Cardiac Pacemaking

Abstract

Six patients 1 to 20 months after surgical implantation of an internal cardiac pacemaker with a fixed ventricular rate of 64 per minute underwent right heart catheterization for hemodynamic studies at rest and during a light exercise stress. At rest all exhibited normal right heart presences with a low cardiac index. In 4 of 6 there was an abnormally large arteriovenous oxygen difference. Arterial oxygen saturation was normal in 4, decreased in 2. Arterial pH, PO2 and PCO2 was normal in all 6 at rest. During the 5th to 8th minute of exercise sufficient to increase resting oxygen consumption 2 to 4 fold, 2 types of responses were observed. Group I (3 patients) exhibited a marked rise in mean pulmonary artery pressure, exaggerated arteriovenous oxygen difference with no significant change in cardiac index or stroke volume. Group II maintained normal pulmonary pressures, increased cardiac index by increasing the ventricular rate in 1 (appearance of sinus rhythm) and by increasing stroke volume in the other. All, however, demonstrated an abnormal increase in arteriovenous oxygen difference during exercise. The importance of atrial transport to ventricular filling and cardiac output, particularly in the presence of a diseased myocardium, is discussed. It is suggested that improvement in cardiac function in patients with a fixed ventricular rate might result from optimum setting of ventricular rate, i.e. 75-80/minute, utilization of the contribution of atrial contraction to ventricular filling and thereby to stroke volume by use of a synchronous pacemaker.