 to Gordon, et al., describes a method of
determining blood pressure by measuring a single harmonic of the
frequencies and displacements of the patient's arterial wall. In Gordon,
initial (absolute) blood pressure values arc obtained with a cuff and
stethoscope or via an intermittent automated cuff machine, and manually
entered into the device as initial reference values. A continuous detector
signal is supplied by a noninvasive detector attached to the patient's
skin above an artery. The detector signal is filtered, amplified and then
sampled. This time sampled detector data is then Fourier transformed into
the frequency domain and normalized.
As blood pressure changes, the reported frequencies and their relative
amplitudes change. A comparison is made between the fundamental frequency
of the present signal and the initial signal. For each shift in frequency
(+or -) of 1 Hz, the offset is adjusted correspondingly to yield a change
of 5 mm Hg. Thus, Gordon shows a device in which the patient's blood
pressure is determined based on the difference in position of the
fundamental frequency of the detector signal and initial signal.
The technique described by Gordon does not adequately account for the
plurality of factors that can reflect a change in blood pressure. There is
a multitude of waveshapes that can accompany a given set of blood pressure
values, and the Gordon technique is limited by its function of comparing
the frequency with the maximum amplitude of the current signal to that of
the initial signal to determine blood pressure.
There is thus a widely recognized need for, and it would be highly
advantageous to have, a system and method for accurately and noninvasively
monitoring continuous beat-to-beat blood pressure and other important
hemodynamic vascular parameters of a patient which is devoid of the above
limitations.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a system
for non-invasively monitoring at least one hemodynamic vascular parameter
of an individual, the system comprising (a) at least two infrared
detectors being positionable in a spaced apart configuration against a
region of a skin of the individual above at least one blood vessel, each
of said at least two infrared detectors being for individually collecting
infrared spectral data from said region of the skin, said infrared
spectral data | | |
