Effect of rate-adapting atrioventricular delay on stroke volume and cardiac output during atrial synchronous pacing

The need to adjust atrioventricular delay relative to pacing rate in atrial synchronous pacemakers was assessed in eight subjects treated for complete heart block (mean age 61 +/- 3.4 years). First, an inverse relationship between heart rate and PR interval was confirmed in two groups of healthy subjects and one group of patients recovering from myocardial infarction. Due to the limitations of the available pacemakers this relationship could not be precisely reproduced. Subjects with pacemakers performed an initial exercise test on a bicycle ergometer to determine heart rate response; this information was used in part 2 to program atrioventricular delay relative to heart rate. During a second exercise test, a rate-adapting atrioventricular delay was compared to a constant atrioventricular delay of 200 ms at matched heart rates. Cardiac output was measured noninvasively by impedance cardiography. The results of part 1 showed an abnormal pattern in the response of stroke volume to exercise in seven subjects. Part 2 results demonstrated no significant difference (P greater than 0.05) between a rate-adapting atrioventricular delay and a constant delay of 200 ms for heart rate, stroke volume, cardiac output, blood pressure or Borg scale. A trend towards increased stroke volume with a rate-adapting atrioventricular delay was observed. Stroke volume was 13% larger with a rate-adapting atrioventricular delay of 125 ms at a heart rate of 125 beats/min. These results suggest that subjects with atrial synchronous pacemakers have a variable stroke volume response to exercise. It appears that in these subjects the ability to increase heart rate is the key factor for raising cardiac output during exercise.