U.S. patent application number 11/707095 was filed with the patent office on 2008-08-21 for method and device for measuring parameters of cardiac function.
Invention is credited to Xuefeng Cheng.
Application Number | 20080200784 11/707095 |
Document ID | / |
Family ID | 39689584 |
Filed Date | 2008-08-21 |
United States Patent
Application |
20080200784 |
Kind Code |
A1 |
Cheng; Xuefeng |
August 21, 2008 |
Method and device for measuring parameters of cardiac function
Abstract
A device for non-invasively measuring parameters of a cardiac
blood vessel in a patient is provided. The device comprises at
least one light source that emits light in the 400 nm to 1000 nm
wavelength range and at least one photodetector adapted to receive
light emitted by the light source, which light is reflected from or
transmitted through tissue of the patient, the output of said
photodetector correlating with a parameter of the blood vessel. The
device also includes a probe which permits delivery of light from
the light source to an external tissue site on the patient in the
proximity of a cardiac blood vessel and permits the photodetector
to receive light originating from the light source which has been
reflected from or transmitted through tissue at the patient
site.
Inventors: |
Cheng; Xuefeng; (Waterloo,
CA) |
Correspondence
Address: |
VALENTINE A COTTRILL;SUSAN TANDAN
50 QUEEN STREET NORTH, STE. 1020, P.O. BOX 2248
KITCHENER
ON
N2H6M2
omitted
|
Family ID: |
39689584 |
Appl. No.: |
11/707095 |
Filed: |
February 16, 2007 |
Current U.S.
Class: |
600/322 ;
600/323; 600/500 |
Current CPC
Class: |
A61B 2562/043 20130101;
A61B 5/021 20130101; A61B 5/1455 20130101; A61B 5/6833 20130101;
A61B 5/14552 20130101; A61B 5/4884 20130101; A61B 5/0205 20130101;
A61B 5/029 20130101; A61B 2562/046 20130101; A61B 5/02116 20130101;
A61B 2562/0233 20130101 |
Class at
Publication: |
600/322 ;
600/323; 600/500 |
International
Class: |
A61B 5/00 20060101
A61B005/00; A61B 5/02 20060101 A61B005/02 |
Claims
1. A device for non-invasively measuring at least one parameter of
a cardiac blood vessel in a patient comprising: at least one light
source that emits light in the 400 nm to 1000 nm wavelength range;
at least one photodetector adapted to receive light emitted by the
light source, wherein said light is reflected from or transmitted
through tissue of the patient, the output of said photodetector
correlating with a parameter of the blood vessel; and a probe which
permits delivery of light from the light source to an external
tissue site on the patient in the proximity of a cardiac blood
vessel and permits transfer of light from said light source which
is reflected from or transmitted through said patient site.
2. A device as defined in claim 1, additionally comprising a
signal-producing means in communication with the photodetector,
said signal-producing means being capable of translating the light
received from the photodetector into a recordable signal to
generate a wave form of the blood vessel.
3. A device as defined in claim 1, adapted to emit and receive
light of at least two different wavelengths in the 400 nm to 1000
nm wavelength range.
4. A device as defined in claim 1, comprising at least two
photodetectors, each of said photodetectors adapted to receive
light at a distinct wavelength, 5. A method for measuring the
waveform of a cardiac blood vessel in a patient comprising the
steps of: directing a beam of light having a wavelength in the
range of 400 nm to 1000 nm at an external tissue site on the
patient that is in the proximity of the blood vessel; detecting
light reflected from the tissue site or transmitted through the
tissue site; and translating the detected light into an output
signal against time to generate a waveform for the selected blood
vessel.
6. A method as defined in claim 5, comprising directing more than
one beam of light at the same or different tissue site on a
patient, each beam having a different wavelength in the range of
400 nm to 1000 nm.
7. A method as defined in claim 6, wherein the light of each
wavelength is detected to generate a waveform for two different
blood vessels.
8. A method for measuring the blood content of a chromophore in a
patient comprising: directing light having at least first and
second selected wavelengths at an external tissue site on the
patient that is in the proximity of a cardiac blood vessel, wherein
said selected wavelengths are based on the absorption
characteristics of the chromophore; detecting light reflected from
the tissue or transmitted through the tissue at the selected
wavelengths; and translating the detected light into an output
signal against time to generate a waveform for each selected
wavelength in order to determine the blood content of said
chromophore.
9. A device as defined in claim 1, wherein the parameter is
selected from the group consisting of blood vessel pulse, blood
vessel volume, blood vessel oxygenation, blood vessel flow and
blood vessel content.
10. A device as defined in claim 1, wherein the device produces a
waveform of the blood vessel.
Description
FIELD OF THE INVENTION
[0001] The present invention is related to techniques for
monitoring vital functions of the human body, including cardiac
functions such as cardiac output and central venous blood
oxygenation. It relates, in particular, to an optical method and
device for the non-invasive and continuous monitoring of cardiac
parameters such as blood flow, blood volume and blood oxygen
saturation.
BACKGROUND OF THE INVENTION
[0002] The evaluation of jugular venous pulse has been an integral
part of cardiovascular examination and has important clinical
diagnostic values [1-2]. Jugular venous pulse is produced by the
changes in blood flow and pressure in central veins caused by right
atrial and ventricular filling and contraction. The two main
objectives of the bedside examination of jugular vein pulse include
the estimation of central venous pressure and the inspection of the
waveform. Because of its more direct route to the right atrium, the
right internal jugular vein is superior for the purpose. Based upon
these measurements, physicians can access hemodynamic events in the
right atrium and thus diagnose heart diseases and abnormalities.
For example, the most common cause of elevated jugular venous
pressure is an increase in right ventricular pressure such as
occurs in patients with pulmonary stenosis, pulmonary hypertension,
or right ventricular failure secondary to right ventricular
infarction. The venous pressure also is elevated when obstruction
to right ventricular inflow occurs, such as with tricuspid stenosis
or right atrial myxoma, or when constructive pericardial disease
impedes right ventricular inflow. It may also result from vena
caval obstruction and, at times, an increase blood volume. Patients
with obstructive pulmonary disease may have an elevated venous
pressure only during expiration.
[0003] The conventional technique for measuring venous pulse and
waveform has been described in the literature [3]. The patient is
examined at the optimum degree of trunk elevation for visualization
of venous pulsations. The venous pressure is measured by a ruler as
the vertical distance from the top of the oscillating venous
column, to the level of the sternal angle plus vertical distance to
the right atrium Due to the fact that the venous pulse is in
generally very small, and due to complications with patients, this
method is challenging for physicians to use and provides
approximate values only.
[0004] Cardiac output is defined as the volume of blood circulated
per minute. It is equal to the heart rate multiplied by the stroke
volume (the amount ejected by the heart with each contraction).
Cardiac output is of central importance in the monitoring of
cardiovascular health [4]. Accurate clinical assessment of
circulatory status is particularly desirable in critically ill
patients in the ICU and patients undergoing cardiac, thoracic, or
vascular interventions, and has proven valuable in long term
follow-up of outpatient therapies. As a patient's hemodynamic
status may change rapidly, continuous monitoring of cardiac output
will provide information that allows rapid adjustment of therapy.
Measurements of cardiac output and blood pressure can also be used
to calculate peripheral resistance.
[0005] Jansen (J. R. C. Jansen, "Novel methods of
invasive/non-invasive cardiac output monitoring", Abstracts of the
7th annual meeting of the European Society for Intravenous
Anesthesia, Lisbon 2004) describes eight desirable characteristics
for cardiac output monitoring techniques; accuracy, reproducibility
or precision, fast response time, operator independency, ease of
use, continuous use, cost effectiveness, and no increased mortality
and morbidity.
[0006] Pulmonary artery catheter (PAC) thermodilution method is
generally accepted as the clinical standard for monitoring cardiac
output, to which all other methods are compared as discussed by
Conway and Lund-Johansen [6]. As this technology is highly
invasive, complicated, and expensive, many new methods have been
developed in an attempt to replace it, but none have so far gained
acceptance. A recent review of the various techniques for measuring
cardiac output is given in Linton and Gilon [5]. This article lists
both non/minimally invasive and invasive methods and compares the
advantages and disadvantages of each. A brief description of some
of these techniques follows.
[0007] Indicator dilution techniques There are several indicator
dilution techniques including transpulmonary thermodilution (also
known as PiCCO technology, Pulsion Medical Technologies of Munich,
Germany), transpulmonary lithium dilution method (LiDCO Group plc
of London, UK), PAC based thermo-dilution and other methods
(Vigilance, Baxter; Opti-Q, Abbott; and TruCCOMS, AorTech).
Application of such techniques assumes three major conditions,
namely, complete mixing of blood and indicator, no loss of
indicator between place of injection and place of detection, and
constant blood flow. The errors associated with indicator dilution
techniques are primarily related to the violation of these
conditions, as discussed by Lund-Johansen [7-8].
[0008] Fick principle. The direct oxygen Fick approach is currently
the standard reference technique for cardiac output measurement as
discussed by Keinanen et al [9-10]. It is generally considered the
most accurate method currently available. The NICO (Novametrix)
system is a non-invasive device that applies Fick's principle and
relies solely on airway gas measurement as described by Botero et
al [11]. This method shows a lack of agreement between
thermodilution and CO.sub.2-rebreathing cardiac output as described
in Nielsson et al [12], due to unknown ventilation/perfusion
inequality in patients.
[0009] Bio-Impedance and conduction techniques. The bio-impedance
method was developed as a simple, low-cost method that gives
information about the cardiovascular system and/or (de)-hydration
status of the body in a non-invasive way. Over the years, a
diversity of thoracic impedance measurement systems have also been
developed. These systems determine CO on a beat-to-beat time basis.
Studies have been reported with mostly poor results, but in some
exceptional cases, there was good correlation with a reference
method. Many of these studies refer to the poor physical principles
of the thoracic impedance method as described in Patterson
"Fundamentals of impedance cardiography", IEEE Engineering in
Medicine and Biology 1989; 35 to explain the discrepancies.
[0010] Echo-Doppler ultrasound. This technique uses ultrasound and
the Doppler Effect to measure cardiac output. The blood velocity
through the aorta causes a `Doppler shift` in the frequency of the
returning ultrasound waves. Echo-Doppler probes positioned inside
the esophagus with their echo window on the thoracic aorta may be
used for measuring aortic flow velocity, as discussed by Schinidlin
et al [13]. Aortic cross sectional area is assumed in devices such
as the CardioQ, made by Deltex Medical PLC, Chichester, UK, or
measured simultaneously as, for example, in the HemoSonic device
made by Arrow International. With these minimally invasive
techniques what is measured is aortic blood flow, not cardiac
output. A fixed relationship between aortic blood flow and cardiac
output is assumed. Echo-Doppler ultrasound requires an above
average level of skill on the part of the operator of the
ultrasound machine to get accurate reliable results.
[0011] Arterial pulse contour analysis. The estimation of cardiac
output based on pulse contour analysis is an indirect method, since
cardiac output is not measured directly but is computed from a
pressure pulsation on the basis of a criterion or model [14-17].
Three pulse contour methods are currently available; PiCCO
(Pulsion), PulseCO (LiDCO) and Modelflow (TNO/BMI). All three of
these pulse contour methods use an invasively measured arterial
blood pressure and they need to be calibrated. PiCCO is calibrated
by transpulmonary thermodilution, LIDCO by transpulmonary lithium
dilution and Modelflow by the mean of 3 or 4 conventional
thermodilution measurements equally spread over the ventilatory
cycle.
[0012] Near infrared spectroscopy has been used to non-invasively
measure various physiological properties in animal and human
subjects. The basic principle underlying near infrared spectroscopy
is that a physiological medium such as tissues includes a variety
of light-absorbing (chromophores) and light-scattering substances
which can interact with transmitted low energy near infrared
photons. For example, deoxygenated and oxygenated hemoglobins in
human blood are the most dominant chromophores in the spectrum
range of 400 nm to 1000 nm. Therefore, diff-use optical
spectroscopy has been applied to non-invasively measure oxygen
levels in the physiological medium in terms of tissue hemoglobin
oxygen saturation. Technical background for diffuse optical
spectroscopy has been discussed in, e.g., Neuman, M. R., Pulse
Oximetry: Physical Principles, Technical Realization and Present
Limitations,@ Adv. Exp. Med. Biol., vol. 220, p. 135-144, 1987 and
Severinghaus, J. W., History and Recent Developments in Pulse
Oximetry,@ Scan. J. Clin. and Lab. Investigations, vol. 53, p.
105-111, 1993.
[0013] Because of the highly scattering nature of tissue to the
visible and near infrared light (400 nm-1000 nm), it is difficult
to apply diffuse optical spectroscopy non-invasively to select
blood vessels within a tissue to calculate blood oxygenation. Thus,
diff-use optical spectroscopy has only been used to measure the
combined or average oxygenation of blood from arteries, veins, and
capillaries within a tissue medium. However, in many clinical
applications, it is desirable to know the blood oxygenation of
particular blood vessels. To do so, various invasive methods have
been developed which involve the use of catheters that must be
inserted into a targeted blood vessel to make the measurement.
[0014] None of the above-mentioned techniques of measuring cardiac
output combines all of the eight "Jansen" criteria mentioned above
and, thus, none can displace the conventional thermodilution
technique as described by Jansen et al [18]. Although highly
invasive, complicated and expensive, the conventional
thermodilution method remains the method of choice for measuring
cardiac output. Given the foregoing, it would be highly desirable
to develop a non-invasive method for real-time monitoring of
cardiac output in a clinical setting which is accurate, reliable,
cost effective and easy to use.
SUMMARY OF THE INVENTION
[0015] The present invention provides a device and method by which
cardiac output can be continuously monitored in a non-invasive
manner by the optical measure of venous blood flow and blood
content including oxygenation.
[0016] In one aspect of the invention, a device for measuring the
at least one parameter of a cardiac blood vessel in a patient is
provided. The device comprises:
[0017] at least one light source that emits light in the 400 nm to
1000 nm wavelength range;
[0018] at least one photodetector adapted to receive light emitted
by the light source, wherein said light is reflected from or
transmitted through tissue of the patient, the output of said
photodetector correlating with a parameter of the blood vessel;
and
[0019] a probe which permits delivery of light from the light
source to an external tissue site on the patient in the proximity
of a cardiac blood vessel and permits the photodetector to receive
light originating from the light source which has been reflected
from or transmitted through said patient site.
[0020] In another aspect of the invention, a method of generating a
waveform of a cardiac blood vessel in a patient is provided. The
method comprises the steps of:
[0021] directing a beam of light having a wavelength in the range
of 400 nm to 1000 nm at an external tissue site in the proximity of
the cardiac blood vessel of the patient;
[0022] detecting light reflected from the tissue or transmitted
through the tissue; and
translating the detected light into an output signal against time
to generate a waveform for the selected blood vessel.
[0023] In another aspect of the present invention, there is
provided a method of measuring blood content of a chromophore in a
patient. The method comprises the steps of:
[0024] directing light having at least first and second selected
wavelengths at an external tissue site on the patient that is in
the proximity of a cardiac blood vessel, wherein said selected
wavelengths are based on the absorption characteristics of the
chromophore;
[0025] detecting light reflected from the tissue or transmitted
through the tissue at the selected wavelengths; and
[0026] translating the detected light into an output signal against
time to generate a waveform for each selected wavelength in order
to determine the blood content of said chromophore.
[0027] The waveform is the time varying component of optical signal
associated with cardiac activities, which can be also translated
into dynamic information of such as blood flow, blood volume and
blood content within the vessel or physical displacement of blood
vessel.
[0028] These and other aspects of the present invention will become
apparent by reference to the detailed description that follows, and
the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 illustrates a top view of a device for monitoring
cardiac output in accordance with an aspect of the invention and
placement of the device relative to cardiac vessels;
[0030] FIG. 2 illustrates a side view of a device as in FIG. 1;
[0031] FIG. 3 illustrates a system incorporating the device of FIG.
1;
[0032] FIG. 4 illustrates a signal or waveform produced using a
device as in FIG. 1;
[0033] FIG. 5(A and B) is a block diagram of a system incorporating
a device as in FIG. 1;
[0034] FIG. 6 illustrates probes (A, B and C) for use in the
device;
[0035] FIG. 7 illustrates a top view of embodiments of the
invention (A, B) comprising multiple light sources and
photodetectors;
[0036] FIG. 8 is a block diagram of a system incorporating a device
as in FIG. 7;
[0037] FIG. 9 illustrates a top view of embodiments of the
invention (A, B, C) comprising multiple photodetectors per light
source;
[0038] FIG. 10 illustrates a dual signal (waveform) generated by an
embodiment as in FIG. 9;
[0039] FIG. 11 illustrates a top view of a cardiac monitoring
device according to an embodiment of the invention comprising
multiple sensor patches; and
[0040] FIG. 12 illustrates a waveform obtained using a device in
accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0041] A device 10 for measuring a parameter of cardiac function in
a patient is provided as shown in FIG. 1, comprising a light source
20 that emits light in the 400 nm to 1000 nm wavelength range, e.g.
visible and infra-red light, a photodetector 30 adapted to receive
transmitted and reflected light (as shown in FIG. 2) and a patch
probe 28 for placement on a patient at an external site in the
vicinity of a cardiac blood vessel (as shown in FIG. 1) which
functions as the interface of the device between light source
20/photodetector 30 and a selected external patient site. Thus, the
probe 28 permits delivery of light emitted by the light source 20
to the selected patient site and transfer of light reflected from
or transmitted through the patient site to the photodetector 30. To
generate a visual signal, the device 10 may additionally comprise a
signal-producing means 40 (FIG. 3) which communicates with the
photodetector 30 to translate light received by the photodetector
30 into a recordable visual signal or waveform of the cardiac
vessel, or central venous pulse, and time course plot of the
cardiac parameter (FIG. 4).
[0042] The light source 20 may be any suitable light source such as
a laser diode (e.g. RLT7605G, 760 nm, 5 mW, sm, 9.0 mmh, or
RLT8510MG, 850 nm, 100 mW, sm, 5.6 mm), a light emitting diode
(LED) or a broadband light source emitting a selected wavelength in
the range of 400 nm to 1000 nm. In an embodiment of the invention,
the light source 20 emits light having a wavelength in the range of
780 nm and 850 nm. The light source 20 is powered by an appropriate
power supply 18 such as a 12V DC power supply. Light from the light
source 20 is directed to at least one external tissue site on the
patient that is within close proximity to a cardiac blood vessel,
such as the internal jugular vein, the external jugular vein and
the carotid artery, while the internal jugular vein is preferred.
The neck, for example, represents a suitable site for monitoring a
cardiac parameter.
[0043] As shown in FIG. 5A/5B, in one embodiment, light from the
light source 20 may be directed or focussed by an optical lens 22
into a transmitting means 24, such as transmitting optical fiber
bundles, for transmission to the selected patient site. Receiving
means 26, such as optical fiber bundles 26, may also be used to
receive light that is reflected/transmitted from the patient site
and convey this light to photodetector 30 (FIG. 5A). As one of
skill in the art will appreciate, each fibre optic bundle will
incorporate fibres manufactured of material appropriate for the
transmission of the wave-length of the light emitted from the light
source 20. For example, if the light source 20 emits in the visible
wavelength range, both multiple mode plastic and glass optical
fibres may be used. The number and diameter of the fibers in the
fiber optic bundle is optimized empirically to provide the highest
signal to noise ratio in a given application. In this embodiment,
the transmitting and receiving optical fiber bundles 24, 26 are set
in the patch probe 28, at distinct spaced sites or may be combined
together at a single site. Optical mirrors 29 may be utilized to
direct or reflect light from the transmitting fiber bundle 24 into
the tissue at the selected patient site, and to direct reflected or
transmitted light from the patient site into the receiving fiber
bundle 26 (FIG. 6A).
[0044] In an alternative embodiment, the light source 20 and
photodetector 30 may be set directly in the patch probe 28
obviating the need for optical fibers as shown in FIG. 6B. In yet
another embodiment, a combination of the foregoing embodiments may
be utilized in which the light source 20 is set directly in the
probe 28 to deliver light to the patient site, while the
reflected/transmitted light is received by optical fibers 26 for
delivery to the photodetector 30. A converse embodiment may also be
used in which the probe 28 comprises transmitting optical fibers 24
to deliver light from the light source, and a photodetector 30 set
directly in the probe 28 (FIG. 6C).
[0045] The light source 20 or transmitting optical fibers 24 may be
set in the same patch probe 28 as the photodetector 30 or receiving
optical fibers 26, or in a separate patch probe 28 for placement at
a distinct site on the patient that is within a suitable distance
from the photodetector 30 or receiving optical fibers 26 to permit
detection of reflected/transmitted light. The distance between the
component delivering light to the patient site (light source or
transmitting optical fibers) and the component receiving light from
the patient site (photodetector or receiving optical fibers) may
vary depending on the nature of each of the components, while a
typical distance is generally between 2 and 4 cm, for example, 3
cm.
[0046] The patch probe 28 may be made out of any material suitable
to support the electronic/optical components it houses, e.g. light
source, photodetector, optical fibers or mirrors, and which is
compatible for placement on the skin. An example of one such
suitable material is medical rubber. The patch 28 may be held in
position manually, may be held in position by adhesives (one side
of the patch may be coated with a material that is adhesive to skin
such as a hydro gel adhesive) or may be adapted to be held in place
with straps at either end thereof that can be tied. Opposing ends
of the band may also include an adhesive material such as Velcro to
facilitate their attachment and hold the device in place.
[0047] The photodetector 30 translates received
reflected/transmitted light into a recordable output such as
current or voltage. An example of a suitable photodetector 30 for
use in the present device is a silicon photo diode (e.g. Hamamatsu
S8553). Condensor lenses may be incorporated, if required, to
refocus the reflected or transmitted beam of light. The device 10
may be connected to a signal producing means 40 to provide a visual
output. The signal producing means 40 may comprise a microprocessor
(e.g. digital signal processor, Texas Instruments) or digital
acquisition board 42 to digitize the signal from the photodetector
30, and a display unit 44, such as a monitor, which is connected to
the microprocessor 42 (FIG. 5), to display the signal as a
waveform. For convenience, the monitor may be portable, and battery
operated. The absorbance values collected at regular
user-determined intervals, for example, 10 data points/mm, are
stored as a spreadsheet associated with a cardiac parameter or
cardiac output. The display unit 44 functions in real-time to
display the selected blood vessel waveform against time which can
be used as described below to calculate the cardiac parameter or
cardiac output.
[0048] A sample display of the signal or waveform obtained using
the present device is shown in FIG. 4. As can be seen, there is a
time course variation in the signal detected by the photodetector
30 that results from a selected blood vessel pulse, changes in the
blood volume and content (such as oxygen saturation) inside the
blood vessel. The blood volume and content in the selected blood
vessel affects the absorption of light, thereby resulting in a
signal with varying amplitude. For example, as the jugular vein
pulse increases and decreases the blood volume in the jugular vein,
the amplitude of the detected optical signal will decrease and
increase. The time course plot of the amplitude of the recorded
light reflects the waveform of the jugular vein pulse.
[0049] In another embodiment of the present invention, a device
100, useful to measure blood content, such as the blood oxygen
saturation of central venous blood, is provided. As the jugular
vein, especially the right internal jugular vein, is directly
connected to the superior vena cava as shown in FIG. 1, the jugular
vein waveform is representative of the parameters of central venous
blood.
[0050] For this utility, the device 100, as shown in FIG. 7,
comprises at least two light sources 120, each emitting light of a
different wavelength within the range of 400 nm to 1000 nm. The
device also comprises a photodetector 130 for each light source 120
to receive the transmitted or reflected light at each wavelength.
As set out above, each light transmitting component (e.g. light
source 120 or transmitting optical fibers) and light receiving
component (e.g. photodetector 130 or receiving optical fibers) is
set in a patch 128, and may be arranged as shown in FIG. 7A or 7B;
however, as one of skill in the art will appreciate, alternative
arrangements of the light transmitting components and light
receiving components exist which will not affect the function of
the device 100. For example, the device 100 may comprise multiple
patches 128, each of which includes a light transmitting component
and a light receiving component. Alternatively, the device 100 may
comprise a single patch 128 including multiple light transmitting
components and light receiving components. In another alternative,
the device 100 may comprise a first patch 128 with one or more
light tranmitting components and light receiving components, and a
second patch with one or more light transmitting components and
corresponding light receiving components. As set out above,
regardless of the number of patches and arrangements thereof, the
device will also include the circuitry necessary to power the
device, and other electronic and/or optical components necessary
for its function as previously described.
[0051] The time course variation in the detected signal associated
with a cardiac vessel pulse at different wavelengths may be used to
calculate the blood content, such as blood oxygen saturation, and
other parameters associated with the cardiac vessel pulse. There
are various ways to calculate blood oxygen saturation as a function
of variations in the detected signal caused by cardiac vessel pulse
at multiple light wavelengths, e.g. at 780 nm and 850 nm. As one of
skill in the art will appreciate, the selected wavelengths for use
in blood content determination will vary with the blood entity
being determined. This function can be determined through
experiment or derived through photon diffusion equations, photon
transportation equations or Modified Beer Lambert's Law.
Modified Beer Lambert's Law
[0052] The detected signal can be expressed as:
I .lamda. 1 = I 0 , .lamda. 1 - B [ Hb , .lamda. 1 ( C Hb + .DELTA.
C Hb ) + HbO , .lamda. 1 ( C HbO + .DELTA. C HbO ) ] L + A ( 1 )
##EQU00001##
where: [0053] I.sub.80 .sub.1 is the signal detected by the
photodetector at wavelength .lamda..sub.1,, [0054]
I.sub.0,.lamda..sub.1 is the signal from the light source at
wavelength, .lamda..sub.1,, [0055] C.sub.Hb, C.sub.HbO are the
concentrations of deoxygenated and oxygenated hemoglobin of steady
tissue medium blood; [0056] .DELTA.C.sub.Hb, .DELTA.C.sub.HbO are
the changes in the concentrations of deoxygenated and oxygenated
hemoglobin caused by the jugular vein pulse; [0057]
.epsilon..sub.Hb,.lamda..sub.1, .epsilon..sub.HbO,.lamda..sub.1 are
the absorption properties of deoxygenated and oxygenated hemoglobin
at wavelength .lamda..sub.1 for the purposes of calculating blood
oxygen saturation. Blood saturation of other chromophores can also
be calculated by substituting into the equation the appropriate
extinction coefficients (.epsilon.) for the selected chromophore
including, for example, water, cytochromes such as cytochrome
oxides, and cholesterol; and [0058] A, B are constants determined
by boundary conditions.
[0059] The relative change in signal from the signal emitted from
the light source to the signal detected by the photodetector which
is caused by the jugular vein pulse is represented for a first
wavelength by:
.DELTA. I .lamda. 1 = - B [ Hb , .lamda. 1 ( .DELTA. C Hb ) + HbO ,
.lamda. 1 ( .DELTA. C HbO ) ] L ; ( 2 ) ##EQU00002##
or as
OD .lamda. 1 = ln ( .DELTA. I .lamda. 1 ) = - B ( Hb , .lamda. 1
.DELTA. C Hb + HbO , .lamda. 1 .DELTA. C HbO ) . ( 3 )
##EQU00003##
[0060] Similarly, the change in signal between emitted and detected
for a second light wavelength is represented by:
OD.sub..lamda..sub.2=ln(.DELTA.I.sub..lamda..sub.2)=-B(.epsilon..sub.Hb,-
.lamda..sub.2.DELTA.C.sub.Hb+.epsilon..sub.HbO,.lamda..sub.2.DELTA.C.sub.H-
bO) (4)
[0061] Blood oxygenation derived from jugular vein pulse is then
determined using the following equation:
S jv O 2 = .DELTA. C HbO .DELTA. C Hb + .DELTA. C HbO = Hb ,
.lamda. 1 OD .lamda. 2 - Hb , .lamda. 2 OD .lamda. 2 ( Hb , .lamda.
1 - HbO , .lamda. 2 ) OD .lamda. 2 - ( Hb , .lamda. 2 - HbO ,
.lamda. 2 ) OD .lamda. 1 ( 5 ) ##EQU00004##
[0062] In use, the patch 28 of device 10 comprising light source
(s) 20 and photodetector (s) 30 is generally placed on the neck of
the patient at a site near a selected blood vessel, for example,
the internal jugular vein. It is desirable for the patient to be
lying down at about a 30 degree incline. The patient maintains
regular breathing during the process of measuring the pulse of the
blood vessel. Light from the light source 20 is either reflected
off of, or transmitted through, the target site on the patient's
neck, and detected by the photodetector 30. The photodetector 30
translates the detected light into an output signal that may be
digitized for expression as amplitude against time to result in a
waveform of the selected blood vessel pulse. The amplitude of
signals obtained using different wavelengths is used according to
Lambert's law as above to determine blood oxygenation.
[0063] In another embodiment, illustrated in FIG. 9(A-C), a device
200 is provided comprising 1 or more light sources 220, each
emitting selected wavelengths of light in the 400 nm to 1000 nm
range. Each light source 220 is coupled with at least two
photodetectors 230 each adapted to receive light emitted at a given
frequency. As discussed above, the circuitry required for the
function of the device 200 is included in the device such as power
supply 18, as well as the necessary electronic/optical
components.
[0064] The device 200 is useful to simultaneously measure multiple
cardiac blood vessel pulses, such as jugular venous pulse as well
as carotid arterial pulse, thereby generating a dual waveform as
illustrated in FIG. 10, and thus, has utility to simultaneously
measure arterial blood oxygenation, S.sub.aO.sub.2, in addition to
central venous oxygenation, S.sub.jvO.sub.2, as described above. As
one of skill in the art will appreciate, in the case of multiple
light sources 220, each light source is turned on in sequence, and
the amplitude of light emitted from the light source(s) is
modulated at a selected frequency, such as 10 kHz. Light emitted by
a single light source 220 can be sequentially modulated at two
alternating frequencies. The output from the photodetectors is
filtered at a frequency selected to correlate with a given
frequency emitted from a light source.
[0065] In another embodiment, cardiac output may be measured or
monitored. As the jugular vein pulse represents central venous
blood and correlates well with mixed venous blood, the trend of
cardiac output can be calculated through Fick's Law as follows:
COI = OCR S o O 2 - S v O 2 ( 6 ) ##EQU00005## [0066] where COI is
the cardiac output index which is the cardiac output (CO) per unit
body surface; [0067] OCR is the oxygen consumption rate which is
oxygen consumption (OC) per unit body surface; [0068]
S.sub.aO.sub.2 is the arterial blood oxygen saturation; and [0069]
S.sub.jvO.sub.2 is the venous blood oxygen saturation.
[0069] Or: CO = OC S a O 2 - S jv O 2 ( 7 ) ##EQU00006##
[0070] As the oxygen consumption or oxygen consumption rate are
constant during many clinical procedures, the trend of cardiac
output index or cardiac output can be reliably monitored.
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* * * * *
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