The technology of cardiac pacemakers has developed to a high level of sophistication of system performance. The current generation of cardiac pacemakers incorporate microprocessors and related circuitry to sense and stimulate heart activity under a variety of physiological conditions. These pacemakers may be programmed to control the heart in correcting or compensating for various heart abnormalities which may be encountered in individual patients. A detailed description of modern cardiac pacemaker technology is set forth in International Application No. PCT/US85/02010, entitled STIMULATED HEART INTERVAL MEASUREMENT, ADAPTIVE PACER AND METHOD OF OPERATION, assigned to the assignee hereof. The disclosure of that application is incorporated herein by reference.
A demand-type pacemaker is one that provides a stimulation pulse only when the heart fails to produce a natural depolarization on its own within a prescribed escape interval. In a dual chamber pacemaker, this is realized by placing electrodes in both the right atrium and right ventricle of the heart. These electrodes are coupled through intravenous and/or epicardial leads to sense amplifiers housed in an implanted pacemaker. Electrical activity occurring in these chambers can thus be sensed. When electrical activity is sensed, the pacemaker assumes that a depolarization or contraction of the indicated chamber has occurred. If no electrical activity is sensed within a prescribed time interval, typically referred to as an atrial or ventricular escape interval, then a pulse generator, also housed within the pacemaker housing, generates a stimulation pulse that is delivered to the indicated chamber, usually via the same lead or electrode as is used for sensing. This stimulation pulse causes or forces the desired depolarization and contraction of the indicated chamber to occur. Thus, with a demand pacer, the heart will either beat on its own (without stimulation from the pacemaker) at a rate that is at least just slightly faster than the stimulation rate defined by the escape interval, or the heart will be stimulated by the pacer at a rate controlled by the escape interval. The stimulation rate provided by the pacemaker is typically referred to as the "programmed rate."
As noted, most pacemakers include a sensor circuit that looks for electrical signals from spontaneous heart activity. On detection of such activity, the pacemaker stimulation action is modified, depending upon the functional mode or type of pacemaker. For example, in the VVI mode (ventricle paced and sensed, response inhibited mode), sensing of heart activity under certain time restrictions is interpreted as normal heart activity such that the stimulating action is inhibited.
The discussion thus far has followed the assumption that a pacemaker and its associated circuitry operate without malfunction. By the very nature of manmade devices, such is not always the case. Whereas electronic circuitry can be, and is, incorporated within the pacemaker itself for exercising or testing various circuit components, the status of battery power sources, and the effectiveness of various amplifiers, waveform shaping stages and the like, it is often more difficult to test the integrity of the leads and implanted electrodes to which the pacemaker is coupled for pacing operation.
At the implanting of the pacemaker and electrode system, minor damage is sometimes incurred which may affect the system's electrical insulation. This type of damage may go undetected and be without present effect on the implanted system, but the condition may manifest itself after extended time in service. When a breakdown or significant degradation of the pacemaker lead insulation occurs, it can have serious or even disastrous results, depending upon whether or not the breakdown is of a catastrophic nature. Various types of lead damage may produce different types of failure or degradation. For example, an insulation defect on the stimulation electrode shunts the energy intended for the heart to some other point. A lead breakage can in some environmental situations temporarily or permanently reduce the stimulation output, sometimes drastically. Another type of detectable error relates to the failure of the electrode tip to be in proper contact with the heart wall.
While the lead defects which have been mentioned thus far are more in the nature of catastrophic failures, there may also occur failures or degradation of a less drastic nature which may be intermittent or which may build up over time. In a common situation, errors often start as temporary or intermittent errors. These can be virtually impossible to discover with commonly used techniques. It is not permissible to test on a random or periodic basis for lead faults in the manner in which power lines or telephone lines, for example, may be tested as, for example, by applying an over-voltage to a suspected circuit, simply because the breakdown of insulation in pacemaker leads under such condition may produce catastrophic results in the patient. It would be desirable to be able to use the signals encountered in the normal operation of an implanted pacemaker in the process analysis to determine impending failure or serious degradation from the temporary or intermittent errors which may be detected. A system for performing such a function would be expected to monitor standard heart operation and to use detected deviations or departures from signal norm to indicate the occurrence of such.
There may be various approaches to the regular monitoring of heart signals for the detection of abnormalities, some of which may relate to the circuitry employed in the implanted pacemaker system. One asserted pacemaker function analyzer for automatic evaluation and indication of the quality of performance of cardiac pacing systems is the subject of U.S. Pat. No. 4,527,567 of Fischler et al. The analyzer of that patent is said to provide a comprehensive examination of asynchronous, demand and demand-hysteresis pacemakers of all makes, including the state of the pacemaker's battery, the intactness of the electronic circuitry and of the electrodes, and the proper location of the electrodes in the heart. The manner in which this analysis is performed by the circuitry of that patent is entirely different from the operation of the lead impedance scanning system of the present invention.