Potential benefit from an H1-receptor antagonist on postexercise syncope in the heat

H1-receptors mediate the early portion (i.e., first 30 min after exercise) of postexercise hypotension. Immediately after exercise, syncope can occur due to an exaggerated form of postexercise hypotension. Therefore, we hypothesized that orthostatic hypotension occurring immediately after exercise would be attenuated with an H1-receptor antagonist.

We studied 15 endurance exercise-trained men and women in an environmental chamber set at 35 degrees C and 30.0% humidity. Subjects were studied in the supine position before a 45-min bout of treadmill running at 50% of VO2max. Immediately after exercise, measurements were taken in the supine position before the subjects were moved from a supine to a 60 degrees head-up tilt. Measurements included arterial pressure, heart rate, and brachial and cutaneous blood flow on a control and an H1-receptor antagonist (blockade) day.

Mean arterial pressure was reduced 1 min into the tilt compared with preexercise values on the control day (76.2 +/- 0.5 vs 74.2 +/- 0.5 mm Hg; P < 0.05). This reduction was not seen on the blockade day (75.2 +/- 0.3 vs 75.0 +/- 0.5 mm Hg; P > 0.41). There were no differences in brachial vascular conductance (calculated as flow/pressure) in response to the head-up tilt between the study days (P > 0.23). The length of the head-up tilt was compared between study days for each subject. When contrasting this difference, the blockade lengthened the mean tilt time by 94 s (P = 0.098).

These data suggest that an H1-receptor antagonist could potentially benefit postexercise syncope in a hot environment.