Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-12-22
2002-10-29
Schaetzle, Kennedy (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C600S509000, C607S013000, C128S901000
Reexamination Certificate
active
06473649
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates generally to a device for stimulating cardiac tissue, and more particularly relates to an implantable cardiac rhythm management device capable of automatically detecting intrinsic and evoked response of a patient's heart. The device of the present invention may operate in an automatic capture verification mode, wherein an electrocardiogram signal of a patient's heart is received and used by the device to determine whether a stimulation pulse evokes a response by the patient's heart. The rhythm management device may automatically adjust the detection threshold during a normal mode or an automatic capture verification mode. Also, the device may suspend the automatic capture verification mode and/or adjust the detection threshold dependent upon detected and/or measured noise, a determined amplitude of evoked response, a determined modulation in the evoked response, or detected and/or measured artifact. Further, the sensing circuit of the rhythm management device of the present invention reduces afterpotentials that result due to delivery of the stimulation pulses.
II. Discussion of the Prior Art
Cardiac rhythm management devices have enjoyed widespread use and popularity over the years as a means for supplanting some or all of an abnormal heart's natural pacing functions. The various heart abnormalities remedied by these stimulation devices include total or partial heart block, arrhythmias, myocardial infarctions, congestive heart failure, congenital heart disorders, and various other rhythm disturbances within the heart. The cardiac rhythm management devices generally include a pulse generator for generating stimulation pulses to the heart electrically coupled to an electrode lead arrangement (unipolar or bipolar) positioned adjacent or within a pre-selected heart chamber for delivering pacing stimulation pulses.
Regardless of the type of cardiac rhythm management device that is employed to restore the heart's natural rhythm, all operate to stimulate excitable heart tissue cells adjacent to the electrode of the lead. Myocardial response to stimulation or “capture” is a function of the positive and negative charges found in each myocardial cell within the heart. More specifically, the selective permeability of each myocardial cell works to retain potassium and exclude sodium such that, when the cell is at rest, the concentration of sodium ions outside of the cell membrane is significantly greater than the concentration of sodium ions inside the cell membrane, while the concentration of potassium ions outside the cell membrane is significantly less than the concentration of potassium ions inside the cell membrane.
The selective permeability of each myocardial cell also retains other negative particles within the cell membrane such that the inside of the cell membrane is negatively charged with respect to the outside when the cell is at rest. When a stimulus is applied to the cell membrane, the selective permeability of the cell membrane is disturbed and it can no longer block the inflow of sodium ions from outside the cell membrane. The inflow of sodium ions at the stimulation site causes the adjacent portions of the cell membrane to lose its selective permeability, thereby causing a chain reaction across the cell membrane until the cell interior is flooded with sodium ions. This process, referred to as depolarization, causes the myocardial cell to have a net positive charge due to the inflow of sodium ions. The electrical depolarization of the cell interior causes a mechanical contraction or shortening of the myofibril of the cell. The syncytial structure of the myocardium will cause the depolarization originating in any one cell to radiate through the entire mass of the heart muscle so that all cells are stimulated for effective pumping. Following heart contraction or systole, the selective permeability of the cell membrane returns and sodium is pumped out until the cell is re-polarized with a negative charge within the cell membrane. This causes the cell membrane to relax and return to the fully extended state, referred to as diastole.
In a normal heart, the sino-atrial (SA) node initiates the myocardial stimulation of the atrium. The SA node comprises a bundle of unique cells disposed within the roof of the right atrium. Each cell membrane of the SA node has a characteristic tendency to leak ions gradually over time such that the cell membrane periodically breaks down and allows an inflow of sodium ions, thereby causing the SA node cells to depolarize. The SA node cells are in communication with the surrounding atrial muscle cells such that the depolarization of the SA node cells causes the adjacent atrial muscle cells to depolarize. This results in atrial systole wherein the atria contract to empty blood into the ventricles.
The atrial depolarization from the SA node is detected by the atrioventricular (AV) node which, in turn, communicates the depolarization impulse into the ventricles via the Bundle of His and Purkinje fibers following a brief conduction delay. In this fashion, ventricular systole lags behind atrial systole such that the blood from the ventricles pumps through the body and lungs after being filled by the atria. Atrial and ventricular diastole follow wherein the myocardium is re-polarized and the heart muscle relaxed in preparation for the next cardiac cycle. It is when this system fails or functions abnormally that a cardiac rhythm management device may be needed to deliver an electronic stimulation pulse for selectively depolarizing the myocardium of the heart so as to maintain proper heart rate and synchronization of the filling and contraction of the atrial and ventricular chambers of the heart.
The success of a stimulation pulse in depolarizing or “capturing” the selected chamber of the heart hinges on whether the output of the stimulation pulse as delivered to the myocardium exceeds a threshold value. This threshold value, referred to as the capture threshold, is related to the electrical field intensity required to alter the permeability of the myocardial cells to thereby initiate cell depolarization. If the local electrical field associated with the stimulation pulse does not exceed the capture threshold, then the permeability of the myocardial cells will not be altered enough and thus no depolarization will result. If, on the other hand, the local electrical field associated with the stimulation pulse exceeds the capture threshold, then the permeability of the myocardial cells will be altered sufficiently such that depolarization will result.
Changes in the capture threshold may be detected by monitoring the efficacy of stimulating pulses at a given energy level. If capture does not occur at a particular stimulation energy level which previously was adequate to effect capture, then it can be surmised that the capture threshold has increased and that the stimulation energy should be increased. On the other hand, if capture occurs consistently at a particular stimulation energy level over a relatively large number of successive stimulation cycles, then it is possible that the capture threshold has decreased such that the stimulation energy is being delivered at level higher than necessary to effect capture. Alternatively, the electrocardiogram signal may be utilized to surmise whether a change in amplitude in the electrocardiogram signal at a particular time is the result of an intrinsic event or evoked response.
The ability of a rhythm management device to detect capture is desirable in that delivering stimulation pulses having energy far in excess of the patient's capture threshold is wasteful of the rhythm management device's limited power supply. In order to minimize current drain on the power supply, it is desirable to automatically adjust the device such that the amount of stimulation energy delivered to the myocardium is maintained at the lowest level that will reliably capture the heart. To accomplish this, a process known as captur
Gryzwa Mark
Zhu Qingsheng
Cardiac Pacemakers Inc.
Mesereau C. G.
Nikolai & Mesereau, P.A.
Schaetzle Kennedy
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