The Exercise-Responsive Cardiac Pacemaker

The heart has two properties: rhythmicity and contractility. Rhythmic contractions are initiated by the heart's pacemaker, the sinoatrial node, which lies in the right atrium. If the sino-atrial node fails, or if its electrical excitations are not propagated to the ventricles (the main pumping chambers), they wil still contract rhythmically but at a rate that is sometimes too slow to provide enough cardiac output to sustain consciousness. It is for this reason that rhythmic electrical stimuli are delivered to the ventricles in order to increase cardiac output to a level adequate to permit the subject to perform routine daily tasks. The technique of applying rhythmic electrical stimuli to the ventricles is called cardiac pacemaking; it can be achieved in several interesting ways, as this paper will describe.