Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-03-19
2003-01-21
Jastrzab, Jeffrey R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06510343
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to cardiac stimulators and, more particularly, to dual-chamber cardiac stimulators that have an improved ability to detect tachyarrhythmias.
2. Description of the Related Art
As most people are aware, the human heart is an organ having four chambers. A septum divides the heart in half, with each half having two chambers. The upper chambers are referred to as the left and right atria, and the lower chambers are referred to as the left and right ventricles. Deoxygenated blood enters the right atrium through the pulmonary veins. Contraction of the right atrium and of the right ventricle pump the deoxygenated blood through the pulmonary arteries to the lungs where the blood is oxygenated. This oxygenated blood is carried to the left atrium by the pulmonary veins. From this cavity, the oxygenated blood passes to the left ventricle and is pumped to a large artery, the aorta, which delivers the pure blood to the other portions of the body through the various branches of the vascular system.
In the normal human heart, the sinus node (generally located near the junction of the superior vena cava and the right atrium) constitutes the primary natural pacemaker by which rhyhnic electrical excitation is developed. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers. In response to this excitation, the atria contract, pumping blood from those chambers into the respective ventricles. The impulse is transmitted to the ventricles through the atrioventricular (AV) node to cause the ventricles to contract. This action is repeated in a rhythmic cardiac cycle in which the atrial and ventricular chambers alternately contract and pump, then relax and fill. One-way valves between the atrial and ventricular chambers in the right and left sides of the heart and at the exits of the right and left ventricles prevent backflow of the blood as it moves through the heart and the circulatory system.
The sinus node is spontaneously rhythmic, and the cardiac rhythm originating from the sinus node is referred to as sinus rhythm. This capacity to produce spontaneous cardiac impulses is called rhythmicity. Some other cardiac tissues also possess this electrophysiologic property and, hence, constitute secondary natural pacemakers. However, the sinus node is the primary pacemaker because it has the fastest spontaneous rate and because the secondary pacemakers tend to be inhibited by the more rapid rate at which impulses are generated by the sinus node.
The resting rates at which sinus rhythm occurs in normal people differ from age group to age group, generally ranging between 110 and 150 beats per minute (“bpm”) at birth, and gradually slowing in childhood to the range between 65 and 85 bpm usually found in adults. The resting sinus rate, typically referred to simply as the “sinus rate,” varies from one person to another and, despite the aforementioned usual adult range, is generally considered to lie anywhere between 60 and 100 bpm (the “sinus rate range”) for the adult population.
A number of factors may affect the sinus rate, and some of those factors may slow or accelerate the rate sufficiently to take it outside of the sinus rate range. Slow rates (below 60 bpm) are referred to as sinus bradycardia, and high rates (above 100 bpm) are referred to as sinus tachycardia. In particular, sinus tachycardia observed in healthy people arises from various factors which may include physical or emotional stress, such as exercise or excitement, consumption of beverages containing alcohol or caffeine, cigarette smoking, and the ingestion of certain drugs. The sinus tachycardia rate usually ranges between 101 and 160 bpm in adults, but has been observed at rates up to (and in infrequent instances, exceeding) 200 bpm in younger persons during strenuous exercise.
Sinus tachycardia is sometimes categorized as a cardiac arrhythmia, since it is a variation from the normal sinus rate range. Arrhythmia rates which exceed the upper end of the sinus rate range are termed tachyarrhythmias. Healthy people usually experience a gradual return to their normal sinus rate after the removal of the factors giving rise to sinus tachycardia. However, people suffering from disease may experience abnormal arrhythmias that may require special, and in some instances immediate, treatment. In this text, we typically refer to abnormally high rates that have not yet been determined to be caused by myocardial malfunction as tachycardias and to abnormally high rates that have been determined to be caused by myocardial malfunction as tachyarrhythmias.
It should also be appreciated that an abnormal tachyarrhythmias may initiate fibrillation. Fibrillation is a tachyarrhythmia characterized by the commencement of completely uncoordinated random contractions by sections of conductive cardiac tissue of the affected chamber, quickly resulting in a complete loss of synchronous contraction of the overall mass of tissue and a consequent loss of the blood-pumping capability of that chamber.
In addition to rhythmicity, other electrophysiologic properties of the heart include excitability and conductivity. Excitability, which is the property of cardiac tissue to respond to a stimulus, varies with the different periods of the cardiac cycle. As one example, the cardiac tissue is not able to respond to a stimulus during the absolute refractory phase of the refractory period, which is approximately the interval of contraction from the start of the QRS complex to the commencement of the T wave of the electrocardiogram. As another example, the cardiac tissue exhibits a lower than usual response during another portion of the refractory period constituting the initial part of the relative refractory phase, which is coincident with the T wave. Also, the excitability of the various portions of the cardiac tissue differs according to the degree of refractoriness of the tissue.
Similarly, the different portions of the heart vary significantly in conductivity, which is a related electrophysiologic property of cardiac tissue that determines the speed with which cardiac impulses are transmitted. For example, ventricular tissue and atrial tissue are more conductive than AV junction tissue. The longer refractory phase and slower conductivity of the AV junction tissue give it a significant natural protective function, as described in more detail later.
For a variety of reasons, a person's heart may not function properly and, thus, endanger the person's well-being. Most typically, heart disease affects the rhythmicity of the organ, but it may also affect the excitability and/or conductivity of the cardiac tissue as well. As most people are aware, medical devices have been developed to facilitate heart function in such situations. For instance, if a person's heart does not beat properly, a cardiac stimulator may be used to provide relief. A cardiac stimulator is a medical device that delivers electrical stimulation to a patient's heart. A cardiac stimulator generally includes a pulse generator for creating electrical stimulation pulses and a conductive lead for delivering these electrical stimulation pulses to the designated portion of the heart. As described in more detail below, cardiac stimulators generally supply electrical pulses to the heart to keep the heart beating at a desired rate, although they may supply a relatively larger electrical pulse to the heart to help the heart recover from fibrillation.
Early pacemakers were devised to treat bradycardia. These pacemakers did not monitor the condition of the heart. Rather, early pacemakers simply provided stimulation pulses at a fixed rate and, thus, kept the heart beating at that fixed rate. However, it was found that pacemakers of this type used an inordinate amount of energy due to the constant pulse production. Even the sinus node of a heart in need of a pacemaker often provides suitable rhythmic stimulation occasionally. Accordingly, if a heart, even for a short period, is able
Armstrong Randolph Kerry
Cook Douglas Jason
Intermedics Inc.
Jastrzab Jeffrey R.
Schwegman Lundberg Woessner & Kluth P.A.
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