Further study on the carotid baroreflex system in the cardiovascular deconditioning induced by head-down tilt

The responses of the carotid baroreflex and of the peripheral sympathetic system to stimulations induced by either lower body negative pressure (LBNP -40 mmHg) or cold pressor test were investigated in eight volunteers before and after 48 h in the -6 degrees head-down tilt (HDT).

Geometry (diastolic diameter and pulsatile distention) and dynamics (cross-sectional compliance and tangential tension) of the bulb and the common carotid artery were investigated using ultrasonic devices, echotracking and aplanation tonometry. The activity of the sympathetic system was evaluated through measurements of plasma concentrations of catecholamines (CAs) and 3,4-dihydroxyphenyl glycol (DHPG).

During LBNP -40 mmHg, the pulsed tangential tension was decreased and the pressure amplification, induced by the reflexion of the pressure wave, was increased with no difference between before and after HDT. Since cross-sectional compliance and distensibility coefficient remained unchanged and the carotid contour of the waveform unaltered, it is concluded that the carotid reflexogenic area reads the same message during LBNP whether the cardiovascular system was deconditioned or not. Nonetheless, during LBNP after 48 h HDT, the heart rate accelerated faster and CAs and DHPG concentrations increased out of proportion, suggesting that the peripheral sympathetic activity was more reactive after HDT than before. Finally, forearm vascular resistances were measured in response to cold pressor test; they increased in the same proportion after HDT when compared with before.

Results indicate that the carotid baroreflex and the peripheral sympathetic system were not deficient after 48 h HDT.