SLOWING OF CAROTID-CARDIAC BAROREFLEX WITH STANDING AND WITH ISOMETRIC AND DYNAMIC MUSCLE-ACTIVITY

We hypothesized that the carotid-cardiac baroreflex becomes slowed in conditions with increased sympathetic activity. Changes in heart rate (HR) and blood pressure in response to 10-s trains of 50-mmHg pulses o f neck suction (NS) were studied in six male subjects during supine re st, upright rest, isometric arm exercise at 30% of maximum voluntary c ontraction, and dynamic leg exercise at 100 W in the sitting position. Estimated mean carotid distending pressure increased by similar to 20 mmHg with 50-mmHg, QRS-triggered, pulsatile NS. Repeated NS sequences were performed in each condition. The amplitude of the bradycardic re sponse was highly variable among the subjects and did not differ signi ficantly between conditions, mean values ranging from 0.3 to 0.6 beats . min(-1). mmHg(-1). In supine rest, the full bradycardic response ap peared within <1 s, i.e., during or immediately after the R-R interval of the first NS pulse. In the other conditions it took significantly longer, 2-3 s or three to seven R-R intervals, for the full HR respons es to develop. Our results support the notion that the carotid-cardiac baroreflex in humans becomes slowed under conditions of concurrent sy mpathetic stimulation.