Initial adjustment of cardiac output in response to onset of exercise in patients with chronic pacemaking as studied by the measurement of pulmonary blood flow

The increase in cardiac output of normal human subjects during exercise is mainly supported by an increment in heart rate. It was observed in dogs with denervated cardiac nerves, however, that the cardiac output was quickly augmented despite the slow rise in heart rate. 1 It was, therefore, of great interest to study the rate of increase in cardiac output at the beginning of exercise in human patients whose heart rate could not change quickly because of heart block and an implanted pacemaker. In previous publications ~-4 a technique was introduced to determine changes in pulmonary blood flow during transient states Such as the beginning of exercise or elevation of intrathoracic pressure. The technique consisted of the rapid inhalation of a low-concentration C2H~ gas mixture, the expiration of the mixture at a constant flow rate, and continuous analysis of C~H~ fraction in the expired gas by the use of a glow discharge rapid gas analyser. 5-7 Since the rate of decrease in the C~H2 fraction depended on pulmonary blood flow and pulmonary gas volume, the pulmonary blood flow for each sequential small time interval could be detected from the continuous recording of the C~H~ fraction in the expired gasY -4 Five patients with a chronically implanted pacemaker, Medtronic 5942, {Table I) inhaled a