Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-09-22
2001-07-03
Getzow, Scott M. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06256538
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an implantable heart stimulator of the type having control means for controlling the delivery of stimulation pulses and an ischemia detector.
2. Description of the Prior Art
The aortic pressure is produced by the pumping action of the heart. During heart contraction—a systole—the blood pressure inside the heart is equal to or slightly higher than the pressure in the aorta, as long as the aortic valve is open and blood is flowing from the ventricle into the aorta or pulmonary artery. Between the contractions, that is during diastole, the aortic pressure is maintained by the elastic properties of the aortic wall. From the aorta, blood is circulated back to the heart via different parts of the body through the venous system. In this way blood is distributed to different organs of the body depending on the needs, the overall control of this distribution emanating from the autonomous nerve system.
Blood penetration of the heart is possible only in the diastolic phase when the aortic valve is closed. About 60% of the oxygen content inside the heart tissue is consumed during a heart contraction and in order to maintain the efficiency of the heart the consumed oxygen must be refilled till the next contraction.
The effect of an increased workload, due to e.g. movement of the patient, climbing stairs, running etc., is a higher blood flow in the body and a resulting increased need of energy and oxygen. This in its turn results in a more rapid decay of the aortic blood pressure between consecutive heart contractions. In response to such an increased demand a healthy heart is pumping at a higher rate and with an increased pumping force.
An increased heart rate results in only a minor shortening of the systolic phase, that is an increased heart rate results mainly in a shortening of the diastolic phase, which is the period during which oxygen is supplied to the heart as mentioned above.
An ischemic heart has a deficiency of oxygen because of insufficient blood to the heart tissue supply due to congestion and blocking of the coronary vessels by stenosis, emboli or spastic congestion. An increased workload will consequently worsen the situation for an ischemic patient.
In such a situation a symptomatic ischemia, that is angina pectoris, heart insufficiency or infarct, will force the patient, because of the associated pain, to stillness with a reduced heart rate as a consequence. This heart rate reduction can then at best stop the ischemic state.
No pacemaker system able to react on pain or any other ischemic indication is known. As a matter of fact pacemakers normally try to maintain a high stimulation rate, which is appropriate to a normal situation of the patient, and a so called rate response pacemaker system, responding to metabolical, haemodynamical or activity inputs will tend to increase the stimulation rate with increasing workload, thus worsening the ischemic situation of the patient.
In U.S. Pat. No. 5,199,428 a technique is described for detecting ischemia and both affecting stimulation of nerves regulating blood pressure and heart rate to reduce the heart'oxygen requirements while providing pacing therapies to maintain the patient's heart rate within acceptable limits, i.e. to overcome bradyarrhythmias and/or unphysiological AC-delays induced by the nerve stimulation.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a heart stimulator which can be used for therapeutic treatment of an ischemic state of a living subject.
This object is achieved in accordance with the principles of the present invention in an implantable heart stimulator having control means for controlling the delivery of stimulation pulses, and an ischemia detector, wherein the control means are connected to the ischemia detector for reducing the stimulation rate in response to a detected ischemia.
Depending on which kind of ischemia detector is used it is possible to detect an ischemia even before the patient gets any symptoms and thus by therapy prevent the occurrence of symptoms.
In the invention there are different ways of reducing the stimulation rate. According to one embodiment of the stimulator according to the invention the control means inhibit each with stimulation pulse in response to the detection of an ischemia, where n=8, . . . , 2. The resulting prolonged interval between two consecutive stimulation pulses will mimic the situation in normal heart operation where it frequently occurs that one heart beat is suppressed, possibly because of e.g. sudden, pressure changes, low oxygen or low glucose concentrations, and this prolonged interval will improve the supply of oxygen to the heart tissue and improve the possibility of terminating the ischemic state. In addition the prolonged interval will result in an increased blood filling of the heart ventricles, which in its turn will increase the efficiency of the next heart beat and compensate for any drop in the aortic pressure and reduction in blood filling because of the suppressed heart beat. The frequency of suppressed stimulation pulses can be varied depending on the degree of ischemia. To start with e.g. one stimulation pulse out of eight can be inhibited. At an increased degree of ischemia, or sustained ischemia over a period of time despite these suppressed stimulation pulses, one out of four pulses, or one out of two pulses can be suppressed. In the last mentioned example the stimulation rate is halved and the length of the diastolic phase, in which part oxygenic change takes place, is more than doubled, of, the discussion above. According to other advantageous embodiments of the heart stimulator according to the invention said control means are arranged to uniformly reduce the stimulation rate or shift the stimulation rate to a predetermined lower value in response to the detection of an ischemia.
In another embodiment of the heart stimulator according to the invention the control means are arranged to reduce the stimulation rate as a function of the detected degree of ischemia. In the case of a fully developed ischemia the stimulation rate is permitted to drop to a set predetermined minimum rate.
In another embodiment of the heart stimulator according to the invention, the heart stimulator is a rate response controlled rate responsive pacemaker, the control means are arranged to shift the stimulation rate to a base rate below the normal rate response controlled stimulation rate.
In another embodiment of the heart stimulator according to the invention said ischemia detector comprises ischemia analyzing means for detecting an ischemia by analysis of recorded IEGMs or ECGs. In an implantable heart stimulator it is suitable to determine an ischemia by analyzing ECGs since the lead for recording the IEGMs already is in place and the technique for IECG sensing is well established and no special sensors are needed. Body surface ECG diagnostics for determining ischemic states also are well known.
In a further embodiment of the heart stimulator according to the invention the ischemia analyzing means detect an ischemia by analysis of the level of ST-segments and T-wave forms of recorded IEGMs or ECGs. The level of ST-segments and the appearance of T-wave forms are related to repolarization of groups of cells, and the repolarization after a heart contraction is sensitive to haemodynamics, like fuel and oxygen supply.
In a further embodiment of the heart stimulator according to the invention the ischemia analyzing means determine heart rate variability from recorded IEGMs or ECGs. Normally heart beat intervals, AV-conduction intervals and QRS-amplitudes are subject to variations. Heart rate variation is at a maximum for a healthy individual at rest. Activity of the individual or reduction in capacity due to insufficiency or illness is reflected as a decrease in the heart rate variability and this effect is used for ischemia detection.
In a further embodiment of the heart stimulator according to the invention the control means modul
Getzow Scott M.
Pacesetter AB
Schiff & Hardin & Waite
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