CYCLE LENGTH DEPENDENCE OF THE ELECTROPHYSIOLOGICAL EFFECTS OF INCREASED LOAD ON THE MYOCARDIUM

Background Mechanoelectric feedback, the process by which changes in m echanical activity change the electrophysiology of the myocardium, has been linked to the genesis of arrhythmias. We investigated possible a rrhythmogenic mechanisms by measuring changes in steady-state action p otential duration and, more particularly, electrical restitution on a transiently applied load change, because action potential recovery may provide clues to arrhythmogenesis. Methods and Results Pigs were anes thetized and their hearts exposed. A snare was placed around the aorta , and the right atrium was paced. Ventricular pressure, monophasic act ion potential, and segment motion were recorded from the left ventricl e. The action potential duration was measured before and during transi ent aortic occlusion. Electrical restitution curves were constructed f rom the records obtained during normal loading or during transient aor tic occlusion. The degree of shortening of action potential duration o n aortic occlusion decreased with decreases in the steady-state beat-t o-beat interval (P=.0008). Control restitution curves had the typical configuration, with a rapid initial, usually monotonic, rise toward a plateau. Some curves showed a marginal ''supernormal'' section. Increa sed load reduced the action potential duration at the plateau of the r estitution curve (9.4 ms, P<.0001) but increased the action potential duration at the start of the restitution curve (8.7 ms, P=.03). Increa sed loading increased the maximum slope of the electrical restitution curve by 32 ms/100 ms CP=.04). Increased load also increased the super normal period of the electrical restitution curves. Conclusions Mechan oelectric feedback produces changes in rate-dependent electrophysiolog y, which could favor a matrix conducive to arrhythmogenesis.