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Claims  |
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What is claimed is:
1. A method for noninvasively measuring the value of cardiac power index as
a descriptor of performance of a heart comprising the steps of:
noninvasively measuring the left ventricular pressure of the heart with
reference to time; the step of measuring the left ventricular pressure
comprising the steps of measuring the arrival times of cardiac pressure
pulses at a given arterial site displaced form the heart at a plurality of
pressure values;
noninvasively measure the left ventricular volume of the heart with
reference to time;
determining the work performed by the left ventricle as the product of the
left ventricular pressure and the left ventricular volume as a function of
time;
determining the power of the left ventricle as the time derivative of said
product; and
determining the slope fo the time derivative as it rises during the
interval from the onset of systole to the moment of maximum power, thereby
to provide a value of the cardiac power index.
2. A method according to claim 1 in which the step of measuring the left
ventricular pressure also comprises the step of:
concentrating the largest number of pressure measurements in the interval
during the early ejection phase of the left ventricle.
3. A method according to claim 2 in which the step of measuring the left
ventricular pressure also comprises the step of measuring the arrival
times of cardiac pressure pulses at the given site during the time period
during which the left ventricular pressure rises from 100% to 125% of the
end-diastolic value.
4. A method according to claim 1 and also comprising the step of displaying
real-time electrocardiogram and blood pressure wave forms on a
continuously updated basis.
5. A method according to claim 4 and also comprising the steps of
displaying simultaneously and together with said electrocardiogram and
pressure wave forms, left ventricle pressure and corresponding left
ventricular volumetric values.
6. A method according to claim 1 including the step of measuring, during
one or more cardiac cycles, the arrival time for a selected occlusive
pressure, and storing the measured times.
7. A method according to claim 1 and wherein the step of measuring the time
of arrival includes the step of rejecting time values having unacceptable
variance.
8. A method according to claim 1 and wherein the step of measuring the time
of arrival also includes the step of statistical averaging of several
acceptable time measurements to reduce the effects of beat-to-beat
variance, artifactual signals and noise.
9. A method according to claim 1 and wherein said step of measuring left
ventricular volume includes the step of taking at least one measurement
within 15 msec of QRS.
10. A method according to claim 9 and wherein said step of measuring left
ventricular volume includes the step of carrying out multiple volume
measurements within 40 msec of each other.
11. A method according to claim 10 including steps of measuring the
systolic and diastolic blood pressure.
12. A method according to claim 1 and also comprising the step of
calculating the cardiac power index as the slope of the best least squares
regression fit to an entire set of instantaneous power values up to a
maximum power point, excluding points whose values lie outside the range
of variance that is commensurate with the other points.
13. Apparatus for noninvasively measuring the value of cardiac power index
as a descriptor of performance of a heart comprising:
means for noninvasively measuring the left ventricular pressure of the
heart with respect to time, including means for measuring the arrival
times of cardiac pressure pulses at a given arterial site displaced from
the heart;
means for noninvasively measuring the left ventricular volume of the heart
with respect to time;
means for determining the work performed by the left ventricle as the
product of the left ventricular pressure and the left ventricular volume
as a function fo time;
means for determining the power of the left ventricle as the time
derivative of said product; and
means for determining the slope of the time derivative as it rises from the
onset of systole to the time of maximum power of the left ventricle,
thereby to provide a value of the cardiac power index.
14. Apparatus according to claim 13 in which the means for measuring the
left ventricular pressure also comprises means for concentrating the
largest number of pressure measurements in the interval during the early
ejection phase.
15. Apparatus according to claim 14 in which the means for measuring the
left ventricular pressure also comprises means for measuring the arrival
times of cardiac pressure pulses at the given site during the time period
during which the left ventricular pressure rises form 100% to 125% of the
end-diastolic value.
16. Apparatus according to claim 14 and also comprising means for
displaying real-time electrocardiogram and blood pressure wave forms on a
continuously updated basis.
17. Apparatus according to claim 16 including means for displaying,
simultaneously and together with said electrocardiogram and pressure wave
forms, left ventricle pressure and corresponding left ventricular
volumetric values.
18. Apparatus according to claim 17 and further comprising means for
measuring, during at least one cardiac cycle the arrival time for a
selected occlusive pressure, and for storage of the measured ties.
19. Apparatus according to claim 18 and wherein the means for measuring the
time of arrival includes means for rejecting time values having
unacceptable variance.
20. Apparatus according to claim 18 and wherein the means for measuring the
time of arrival also includes means for statistical averaging of several
acceptable time measurements to reduce the effects of beat-to-beat
variance, artifactual signals and noise.
21. Apparatus according to claim 20 and wherein said means for measuring
left ventricular volume includes means for taking at least one measurement
within 15 msec of QRS.
22. Apparatus according to claim 21 and wherein said means for measuring
left ventricular volume includes means for carrying th systolic and
diastolic blood pressure.
23. Apparatus according to claim 20 and further including means for
measuring the systolic and diastolic blood pressure.
24. Apparatus according to claim 23 and also comprising means for
calculating the cardiac power index as the slope of the best least squares
regression fit to an entire set of instantaneous power values up to a
maximum power point, excluding points whose variance is not commensurate
with the other points.
25. Apparatus according to claim 13 wherein said means for noninvasively
measuring left ventricular pressure includes a pulse wave sensor and a
pulse wave processor operative to reject motion artifact in the
measurement of left ventricular pressure.
26. Apparatus according to claim 25 and wherein said means for detecting
the arrival fo the cardiac pressure wave at a given site is a Doppler
ultrasound arterial wall motion sensor.
27. Apparatus according to claim 25, wherein said means for detecting the
arrival fo the cardiac pressure wave at a given arterial site is a Doppler
ultrasound blood flow sensor.
28. Apparatus according to claim 27 characterized by means for rejecting
motion artifact in the measurement of left ventricular pressure, said
means comprising a Doppler sensor holder and means for rejecting low
frequencies from the Doppler audio shift spectrum.
29. Apparatus according to claim 27, wherein said Doppler ultrasound sensor
is held by an armband mount comprising an adjustable sensor fixed to an
adjustable attachment strap.
30. Apparatus according to claim 27, wherein said Doppler ultrasound sensor
is formed as a flat package with Doppler crystals mounted so as to provide
fixed angle of illumination of about 30.degree. to horizontal.
31. Apparatus according to claim 25, wherein said pulse wave processor
includes a high-pass filter separating the high frequencies form the audio
signal and a RMS-amplitude-to-DC-converter measuring the power of the high
frequency spectrum by converting the total RMS value into a proportional
DC voltage.
32. A method for reliably measuring performance of a heart under resting
and/or exercise stress conditions to enable measurement of a cardiac power
index, the method comprising the steps of
noninvasively measuring, through at least a portion of each of a selected
number of cardiac cycles, the left ventricular pressure of the heart,
including measuring at a plurality of pressure values the times of arrival
of cardiac pressure pulses at a given arterial site displaced from the
heart,
noninvasively measuring, through corresponding portions of a corresponding
number of cardiac cycles, the left ventricular volume of the heart,
determining the product of the left ventricular pressure and left
ventricular volume as a function of time, thereby to determine the work
performed by the left ventricle,
determining the time derivative of said product, thereby to determine the
left ventricular power, and
determining the slope of the time derivative as it rises during the
interval from the onset of the systole to the moment of maximum power,
thereby to provide a value of the cardiac power index for that heart.
33. Apparatus for reliably measuring performance of a heart under resting
and/or exercise stress conditions to enable measurement of a cardiac power
index, the apparatus comprising
mans for noninvasively measuring, through at least a portion of each of a
selected number of cardiac cycles, the left ventricular pressure of the
heart, including means for measuring at a plurality of pressure values the
times of arrival of cardiac pressure pulses at a given arterial site
displaced from the heart,
means for noninvasively measuring, through corresponding portions of a
corresponding number of cardiac cycles, the left ventricular volume of the
heart,
means for determining the work performed by the left ventricle as the
product of the left ventricular pressure and left ventricular volume as a
function of time,
means for determining the power of the left ventricle as the time
derivative of said product, and
means for determining the slope of the time derivative as it rises between
the onset of systole to the point of maximum power, thereby to provide a
value of the cardiac power index for that heart.
34. A method for reliably measuring performance of a heart under resting
and/or exercise stress conditions to enable measurement of a cardiac power
index, the method comprising the steps of:
noninvasively measuring, through at least a portion of each of a selected
number of cardiac cycles, the left ventricular pressure of the heart,
including measuring at a plurality of pressure values the times of arrival
of cardiac pressure pulses at a given arterial site displaced from the
heart,
noninvasively measuring, through corresponding portions of a corresponding
number of cardiac cycles, the left ventricular volume of the heart,
effectively representing the variation of left ventricular work with time
through corresponding portions of a corresponding number of cardiac
cycles,
effectively representing as a curve the power of the left ventricle during
at least said portion of said cycles, and
determining a value of the cardiac power index for that heart as the rate
of change of left ventricular power between the onset of systole and the
time of maximum power.
35. A method for noninvasively determining the cardiac power index of a
living heart, the index being defined as the second time derivative of the
work performed by the left ventricle of the heart between the onset of
systole and a point of maximum left ventricular power, the method
comprising the steps of
noninvasively measuring, through at least a portion of each of a selected
number of cardiac cycles, the left ventricular pressure of the heart,
including measuring at a plurality of pressure values the times of arrival
of cardiac pressure pulses at a given arterial site displaced from the
heart,
noninvasively measuring, through corresponding portions of a corresponding
number of cardiac cycles, the left ventricular volume of the heart and
determining the value for the Cardiac Performance Index on the basis of
the measured values for left ventricular pressure and left ventricular
volume. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates to cardiac monitors generally and more
particularly to cardiac monitors which measure left ventricular
performance.
BACKGROUND OF THE INVENTION
Various cardiac monitors are known in the art. The known monitors typically
utilize measurements taken invasively using cardiac catheterization or
noninvasively. The prior art is summarized in an article entitled "Method
for Noninvasive Measurement of Central Aortic Systolic Pressure," by A.
Marmor, et al., Clinical Cardiology, 1987, 10:215, and the references
cited therein.
SUMMARY OF THE INVENTION
The present invention seeks to provide an improved cardiac monitor and
method for cardiac monitoring.
There is thus provided in accordance with a preferred embodiment of the
present invention a method for reliably measuring cardiac performance
under resting and/or exercise stress conditions to enable measurement of
the cardiac power index including the steps of measuring the left
ventricular pressure:
measuring the left ventricular volume;
determining the product of the left ventricular pressure and the left
ventricular volume as a function of time;
determining the time derivative of the product; and
determining the slope of the time derivative, as it rises thereby to
provide an indication of the cardiac power index,
characterized in that the step of measuring the left ventricular pressure
includes the step of:
measuring the arrival times of cardiac pressure pulses at a given site at a
plurality of pressure values, especially a set of optimized pressure
values.
Further in accordance with an embodiment of the present invention the
method is further characterized in that the step of measuring the left
ventricular pressure also comprises the step of employing an optimization
algorithm which concentrates the largest number of pressure measurements
in the interval during the early ejection phase.
Additionally in accordance with a preferred embodiment of the present
invention, the method is additionally characterized in that the step of
measuring the left ventricular pressure also comprises the step of
measuring the arrival times of cardiac pressure pulses at a given site
during the time period during which the left ventricular pressure rises
from 100% to 125% of the end-diastolic value.
The method may also comprise the step of displaying real-time
electrocardiogram and blood pressure wave forms on a continuously updated
basis.
There is also provided a method for reliably measuring cardiac performance
under resting and/or exercise stress conditions to enable measurement of
the cardiac power index including the steps of:
measuring the left ventricular pressure and the left ventricular volume;
determining the product of the left ventricular pressure and the left
ventricular volume as a function of time;
determining the time derivative of said product; and
determining the slope of the time derivative, as it rises thereby to
provide an indication of the cardiac power index,
characterized in that it also includes the step of displaying real-time
electrocardiogram and blood pressure wave forms on a continuously updated
basis.
In accordance with a preferred embodiment of the invention, the method is
also characterized in that it includes the steps of displaying,
simultaneously and together with said electrocardiogram and brachial
pressure wave forms, the calculated delayed left ventricle pressure values
and the calculated corresponding left ventricular volumetric values.
Additionally in accordance with a preferred embodiment of the invention,
the method is further characterized in that it comprises the step of
measuring, during one or more cardiac cycles, the arrival time for the
given occlusive pressure, and storage of the measured times for each
pressure.
Further in accordance with an embodiment of the present invention, the step
of measuring the time of arrival includes the step of rejecting time
values having unacceptable variance.
Additionally in accordance with a preferred embodiment of the invention,
the step of measuring the time of arrival also includes the step of
statistical averaging of several acceptable sample points to reduce the
effects of beat-to-beat variance, artifactual signals and noise.
Further in accordance with an embodiment of the invention, the step of
measuring left ventricular volume includes the steps of taking least one
measurement within 15 msec of QRS.
Additionally in accordance with an embodiment of the invention, the step of
measuring left ventricular volume includes the steps of carrying out
multiple volume measurements within 40 msec of each other.
Further in accordance with an embodiment of the invention, the method is
further characterized by the steps of measuring the systolic and diastolic
blood pressure.
In accordance with a preferred embodiment of the invention, there is also
provided the step of calculating the cardiac power index as the slope of
the best least squares regression fit to an entire set of instantaneous
power values up to a maximum power point, excluding points whose values
lie outside the range of variance that is commensurate with the other
points.
Another preferred embodiment of the inventive method relates to a method of
measurement of the left ventricular pressure as a function of time, i.e.,
according to this embodiment not the cardiac power index based on the
product of pressure and volume as a function of time is ascertained,
rather the arrival times of cardiac pressure pulses at a given site at a
plurality of pressure values, especially a set of optimized pressure
values, are measured, and indices from said arrival times at said
plurality of pressure values are derived, including but not limited to the
time derivative of the pressure. These indices can be taken or evaluated
for the characterization of cardiac performance.
The measured arrival times are preferably used for fitting a curve, said
curve estimating the time varying wave form of the left ventricular
pressure. The slope of the curve is calculated and defines one of the
preferred indices.
An especially preferred embodiment of the inventive method resides in
measuring the arrival times by measurement of Doppler signals of blood
flow at the given site. For this a specific Doppler ultrasound sensor and
processor are used which are described below.
The inventive method has the advantage that cardiac performance can be
reliably measured under exercise stress conditions of the patient. This is
especially achieved by the Doppler blood flow measuring method used
together with a very specific processing of the received Doppler signals
which results in a clear and noise-free characterization of the cardiac
performance, i.e., pressure and volume-time or pressure-time curves.
Additionally in accordance with an embodiment of the invention, there is
provided an apparatus for reliably measuring cardiac performance under
resting and/or exercise stress conditions to enable measurement of the
cardiac power index comprising:
apparatus for measuring the left ventricular pressure;
apparatus for measuring the left ventricular volume;
apparatus for determining the product of the left ventricular pressure and
the left ventricular volume as a function of time;
apparatus for determining the time derivative of said product; and
apparatus for determining the scope of the time derivative, as it rises
thereby to provide an indication of the cardiac power index,
characterized in that the apparatus for measuring the left ventricular
pressure comprises apparatus for measuring the arrival times of cardiac
pressure pulses at a given site at a plurality of pressure values,
especially a set of optimized pressure values.
Further in accordance with an embodiment of the invention, the apparatus
for measuring the left ventricular pressure also comprises apparatus for
employing an optimization algorithm which concentrates the largest number
of pressure measurements in the interval during the early ejection phase.
Additionally in accordance with an embodiment of the invention, the
apparatus is additionally characterized in that the apparatus for
measuring the arrival times of cardiac pressure pulses at a given site
during the time period during which the left ventricular pressure rises
from 100% to 125% of the end-diastolic value.
Additionally in accordance with an embodiment of the present invention,
there is also provided apparatus for displaying real-time
electrocardiogram and blood pressure wave forms on a continuously updated
basis.
Further in accordance with an embodiment of the present invention, there is
provided apparatus for reliably measuring cardiac performance under
resting and/or exercise stress conditions to enable measurement of the
cardiac power index comprising:
apparatus for measuring the left ventricular pressure and the left
ventricular volume;
apparatus for determining the product of the left ventricular pressure and
the left ventricular volume as a function of time;
apparatus for determining the time derivative of said product; and
apparatus for determining the slope of the time derivative, as it rises
thereby to provide an indication of the cardiac power index,
characterized in that it also includes apparatus for displaying real-time
electrocardiogram and blood pressure wave forms on a continuously updated
basis.
Additionally in accordance with a preferred embodiment of the present
invention, the apparatus is also characterized in that it includes
apparatus for displaying, simultaneously and together with said
electrocardiogram and brachial pressure wave forms, calculated delayed
left ventricle pressure values and calculated corresponding left
ventricular volumetric values.
Additionally in accordance with a preferred embodiment of the present
invention, the apparatus is further characterized in that it comprises
apparatus for measuring, during one or more cardiac cycles, the arrival
time for the given occlusive pressure, and storage of the measured times
for each pressure.
Further in accordance with a preferred embodiment of the present invention,
the apparatus for measuring the time of arrival includes apparatus for
rejecting time values lying outside the range of variance of the other
values.
Further in accordance with an embodiment of the present invention, the
apparatus for measuring the time of arrival also includes apparatus for
statistical averaging of several acceptable sample points to reduce the
effects of beat-to-beat variance, artifactual signals and noise.
Additionally in accordance with a preferred embodiment of the present
invention, the apparatus of measuring left ventricular volume includes
apparatus for taking at least one measurement within 15 msec of QRS.
Further in accordance with a preferred embodiment of the present invention,
the apparatus for measuring left ventricular volume includes apparatus for
carrying out multiple volume measurements within 40 msec of each other.
Additionally in accordance with a preferred embodiment of the present
invention, there is also provided apparatus for measuring the systolic and
diastolic blood pressure.
Additionally in accordance with a preferred embodiment of the present
invention, there is also provided a pulse wave sensor and/or pulse wave
processor with reduced motion artifact effects.
Further in accordance with a preferred embodiment of the invention, the
apparatus for detecting the arrival of the cardiac pressure waves at a
given site, preferably at the brachial artery site, is a Doppler
ultrasound arterial wall motion sensor.
According to an especially preferred embodiment of the inventive apparatus,
the means for detecting the arrival of the cardiac pressure waves at a
given site, preferably at the brachial artery site, is a Doppler
ultrasound blood flow sensor. The sensor itself and a corresponding
processing unit combined therewith allow the rejection of motion artifact
effects.
The Doppler ultrasound sensor (transducer) is advantageously held by an
armband mount comprising an adjustable transducer mount fixed to an
adjustable attachment strap. The Doppler ultrasound sensor (transducer) is
preferably formed as a flat package with Doppler crystals mounted so as to
provide fixed angle of illumination, typically 30.degree. to horizontal.
Said pulse wave processor preferably contains a high-pass filter separating
the high frequencies from the audio signal and an RMS-amplitude-to-DC
converter measuring the power of the high frequency spectrum by converting
the total RMS (root mean square) into a proportional DC voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood and appreciated more fully from
the following detailed description, taken in conjunction with the drawings
in which:
FIG. 1 is a functional block diagram of the cardiac power index monitor
(CPIM) constructed and operative in accordance with a preferred embodiment
of the present invention;
FIG. 2 illustrates a system implementation based on the embodiment of FIG.
1;
FIGS. 3A, 3B and 3C illustrate the derivation of points on a pressure-time
curve using a cuff, an ECG, and a distal pulse wave form sensor;
FIGS. 4A, 4B and 4C are a collection of idealized graphs of ECG, brachial
arterial pressure and brachial arterial wall motion as a function of time,
which are useful in understanding the operation of the apparatus of FIG.
1;
FIG. 5 illustrates one possible version of a cuff pressure control
algorithm for optimal decrementing of cuff pressure;
FIGS. 6A, 6B and 6C illustrate the acquisition and synchronization of
composite volume and pressure curves, and the calculation of the resulting
cardiac power curve, from which the cardiac power index (CPI) is derived;
FIG. 7 (comprised of FIGS. 7A-7D) is a flow chart describing the operation
of the apparatus shown in FIGS. 1-6;
FIG. 8 shows a specific embodiment of a pulse wave form sensor together
with holding means;
FIG. 9 shown another embodiment of the holding means for the pulse wave
form sensor;
FIG. 10 is a block diagram of a processor for the pulse wave form sensor;
FIGS. 11a and b are an exact circuit of the processor according to FIG. 10;
FIG. 12 is a block diagram of a cuff pressure control unit; and
FIGS. 13A and B are an exact circuit of the cuff pressure control unit
according to FIG. 12.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
In an article entitled, "Noninvasive Assessment of Myocardial Performance,"
by A. Marmor, et al., published in the Journal of Nuclear Medicine, vol.
30, No. 10, Oct. 1989, the authors define a measure of cardiac performance
known as the ejection rate of change of power, which is referred to herein
as the cardiac power index or CPI. CPI represents the rate at which
cardiac power changes during the period of ejection of blood from the
heart, known as early systole, and is estimated from the cardiac power
curve. The cardiac power curve is obtained by taking the time derivative
of the product of the cardiac left ventricular pressure and volume during
the early part of systole.
Reference is now made to FIG. 1 which illustrates, in block diagram form, a
cardiac power index monitor, constructed and operative in accordance with
the present invention. Reference is also made to FIG. 2, which illustrates
a system implementation based on the embodiment of FIG. 1. The cardiac
monitor, denoted by reference numeral 10, comprises a microcomputer 20,
which is preferably IBM-PC compatible. The microcomputer 20 preferably
controls all monitor functions and drives a physiological data display 22,
such as an EGA graphics video monitor, and a cardiac power index (CPI)
display 24, which may be provided by the same apparatus used for display
22. The microcomputer 20 also stores data in and retrieves data from a
mass storage device 28, preferably a hard disk drive with at least 10
mbytes, and drives a hard copy device 26, preferably an Epson compatible
dot-matrix printer.
The monitor of FIG. 1 also comprises noninvasive blood pressure measurement
(NIBP)/cuff pressure controller (CPC) apparatus 30, such as a Bosch EBM
502 D, for measuring the brachial arterial pressure and heart rate, and
which operates a sphygmomanometric cuff 38. Cuff 38 is preferably a
wrap-around type such as that used in the PediSphyg system by CAS Medical,
Inc. of Branford, Conn., U.S.A., or a Bosch cuff. The cuff pressure
controller incorporates appropriate interface and control circuitry and
software to enable the operation of apparatus 30 in the mode of pressure
control of cuff 38 instead of its conventional mode of operation for blood
pressure measurement. A block diagram of the controller is shown in FIG.
12.
The monitor 10 also includes an ECG monitor 70 and an R-wave detector and
trigger generator 72, both typically contained in standard ECG monitor
system such as a Mennen Horizon 2000 patient monitor.
Also included in monitor 10 is a pulse wave form sensor 40, namely, a
Doppler ultrasound wall motion and blood flow detection sensor, such as
MedaSonics model 94G, attached to the same arm as the cuff 38, and
approximately 1-3 cm distal to it. A pulse waveform processor 42 (shown in
FIG. 10), preferably an analog and/or digital circuit whose input is the
waveform from sensor 40, provides an analog output which is preferably
proportional to the blood flow.
Alternatively, the output may be proportional to the wall motion or the
velocity of wall motion. In either implementation, high-pass filters
eliminate most of the influence of motion artifacts from the output signal
to the A/D converter 44, whose digital data output is read by
microcomputer 20.
A gamma camera 60, which may be a commercial field-of-view gamma camera,
such as an Elscint Model APEX and its associated CPU 62, receives a
gatling R-wave trigger either from an ECG monitor 70 or from its own
internal ECG monitor. In response thereto, camera 60 records a plurality
of frames of several milliseconds duration at intervals of typically 25-40
milliseconds throughout each cardiac cycle, averaging together the frames
from many (typically 300) cycles to obtain the averaged volumetric frame
values along the time curve through the cardiac cycle.
A gamma camera CPU 62 communicates the resulting data values to
microcomputer 20 via a digital link, preferably RS232 or Centronics
parallel, or alternatively via disk transfer.
As illustrated in FIG. 2, cuff 38 is attached preferably above an elbow,
and is controlled by microcomputer 20 via cuff pressure controller 30. An
R-wave detector and trigger generator 72 senses the sharp spike-like wave
of the ECG, known as the QRS complex, and provides a digital trigger pulse
corresponding to the occurrence of the R-wave (the center of the QRS
spike).
It is proposed in the article by A. Marmor, et al., of Annex A to measure a
cardiac power curve and from it to calculate a cardiac power index.
Cardiac power is defined as the time derivative of the product of cardiac
volume and cardiac (or aortic) pressure with time. The cardiac power index
is defined as the slope of the portion of the power versus time curve from
onset of systole to the moment of maximal power.
Determination of the cardiac power curve and cardiac power index (CPI)
using the cardiac monitor 10 is described hereinbelow.
ESTIMATION OF LEFT VENTRICULAR PRESSURE
Occlusion of brachial flow during most of the cardiac cycle creates a
standing fluid column between the aorta and the brachial artery, such that
the rising intra-aortic pressure wave form is transmitted to the brachial
artery with minimal distortion. Accordingly, the pressure values obtained
at the brachial artery very closely represent those in the left ventricle.
In order to enable later combination with left ventricular volume
measurements made at the heart, the brachial pressure values must be
shifted in time to account for the propagation of the cardiac pressure
wave from the heart to the brachial artery. The post-QRS time required for
a cardiac pressure wave to travel from the heart to the brachial artery
measurement site is known herein as the propagation time, as is discussed
below in conjunction with FIG. 5. The propagation time for a given patient
during the examination period is presumed constant under all conditions of
heart activity.
The operation of the cardiac monitor 10, including the calculation of the
CPI, is described in the flow chart of FIG. 7. Patient preparations for
gamma camera ventriculography are completed, and 3-4 ECG electrodes 41 are
attached in standard thoracic montage, for input to ECG apparatus 70.
While the patient is at rest, cuff 38 is applied just above an elbow, and
the pulse wave form sensor 40 its attached 1-3 cm distal to the cuff on
the same arm. The pulse waveform signal is acquired by microcomputer 20
from apparatus 42 and displayed together with the ECG, on the
physiological data display 22, where the quality of both ECG and pulse
waveform signals are used as visual feedback to verify proper signal
acquisition or to guide any required adjustment.
FIGS. 3A, 3B and 3C illustrate the technique by which the sample points on
the composite pressure-time curve are determined, through the relationship
between brachial arterial pressure, cuff pressure, the ECG QRS complex,
and the detection of a pulse wave form distal to the cuff.
Two simplified cardiac cycles are shown with representative parameter
values in FIGS. 3A-3C. In the first cardiac cycle, systolic pressure is
110 and cuff pressure is set to 100 Torr, while in the second cycle,
systolic pressure is 115 and cuff pressure is set to 90 Torr Shown in FIG.
3A are the brachial pressure waveform, the cuff pressure, and the ECG
waveform, indicating the relative timing of the QRS complex of each
cardiac cycle and the resulting brachial pressure waveform.
Point A1 of cardiac cycle 1 occurs at the first instance during the cycle
when brachial pressure exceeds cuff pressure. Referring to FIG. 3B, which
depicts the pulse waveform produced by pulse wave form processor 42, it is
noted that the pulse waveform abruptly rises at point B1, whose occurrence
coincides in time with point A1 of FIG. 3A, as the blood pressure wave
passes the cuff, i.e., breaks through, and causes arterial wall motion
that is sensed by device 42.
The time delay from the QRS complex to the beginning of the abrupt rise of
the pulse waveform, labeled T1 and having a value of 220 msec in FIG. 3B
represents the time, after the QRS complex, when brachial arterial
pressure reached 100 Torr. In FIG. 3C, which represents the composite
pressure-time curve, point C1 has a pressure value of 100 Torr and a time
of 220 msec, in accordance with the pressure and time values of points A1
and B1 above. It is noted that the time scale of FIG. 2C is in msec,
whereas that of both FIGS. 3A and 3B is in seconds.
In similar fashion, in cardiac cycle 2, where systolic pressure is shown as
115 Torr and cuff pressure is shown as 90 Torr, points A2 and B2
correspond to the time when the blood pressure wave breaks through the
cuff, which occurs at 180 msec after the QRS of cardiac cycle 2. In FIG.
3C, point C2 is shown at a pressure of 90 Torr and a time of 180 msec, in
accordance with the pressure and time values of points A2 and B2 above. In
actual implementation, each point on the composite pressure-time curve is
determined by averaging together the delay times measured for a given cuff
pressure maintained over a plurality of cardiac cycles.
While the patient is still in resting position, the operator causes the
cardiac monitor 10 to commence measurement initialization. During
initialization, prior to application of any pressure on cuff 38, the
arterial pressure propagation time from heart to brachial artery is
estimated, and the pulse waveform is characterized.
Cardiac monitor 20 is operated to measure the maximum and minimum pulse
waveform values. Pulse waveform values MAXAMP and MINAMP are the
respective average maximum and minimum values of the pulse waveform output
of detector 42 during a plurality of cardiac cycles, preferably 10. MAXAMP
is preferably obtained by averaging together the maximum amplitude value
of the output of detector 42 from the aforementioned plurality of cardiac
cycles, while MINAMP is preferably obtained by averaging together the
minimum amplitude value of the output of detector 42 from each of the
aforementioned plurality of cardiac cycles.
FIGS. 4A, 4B and 4C illustrate a method for calculating the propagation
time, which is also used for calculating the breakthrough time referred to
below and in Procedure ARRIVAL of FIG. 7. FIGS. 4A, 4B and 4C,
respectively, show the ECG waveform brachial arterial pressure waveform,
and pulse waveform for two idealized cardiac cycles. The propagation time
is calculated by first detecting the steep upswing of the pulse waveform
shown in 4C.
A regression line, labeled S1 in the first cycle and S2 in the second
cycle, is fitted to the early portion of the upswing, preferably to the
samples from the first 30 milliseconds of the upswing. A second regression
line, labeled D1 in the first cycle and D2 in the second cycle, is fitted
to the last portion of the waveform prior to the upswing, preferably to
the samples during the last 30 milliseconds prior to the upswing. The time
interval T1, from the R-wave of QRS 1 until the intersection point B1
between lines S1 and D1, is the arrival time of the pulse wave of cardiac
cycle 1 at the pulse waveform sensor 40. Similarly, the time interval T2,
from the R-wave of QRS 2 until point B2 is the arrival time of the pulse
wave of cardiac cycle 2 at sensor 40. When determining propagation time,
the above arrival times are preferably averaged together from a plurality
of cardiac cycles, preferably 10 cycles.
The operator then causes the apparatus 30 to obtain the diastolic and
systolic pressure values, and the heart rate, via microcomputer 20. A cuff
pressure control algorithm, one embodiment of which illustrated in FIG. 5,
uses the measured diastolic and systolic pressure values, and selects the
pressures to which the cuff is to be inflated.
In a particularly important characteristic of the present invention, the
series of pressure values to be implemented by the cuff 38 are defined
such that the largest number of pressure measurements are concentrated
during the early ejection phase, typically defined as the phase between
100-125% of the end-diastolic pressure. An example optimization algorithm
for defining the pressure values is illustrated in FIG. 5, wherein the
pressures P0 through P9 are set as follows:
for DP=Systolic pressure--Diastolic pressure
P0--1.25 .multidot. Systolic
P1--Systolic pressure
P2--Systolic--0.25 .multidot. DP
P3--Systolic--0.50 .multidot. DP
P4--Systolic--0.45 .multidot. DP
P5--Systolic--0.75 .multidot. DP
P6--Systolic--0.85 .multidot. DP
P7--Systolic--0.90 .multidot. DP
P8--Systolic--0.95 .multidot. DP
P9--Diastolic pressure
The number of points, and their precise dependence on systolic and
diastolic pressure, may vary from the foregoing, so long as there are a
plurality of points in the pressure range from the end-diastolic point to
midway up the systolic rise, i.e., from diastolic pressure to
(systolic--0.5 .multidot. DP). In response to an operator instruction to
monitor 10, cuff 38 is inflated to pressure PO, and the pulse detector
output used to verify occlusion of flow by the cuff.
The threshold for confirmation of occlusion is when the output amplitude of
pulse waveform detector 42 is less than a fraction of the difference
between aforementioned MAXAMP and MINAMP, preferably 0.05
.multidot.(MAXAMP-MINAMP). If the original cuff pressure PO does not
reduce the output of detector 42 per above, the value of PO is increased,
preferably by 10% of its previous value, and the confirmation procedure
repeated. The above is repeated until occlusion is confirmed or until PO
reaches a maximum of 150% of systolic pressure. Once occlusion is
confirmed, the detected pulse waveform values are averaged together over a
plurality of cardiac cycles, typically 10, to obtain an average baseline
value AMP.
The operator then operates monitor 10 to commence the measurement of the
pressure-time curve. Cuff pressure is reduced to value P1, intended to
allow breakthrough only near the systolic peak. Microcomputer 20 analyses
the pressure waveform signal in real time during the current cardiac cycle
to determine if and when breakthrough occurs. Breakthrough is typically
defined as the point when the waveform value first rises significantly
above the baseline, which in the preferred embodiment is defined as a rise
of more than three standard deviations above the aforementioned baseline
average value AMP.
If and when breakthrough is detected, the method described above in
determining propagation time is used to estimate the breakthrough time.
The above procedure is repeated during at least 2, typically 5-10, cardiac
cycles for the same cuff pressure setting, providing at least 2, typically
5-10, estimates of the breakthrough time for the pressure, from which mean
and variance are calculated for said breakthrough time. Before proceeding
to a new cuff pressure value, the set of breakthrough time estimates is
reviewed, and outlying values (typically those lying more than three
standard deviations from the mean) are excluded form the set, and a new
final mean value calculated. The final mean value is the one stored in the
pressure-volume curve for the cuff pressure value used.
Once the final pressure-time point has been determined for a given cuff
pressure value, the cuff pressure is then reduced to the next value
determined in the cuff pressure control algorithm, until the last value
has been completed.
It will be appreciated from a consideration of FIG. 3 that at low
pressures, such as those close to the diastolic pressure, the
above-mentioned method may be unreliable as the required standing column
of blood is not well established prior to the onset of systole. Hence, the
pressure-time value for onset of systole is taken to be the most recently
measured diastolic pressure value and its time is taken to be the
aforementioned propagation time determined when the patient was at rest.
The set of pressure values then obtained is interpolated typically by a
piecewise polynomial curve fit by least squares minimization to provide
estimated pressure values at any desired time point during the systolic
portion of the cardiac cycle. The pressure curve a shown in FIG. 6B, which
typically comprises an average of pressure values recorded over a
multiplicity of cardiac cycles as described hereinabove, is then shifted
by the amount of the propagation delay, thereby producing an estimated
left ventricular pressure curve.
LEFT VENTRICULAR VOLUME DETERMINATION
Reference is now made again to FIG. 1. As noted above, in the preferred
embodiment, the invention additionally comprises a field-of-view gamma
camera 60, such one commercially available form Elscint of Haifa, Israel,
and its associated CPU 62. The gamma camera 60 and CPU 62 measure the
volume fo the left ventricle using gated radionuclide ventriculography
according to the count rate method as described in "Left Ventricular
Pressure-Volume Diagrams and End-systolic Pressure-Volume Relations in
Human Beings," by McKay, R.G., et al., and published in Journal of the
American College of Cardiology, vol. 3, 1984.
In accordance with a preferred embodiment of the invention, the R-wave
detector 72 detects the R-wave of the ECG signal. Alternatively, if gamma
camera 72 incorporates an ECG apparatus and associated QRS or R-wave
detector, the QRS or R-wave is detected by the detector of the gamma
camera.
A predefined amount of time later, typically 10-20 msec, the gamma camera
60 counts the number of gamma rays coming from the left ventricle during a
predefined time frame, typically 5-10 msec. The gamma camera 60 repeats
the measurement every typically 20-50 ms, producing sampled points on a
curve of the left ventricular volume with time. The volume curve thus
produced is typically synchronized to the QRS complex via the R-wave
detector, and is illustrated in FIG. 6A.
Typically, the volume curve will have only a few points and, thus, it is
typically interpolated by least squares piecewise polynomial curve-fitting
methods. Thus, an interpolated volume curve, illustrated in FIG. 6A, is
calculated which has data at the same time points as the pressure curve
calcul | | |
