Surgery – Diagnostic testing
Reexamination Certificate
1999-09-22
2001-07-24
Nasser, Jr., Robert L. (Department: 3736)
Surgery
Diagnostic testing
C600S301000, C600S484000, C600S529000, C600S534000, C607S019000, C607S020000
Reexamination Certificate
active
06264606
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ischemia detector and to an implantable heart stimulator having such an ischemia detector.
2. Description of the Prior Art
The blood flow and penetration in the circulatory system of a living subject are dependent on the arterial muscular tension, the so called tonus. Thus the blood flow is controlled by the tonus, the driving force of the flow is the blood-pressure in the elastic aorta and the pressure in the aorta is maintained by the pumping action of the heart. For this pumping action the heart needs energy in the form of oxygen and glucose. About 60% of the oxygen in the heart interstitial fluid is consumed within one heart beat. If the energy supplied to the heart is disturbed the heart contractibility and the pumping action of the heart are severely deteriorated and an oxygen shortage or ischemic situation will rapidly develop. Ischemia results from insufficient blood flow through the heart muscle. Due to blocking or passage congestion of coronary blood vessels of the heart. I ischemia is experienced by the patient as a severe chest pain and is one of the most stressing factors known to the organism. Several techniques for detecting ischemia are known. In U.S. Pat. No. 5,156,148 a system for treating a malfunctioning heart, e.g. ischemia, is known using the variation of selected physiologic parameters. In U.S. Pat. No. 4,821,735 a method and an apparatus for detecting myocardial ischemia are described, wherein the systemic vascular resistance (SVR) in a subject is monitored and the presence of myocardial ischemia is detected when the SVR increases by at least 60% over a base line value.
In U.S. Pat. No. 5,497,780 an apparatus is described for determining an ischemia by measurements of electric potentials between at least three implanted measuring electrodes, two of these electrodes being implanted with their poles in the heart and the third electrode being implanted with its pole lying outside the heart.
In U.S. Pat. No. 5,199,428 a technique is described for detecting ischemia and effecting stimulation of nerves regulating blood pressure and heart rate to reduce the heart's oxygen requirements while providing pacing therapies to maintain the patient's heart rate within acceptable limits to avoid bradyarrhythmias and/or unphysiological AV delays induced by the nerve stimulation. The ischemia detection is based on the occurrence of changes in the ST-segment variation different from pre-determined or programmed threshold levels, or on changes in the pH and/or in the dissolved blood oxygen in venous return blood in the coronary sinus region of the patient's heart.
An ischemic state can also be detected by analysis of recorded IECG's or surface ECG's to determine the heart rate variability. Ischemia can be detected by a lead bend sensor located at the distal end portion of an implanted heart stimulator lead. As the heart wall is thickening and stiffening as the result of an ischemic state, the accompanying change in the moving pattern of the heart wall can be detected in this way. Also sound absorption is affected by changes in the stiffness of the heart tissue and by measuring the absorption of sound waves, generated e.g. at the heart valve closure, on their way from the upper portion of the ventricle to the apex region, an ischemic situation can be detected. Ischemia deteriorates the efficiency of the heart's pumping and an ischemic situation can therefore be detected by studying blood pressures and cardiac outputs, too. Thus, by measuring the difference between the systolic and diastolic pressures and comparing this difference obtained from one heartbeat to the difference obtained from the next heartbeat an ischemic can be detected. An ischemic state can be detected using a flow sensor for measuring cardiac output, as well.
As mentioned above an ischemic state is normally associated with severe pain forcing the patient to sit down or lie down with a reduced heart rate as a consequence. At the same time the patient feels a need for forced breathing, so called hyperventilation.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an ischemia detector the functioning of which is based on the above mentioned needs of a patient experiencing ischemia.
The above object is achieved in accordance with the principles of the present invention in an ischemia detector having a patient workload sensor and a patient breathing sensor, which respectively emit signals which are supplied to a detector unit, the detector unit detecting a state of ischemia dependent on the occurrence of a predetermined relation between the sensed workload and the sensed breathing activity, this predetermined relation being a sensed low workload and a simultaneously sensed high breathing activity.
For a healthy person the need of an increased respiration rate normally results from an increased effort or workload of the person. The ischemia detector according to the invention is based on the occurrence of the abnormal combination of low workload and high breathing activity, which is typical of ischemic patients.
In an advantageous embodiment of the ischemia detector according to the invention an averaging unit is connected to the workload sensor and breathing sensor which forms average values during time periods of pre-determined lengths of workload and breathing activity respectively, and delivers corresponding average signals to said detector unit. In this way false detections due to accidental variations of transitory nature in the workload and breathing activity are avoided.
In another embodiment of the detector according to the invention an alerting unit is activated in response to a detected ischemia.
According to another aspect of the invention an implantable heart stimulator has means for varying the stimulation rate and an ischemia detector as defined above.
The heart stimulator can have a control unit connected to the ischemia detector for controlling the stimulation rate varying means to lower the stimulation rate in response to the detection of an ischemia.
REFERENCES:
patent: 4821735 (1989-04-01), Goor et al.
patent: 5025786 (1991-06-01), Siegal
patent: 5156148 (1992-10-01), Cohen
patent: 5199428 (1993-04-01), Obel et al.
patent: 5497780 (1996-03-01), Zehender
patent: 5515859 (1996-05-01), Paz
patent: 5833713 (1998-11-01), Moberg
patent: 6024705 (2000-02-01), Schlager et al.
Ekwall Christer
Noren Kjell
Nasser Jr. Robert L.
Pacesetter AB
Schiff & Hardin & Waite
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