Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2001-01-17
2003-05-13
Wolfe, Willis R. (Department: 3747)
Surgery
Diagnostic testing
Cardiovascular
C600S536000, C600S547000
Reexamination Certificate
active
06561986
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the field of hemodynamic analysis of living subjects, and particularly to an apparatus and method for non-invasively detecting and evaluating fiducial points within waveforms such as those present in the impedance cardiograms and electrocardiograms of the subject.
2. Description of Related Technology
The study of the performance and properties of the cardiovascular system of a living subject has proven useful for diagnosing and assessing any number of conditions or diseases within the subject. The performance of the cardiovascular system, including the heart, has characteristically been measured in terms of several different parameters, including the stroke volume and cardiac output of the heart.
Noninvasive estimates of cardiac output (CO) can be obtained using the well known technique of impedance cardiography (ICG). Strictly speaking, impedance cardiography, also known as thoracic bioimpedance or impedance plethysmography, is used to measure the stroke volume (SV) of the heart. As shown in Eqn. (1), when the stroke volume is multiplied by heart rate, cardiac output is obtained.
CO=SV×
heart rate. (Eqn. 1)
During impedance cardiography, a constant alternating current, with a frequency such as 70 kHz, I(t), is applied across the thorax. The resulting voltage, V(t), is used to calculate impedance. Because the impedance is assumed to be purely resistive, the total impedance, Z
T
(t), is calculated by Ohm's Law. The total impedance consists generally of a constant base impedance, Z
o
, and time-varying impedance, Z
c
(t), as shown in Eqn. 2:
Z
T
(
t
)
=
V
(
t
)
I
(
t
)
=
Z
o
+
Z
c
(
t
)
.
(
Eqn
.
2
)
The time-varying impedance is believed to reflect the change in blood resistivity as it transverses through the aorta.
Stroke volume is typically calculated from one of three well known equations, based on this impedance change:
Kubicek
:
SV
=
ρ
(
L
2
Z
o
2
)
LVET
ⅆ
Z
(
t
)
ⅆ
t
max
,
(
Eqn
.
3
)
Sramek
:
SV
=
L
3
4.25
Z
o
LVET
ⅆ
Z
(
t
)
ⅆ
t
max
,
(
Eqn
.
4
)
Sramek-Bernstein
:
SV
=
δ
(
0.17
H
)
3
4.25
Z
o
LVET
ⅆ
Z
(
t
)
ⅆ
t
max
.
(
Eqn
.5
)
Where:
L=distance between the inner electrodes in cm,
LVET=ventricular ejection time in seconds,
Z
o
=base impedance in ohms,
ⅆ
Z
(
t
)
ⅆ
t
max
=
magnitude
of
the
largest
negative
derivative
of
the
impedance
change
,
Z
c
(
t
)
,
occurring during systole in ohms/s,
&rgr;=resistivity of blood in ohms-cm,
H=subject height in cm, and
&dgr;=special weight correction factor.
Two key parameters present in Eqns. 3, 4, and 5 above are (i)
ⅆ
Z
(
t
)
ⅆ
t
max
and (ii) LVET. These parameters are found from features referred to as fiducial points, that are present in the inverted first derivative of the impedance waveform,
ⅆ
Z
(
t
)
ⅆ
t
.
As described by Lababidi, Z., et al, “
The first derivative thoracic impedance cardiogram
,” Circulation, 41:651-658, 1970, the value of
ⅆ
Z
(
t
)
ⅆ
t
max
is generally determined from the time at which the inverted derivative value has the highest amplitude, also commonly referred to as the “C point”. The value of
ⅆ
Z
(
k
)
ⅆ
t
max
is calculated as this amplitude value. LVET corresponds generally to the time during which the aortic valve is open. That point in time associated with aortic valve opening, also commonly known as the “B point”, is generally determined as the time associated with the onset of the rapid upstroke (a slight inflection) in
ⅆ
Z
(
t
)
ⅆ
t
before the occurrence of the C point. The time associated with aortic valve closing, also known as the “X point”, is generally determined as the time associated with the inverted derivative global minimum, which occurs after the C point, as illustrated in FIG.
1
.
In addition to the foregoing “B”, “C”, and “X” points, the so-called “O point” may be of utility in the analysis of the cardiac muscle. The O point represents the time of opening of the mitral valve of the heart. The O point is generally determined as the time associated with the first peak after the X point. The time difference between aortic valve closing and mitral valve opening is known as the isovolumetric relaxation time, IVRT. However, to date, the O point has not found substantial utility in the stroke volume calculation.
Impedance cardiography further requires recording of the subject's electrocardiogram (ECG) in conjunction with the thoracic impedance waveform previously described. Processing of the impedance waveform for hemodynamic analysis requires the use of ECG fiducial points as landmarks. Processing of the impedance waveform is generally performed on a beat-by-beat basis, with the ECG being used for beat detection. In addition, detection of some fiducial points of the impedance signal may require the use of ECG fiducial points as landmarks. Specifically, individual beats are identified by detecting the presence of QRS complexes within the ECG. The peak of the R wave (commonly referred to as the “R point”) in the QRS complex is also detected, as well as the onset of depolarization of the QRS complex (“Q point”).
Historically, the aforementioned fiducial points in the impedance cardiography waveform (i.e., B, C, O, and X points) and ECG (i.e. R and Q points) were each determined through empirical curve fitting. However, such empirical curve fitting is not only labor intensive and subject to several potential sources of error, but, in the case of the impedance waveform, also requires elimination of respiratory artifact. More recently, digital signal processing has been applied to the impedance cardiography waveform for pattern recognition. One mathematical technique used in conjunction with such processing, the well known time-frequency distribution, utilizes complex mathematics and a well known time-frequency distribution (e.g., the spectrogram). See for example, U.S. Pat. Nos. 5,309,917, 5,423,326, and 5,443,073 issued May 10, 1994, Jun. 13, 1995, and Aug. 22, 1995, respectively, and assigned to Drexel University. As discussed in the foregoing patents, the spectrogram is used for extraction of information relating to the transient behavior of the dZ/dt signal. Specifically, a mixed time-frequency representation of the signal is generated through calculation of the Fast Fourier Transform and multiplication by a windowing function (e.g., Hamming function) to convert the one-dimensional discrete dZ/dt signal into a two-dimensional function with a time variable and frequency variable.
However, the spectrogram (and many of the time-frequency distributions in general) suffers from a significant disability relating to the introduction of cross term artifact into the pattern recognition calculations. Specifically, when a signal is decomposed, the time-frequency plane should accurately reflect this signal. If a signal is turned off for a finite time, some time-frequency distributions will not be zero during this time, due to the existence of interference cross terms inherent in the calculation of the distribution.
Another limitation of the spectrogram is its assumption of stationarity within the windowing function. This assumption is valid if the frequency components are constant throughout the window. However, biological signals, including the ECG and the impedance waveform, are known to be non-stationary.
Additionally, the signal processing associated with such time-frequency distributions by necessity incorporates complex mathematics (i.e., involves operands having both real and imaginary components), which significantly complicates even simple pattern recognition-related computations.
Furthermore
Baura Gail D.
Ng Sau Kuen
CardioDynamics International Corporation
Gazdzinski & Associates
Wolfe Willis R.
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