Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2001-10-31
2004-07-20
Layno, Carl (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
C600S509000, C607S004000
Reexamination Certificate
active
06766190
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
Reference is hereby made to commonly assigned, co-pending U.S. patent application Ser. No. 09/999,890 filed on even date herewith for METHOD AND APPARATUS FOR DISCRIMINATING BETWEEN TACHYARRHYTHMIAS BY Bozidar Ferek-Petric.
FIELD OF THE INVENTION
This invention relates to cardiac implantable medical devices (IMDs) particularly adapted for developing a vectorcardiograph (VCG) from vector lead signals developed across selected pairs of implanted electrodes.
BACKGROUND OF THE INVENTION
The mechanical events of the heart are preceded and initiated by the electrochemical activity of the heart (i.e., the propagation of the action potential). In the healthy heart, the electrical and mechanical operation of the heart is regulated by electrical signals produced by the heart's sino-atrial (SA) node. Each signal produced by the SA node spreads across the atria, causing the depolarization and contraction of the atria, and arrives at the atrioventricular (A-V) node. The signal is then conducted to the “Bundle of His” during which time it is slowed down to allow for the atrium to pump blood into the ventricles and thereafter to the “Bundle Branches” and the Purkinje muscle fibers of the right and left ventricles. The signals propagated through the Bundle Branches effects depolarization and accompanying contraction of the left ventricle and the right ventricle in close order. Following contraction, the myocardial cells repolarize during a short period of time, returning to their resting state. The right and left atria refill with venous and oxygenated blood, respectively, until the cardiac cycle is again commenced by a signal originating from the SA node. At rest, the normal adult SA node produces a signal approximately 60 to 85 times a minute, causing the heart muscle to contract, and thereby pumping blood to the remainder of the body. The electrical signal passes through the heart chambers as a wave front that can be characterized as a plane advancing from cell to cell through the cardiac muscle that separates cells of different electrical potential as a function of the time that it takes to complete the cardiac cycle.
The above-described cardiac cycle is disrupted in diseased or injured hearts, and the chronic or episodic disrupted electrical activity has long been employed to diagnose the state of the heart. A variety of techniques have been developed for collecting and interpreting data concerning the electrical activity of the heart both in the clinical setting and by way of portable external monitors carried by or IMDs implanted in an ambulatory patient to collect data relating to electrical heart function during daily activities of the patient. Such techniques include electrocardiography, vectorcardiography and polarcardiography.
The most commonly used of these techniques is the electrocardiograph (ECG) machine that displays one-dimension tracings of electrical signals of the heart as the depolarization wave front advances across the heart chambers in the cardiac cycle. An ECG machine typically measures and displays and/or records the voltages at various skin electrodes placed about the body relative to a designated “ground” electrode. The paired electrodes are referred to as “leads” and the lead signal is displayed or printed as an ECG lead tracing. The term “lead” would appear to indicate a physical wire, but in electrocardiography, “lead” actually means the electrical signal or vector in space between a designated pair of skin electrodes arranged as described below, wherein the vectors traverse the heart disposed between the skin electrodes.
The cardiac cycle as displayed in an ECG lead tracing reflects the electrical wave front as measured across one such ECG lead, as shown in U.S. Pat. No. 4,587,976, for example, and depicted in FIG.
1
. The portion of a cardiac cycle representing atrial depolarization is referred to as a “P-wave.” Depolarization of the ventricular muscle fibers is represented by “Q”, “R”, and “S” points of a cardiac cycle. Collectively these “QRS” points are called an “R-wave” or a “QRS complex.” Re-polarization of the depolarized heart cells occurs after the termination of another positive deflection following the QRS complex known as the “T-wave.” The QRS complex is the most studied part of the cardiac cycle and is considered to be the most important for the prediction of health and survivability of a patient. However, the time relation of the P-wave to the QRS complex and the height and polarity of the T-wave and S-T segment are also indicators of a healthy or diseased heart. The S-T segment of a healthy heart is usually isoelectric, i.e., neither positive nor negative in deflection from baseline of the ECG lead tracing. This S-T segment is a most important indicator of the health of the ventricular myocardium and is elevated in ischemia and due to infarctions disrupting the depolarization wave front.
The ECG machine typically plots each ECG lead in parallel over an interval of time such that the heart's electrical activity for one or more cardiac cycles is displayed as parallel ECG lead tracings on a visual display screen and/or printed for purposes of monitoring or analysis. The most common ECGs are known as the “12 lead”, the “18 lead,” and a variety of other, fewer, lead combinations that simulate the more complete ECGs.
The 12-Lead system provides much redundant information in the frontal (X, Y) plane and transverse (X, Z) plane of the ECG vector signal. It permits only a rough visual estimate of the vector direction in theses two planes. Moreover, the number of skin electrodes and the bulk of the cables and the ECG machine make 12-lead and 18-lead ECG systems only practical in the clinical setting and impractical for use in a portable monitor for chronic use by a patient. Portable ECG recorders or “Holter monitors” therefore employ fewer cables and electrodes to record at least certain of the above-listed ECG lead tracings.
In order to better explain the novel aspects and unique benefits of the present invention, a brief explanation of vectorcardiography and the numerous steps and processes a physician typically undergoes in order to offer a somewhat accurate diagnosis is relevant.
Vectorcardiography uses a vector description of the progress of the depolarization wave front through the heart during the P-wave or loop, the QRS wave or loop and the T-wave or loop as described and illustrated in U.S. Pat. No. 4,587,976, for example, particularly in reference to
FIGS. 1 and 2
thereof. Vectorcardiography abandons the one dimension time coordinate of the ECG in favor of plots or tracings of the orientation and magnitude of the vector of the depolarization wave front on each of three planes: a vertical, frontal (X,Y) plane plotting an X-axis (right side or arm to left side or arm) against a Y-axis (head to foot); a horizontal or transverse (X,Z) plane plotting the X-axis against a Z-axis (anterior-posterior); and a vertical, sagittal (Y,Z) plane plotting the Y-axis against the Z-axis as shown in FIG.
2
. The resultant xyz-vector is often characterized as comprising the mean P-wave vector, the mean QRS vector and the mean T-wave vector over a cardiac cycle. Each xyz-vector traces a loop during the time of occurrence of the P-wave, QRS complex and T-wave of FIG.
1
. In simplified terms, at least three orthogonal ECG signals are simultaneously obtained from at least three orthogonal ECG leads that are generally co-planar with the frontal X,Y plane, the transverse X,Z plane, and the sagittal Y,Z plane. Signal pairs are combined to form the frontal X,Y plane vector or z-vector, the transverse or horizontal X,Z plane vector or y-vector, and the sagittal Y,Z plane vector or x-vector as shown in FIG.
2
. The visual presentation and measurement of the xyz-vector in 3-D space is difficult. Consequently, the planar x-vector, y-vector and z-vector are typically simultaneously displayed employing three CRT displays or a split screen CRT display. The trained physician viewing the displays can diagnose the state of the
Layno Carl
Medtronic Inc.
Wolde-Michael Girma
LandOfFree
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