Sympathetic discharge and vascular resistance after bed rest

The effect of -6°head-down-tilt bed rest (HDBR) for 14 days on supine sympathetic discharge and cardiovascular hemodynamics at rest was assessed. Mean arterial pressure, heart rate (n = 25), muscle sympathetic nerve activity (MSNA; n = 16) burst frequency, and forearm blood flow (n = 14) were measured, and forearm vascular resistance (FVR) was calculated. Stroke distance, our index of stroke volume, was derived from measurements of aortic mean blood velocity (Doppler) and R-R interval (n = 7). With these data, an index of total peripheral resistance was determined. Heart rate at rest was greater in the post (71 ± 2 beats/min)- compared with the pre-HDBR test (66 ± 2 beats/min; P < 0.003), but mean arterial pressure was unchanged. Aortic stroke distance during post-HDBR (15.5 ± 1.1 cm/beat) was reduced from pre- HDBR levels (20.0 ± 1.5 cm/beat) (P < 0.03). Also, MSNA burst frequency was reduced in the post (16.7 ± 2.8 beats/min)- compared with the pre (25.2 ± 2.6 beats/min)-HDBR condition (P < 0.01). Bed rest did not alter forearm blood flow, FVR, or total peripheral resistance. Thus reductions in MSNA with HDBR were not associated with a decrease in FVR.