ELECTROCARDIOGRAPHIC AND ELECTROPHYSIOLOGIC PROPERTIES OF CARDIAC ALLOGRAFTS

The increasing number of heart transplant patients requires that physi cians be able to recognize the electrocardiographic (ECG) and electrop hysiologic properties of cardiac allografts. Cardiac allografts are ch aracterized by modifications of resting ECGs and frequent arrhythmias in the postoperative period, and the loss of autonomic nervous control illustrated by permanent tachycardia and loss of heart rate variabili ty during 24-hour ambulatory ECG recording. Some clinical and experime ntal observations suggest a mid-term reinnervation of the cardiac allo graft, but this requires histologic confirmation. The electrophysiolog ic characteristics of the denervated myocardium are similar to those o f the innervated myocardium at rest. However, supersensitivity to circ ulating catecholamines has been observed in cardiac allografts as in e xperimentally denervated hearts, which is responsible for a progressiv e increase in heart rate during exercise and a slow decrease during re covery. Supersensitivity of the denervated heart to acetylcholine may explain the high prevalence of donor sinus dysfunction due to impairme nt of its automaticity. More often, the sinus node dysfunction is tran sient and can be treated with an adenosine antagonist, such as theophy lline, before permanent implantation of a pacemaker. In the case of pa cemaker implantation, synchronization of the donor atria with the reci pient atria is desirable, and an endocardial lead implantation is pref erred. Several electrophysiologic changes have been observed during ac ute cardiac allograft rejection. From experimental studies, the most i mportant of these are the disturbance of conduction in the atria and t he atrioventricular node and a decrease in the amplitude of the ventri cular potential. Initial studies on isolated myocytes show profound ch anges in membrane conductance during experimental cardiac rejection. T he development of new noninvasive detection methods of cardiac allogra ft rejection, such as intramyocardial voltage electrogram monitoring a nd high-resolution EGG, could help early diagnosis.