Surgery – Diagnostic testing – Via monitoring a plurality of physiological data – e.g.,...
Reexamination Certificate
2000-08-28
2002-12-17
Getzow, Scott M. (Department: 3762)
Surgery
Diagnostic testing
Via monitoring a plurality of physiological data, e.g.,...
Reexamination Certificate
active
06494832
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to determining cardiac performance in a patient. More specifically, the present invention is related to determining cardiac performance in a patient with a conductance catheter which can be excited with multiple frequencies.
BACKGROUND OF THE INVENTION
Although there are other methods to measure ventricular volumes such as MRI and nuclear technologies, they cannot do so instantaneously. Echocardiography can generate an estimate of instantaneous volume using the modified Simpsons rule or “stack of discs”. Because it utilizes a single tomographic plane to estimate three dimensional volumes, it has limitations when applied to patients with regional wall motion abnormalities. Therefore, only improvements in conductance technology offer the ability to make these precise mechanical measurements.
One conductance apparatus commercially available is the Cardiac Function Analyzer made by CardioDynamics in the Netherlands. This apparatus includes the Leycom Sigma 5, a device which is used to measure instantaneous volume from a conductance catheter. The Leycom Sigma 5 has been able to generate adequate volume data in ventricular chambers of large animals which are smaller than 150 ml. However, in patients with congestive heart failure, hearts may range from 180 to 500 ml. It has been previously shown (Reprint 1) that the Sigma 5 cannot generate a homogeneous electric field for volumes seen in human heart failure. Furthermore, there is no built in mechanism for the Sigma 5 to correct for current leakage into the surrounding conductive structures such as myocardium. As a result, it significantly underestimates the stroke volume (volume of blood pumped by the failing heart) and overestimates end-systolic and end-diastolic volumes. In reprints 2-5, there was an average 2-fold underestimation of the stroke volume. U.S. Patent to Carlson teaches that parallel conductance (current leakage outside the blood volume i.e., heart muscle) will be constant at different frequencies, so that this term can be excluded (see column 4, item (6)). Gwane et al. J Appl Physiology vol 63, pg 872-876, 1987 teaches that parallel conductance does vary with frequency, while By stroke volume is constant. The present invention is based on the discovery that since muscle resistivity does vary with frequency and blood does not, the resistivity ratio of blood and muscle will vary with frequency. Hence, both the field density within the left ventricle and the current leakage to the surrounding heart muscle both vary with frequency. The end result is both stroke volume and parallel conductance varying with frequency, which is in contrast to both the Carlson patent and Gwane paper. The apparatus uses a digitally controlled signal synthesizer to drive any conductance catheter. This results in more consistent control over waveform shape, amplitude, and frequency than known before. The use of the digital synthesizer also allows the user to select any type of waveform over a broad range of frequencies to apply to a conductance catheter. The digital signal synthesizer is a Signametrics Complex DDS Generator. The device can couple with commercially available conductance catheters made by numerous vendors. One includes Millar Instruments in Houston, Tex. They market conductance catheters with an incorporated Mikrotip pressure transducer for small animals including transgenic mice (SPR 719) and humans (SPC 550, 560, and 570).
The ability to delete single genes from small animals (mice and rats) to generate transgenic animals is now possible. This allows the study of the effect of a single gene deletion on the development on congestive heart failure (weak heart muscle) and hypertrophy (thickened heart muscle). Investigators are currently utilizing left ventricular pressure or its first derivative (dP/dt); or dimension and fractional shortening (derived by echocardiography). The problem with these isolated pressure and dimension measurements is that they are altered just by the heart size changes which accompany congestive heart failure and hypertrophy. Conductance catheter pressure-volume measurements miniaturized for the transgenic mouse allows the physiologic endpoint of how weak the heart muscle has become to be accurately determined. See “Cardiac physiology in transgenic mice” by James et al., and another paper demonstrating the technique of conductance PV loops in the mouse (Georgakopoulos et al. Am J Physiology 1998), both of which are incorporated by reference herein.
Conductance measurement offers a method to generate an instantaneous left ventricular volume signal in the mouse (Georgakopoulos D, Mitzner W A, Chen C H, Byrne B J, Millar H D, Hare J M, Kass D A. In vivo murine left ventricular pressure-volume relations by miniaturized conductance micromanometry. Am J Physiol 274 (Heart Circ Physiol 43): H1416-H1422, 1998, incorporated by reference herein). It uses an electric field generated from electrodes at the apex and immediately above the left ventricle to sense the instantaneous conductance change as the left ventricle fills and ejects blood. A signal proportional to the left ventricular blood volume is required for use in physiologic studies. Unfortunately, the presently available instantaneous conductance output is a combination of blood and left ventricular muscle (Boltwood C M, Appleyard R F, Glantz S A. Left ventricular volume measurement by conductance catheter in intact dogs: parallel conductance volume depends on left ventricular size. Circulation 80: 1360-1377, 1989; Burkhoff D, Van Der Velde E, Kass D, Baan J, Maughan W L, Sagawa K. Accuracy of volume measurement by conductance catheter in isolated, ejecting canine hearts. Circulation 72: 440-447, 1985; Cabreriza S E, Dean D A, Jia C X, Dickstein M L, Spotnitz H M. Electrical isolation of the heart: stabilizing parallel conductance of left ventricular volume measurement. ASAIO Journal 43: M 509-M 514, 1997; Lankford E B, Kass D A, Maughan W L, Shoukas A A. Does parallel conductance vary during a cardiac cycle? Am J Physiol 258 (Heart Circ Physiol 27): H1933-H1942, 1990; Szwarc R S, Mickleborough L L, Mizuno S I, Wilson G J, Liu P, Mohamed S. Conductance catheter measurements of left ventricular volume in the intact dog: parallel conductance is independent of left ventricular size. Cardiovas Res 28: 252-258, 1994, all of which are incorporated by reference herein). By developing a conductance system that operates at several simultaneous frequencies, identification and possibly correction for the myocardial contribution to the instantaneous volume signal can be had.
This is based on the assumption that patient myocardial conductivity will vary with frequency, while patient blood conductivity will not. Prior work has shown that blood has constant electrical resistivity over a wide range of frequencies (2 to 100 kHz, 22). In contrast, the resistivity of myocardium is known to change with frequency; specifically, the resistivity of myocardium is lower at increased excitation frequency (Epstein B R, Foster K R. Anisotropy in the dielectric properties of skeletal muscle. Med Biol Eng Comput 21: 51-55, 1983; Schwan H P, Kay C F. Specific resistance of body tissues. Circ Res IV: 664-670, 1956; Steendijk P, Mur G, Van Der Velde E, Baan J. The four-electrode resistivity technique in anisotropic media: theoretical analysis and application on myocardial tissue in vivo. IEEE Trans Bio Med Eng 40: 1138-1148, 1993; Steendijk P, Mur G, Van Der Velde E, Baan J. Dependence of anisotropic myocardium electrical resistivity on cardiac phase and excitation frequency. Basic Res Cardiol 89: 411-426, 1994; Zheng E, Shao S, Webster J G. Impedance of skeletal muscle from 1 Hz to 1 MHz. IEEE Trans Biomed Eng 31: 477-483, 1984, all of which are incorporated by reference herein). See FIG.
1
. At lower frequencies, there is a maximal gradient between the resistivity of blood and myocardium such that the electric field generated will be primarily confined to the left ventricular cavity and to a lesser degree in the myocardium. A
Feldman Marc D.
Pearce John A.
Valvano Jonathan W.
Conductance Technologies, Inc.
Getzow Scott M.
Schwartz Ansel M.
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