The constraints on implantable cardiac pacemaker circuit design are particularly demanding. The pacemaker must stimulate the heart at an appropriate rate for long periods of time, preferably without attention. Its behavior should be predictable, its failure modes benign and the onset of malfunction, particularly battery exhaustion should be readily detectable. These characteristics must be maintained within a warm, humid and electromagnetically complex environment. Finally the size, shape and composition of the materials in contact with the tissues must be readily tolerated by the host.