The electrophysiologic effects of upright posture.

In order to determine the effects of upright posture on the electrophysiologic properties of the human heart, 12 patients underwent electrophysiologic studies in the supine and upright positions. Compared to supine, the upright position significantly reduced basic cycle length from 818 +/- 111 to 680 +/- 141 msec, sinoatrial conduction time from 186 +/- 94 to 135 +/- 56 msec, corrected sinoatrial recovery time from 206 +/- 104 to 108 +/- 55 msec, interatrial conduction time from 76 +/- 17 to 70 +/- 16 msec, and the AV nodal conduction time (AH interval) from 88 +/- 19 to 78 +/- 14 msec (all p less than 0.05). Right intra-atrial and His-Purkinje (HV interval) conduction times were not altered. When the subjects assumed the upright position, the effective refractory periods of the right atrium, atrioventricular node, and right ventricle decreased significantly at basic and paced cycle lengths. For the group as a whole, the upright posture did not significantly augment repetitive responses to atrial and ventricular extrastimuli.