Hemodynamic Effects of Artificial Pacing

The hemodynamic changes associated with complete heart block complicating acute myocardial infarction and the effects of artificial pacing at various rates on these measurements were studied in 13 patients. Studies were performed on the first day of complete block in all patients and were repeated on subsequent days in six. With two exceptions cardiac output was increased by pacing. In patients paced at progressively increasing ventricular rates, the rate associated with the maximal cardiac output exceeded 100/min, with one exception. However, an adequate response usually occurred between 80 and 90/min. Stroke volume fell as the rate was increased with pacing in most patients, but rose in some, suggesting improved myocardial performance. Systemic blood pressure increased, and the change paralleled cardiac output changes fairly closely as pacing rates were increased. The blood pressure did not, however, reflect reliably the absolute level of cardiac output either in heart block or after the return of sinus rhythm. Systemic vascular resistance was high in most patients with low cardiac outputs and fell with pacing at maximal output rates. There was no consistent change in mean right atrial or mean pulmonary arterial pressure with pacing during the periodof study. Tension time index was low in heart block, and increased considerably with pacing, implying a corresponding increase in myocardial oxygen requirements. Pacing improved signs of depressed mental function and poor skin circulation which were invariably associated with a severe reduction in cardiac output. These signs were found to be superior to heart rate and blood pressure in clinical evaluation of the adequacy of the cardiac output in heart block and the response to pacing.