Implantable monitoring systems: Design considerations and challenges

Abstract

The use of electrocardiographic (ECG) information in the diagnosis and follow-up evaluation of cardiac patients has evolved from the simple limb lead surface ECG to more sophisticated instruments using advanced recording and diagnostic methods. Advances in electronic technology have provided continued improvement in the performance of ECG monitoring equipment such as the i2-1ead resting and stress ECG, the interpretive electrocardiograph, the signal-averaged ECG, ambulatory ECG monitors, and body surface potential mapping systems. Holter I reported in 1961 the use of a portable ECG recording device. With the introduction of this first ambulatory monitor, clinicians had the opportunity to identify the presence of various cardiac rhythms while the patient was performing his or her normal daily activities. Applications of these ambulatory monitors have included identification of arrhythmias, evaluation of patient symptoms, quantification of ischemia (for both anginal and silent episodes), and performance evaluation of implantable pacemakers and defibrillators. 2 Ambulatory monitors are presently available with magnetic tape, digital random access memory, or min i -ha rd disk drives as their data storage medium. Some digital recorders also include real-time E C G analysis,3 which allows the ambulatory monitor to store higher-resolution ECG data during detected events. Variants of ambulatory monitors include event recorders and loop monitors. These monitors are typically used by the patients over longer periods of time, primarily for the identification of less frequent arrhythznias. However, their operation requires that any cardiac events be symptomatic to be recorded, because the patient must direct the storage of the event into memory for future retrieval and clinical review. In a similar manner, pacemaker technology has evolved from the simple nonprogrammable single-chamber pulse generators to dual-chamber rate-modulated pacemakers. The 1970s saw the introduction of the first programmable pacemakers, the use of the long-life lithium batteries, and basic memory storage with the storage of paced and sensed events. The 1980s were highlighted by the introduction