FURTHER STUDY ON THE CAROTID BAROREFLEX SYSTEM IN THE CARDIOVASCULAR DECONDITIONING INDUCED BY HEAD-DOWN TILT

Hypothesis: The responses of the carotid baroreflex and of the periphe ral sympathetic system to stimulations induced by either lower body ne gative pressure (LBNP -40 mmHg) or cold presser test were investigated in eight volunteers before and after 48 h in the -6 degrees head-down tilt (HDT). Methods: Geometry (diastolic diameter and pulsatile diste ntion) and dynamics (cross-sectional compliance and tangential tension ) of the bulb and the common carotid artery were investigated using ul trasonic 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). Results: 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 co efficient remained unchanged and the carotid contour of the waveform u naltered, it is concluded that the carotid reflexogenic area reads the same message during LBNP whether the cardiovascular system was decond itioned or not. Nonetheless, during LBNP after 48 h HDT, the heart rat e accelerated faster and CAs and DHPG concentrations increased out of proportion, suggesting that the peripheral sympathetic activity was mo re reactive after HDT than before. Finally, forearm vascular resistanc es were measured in response to cold presser test; they increased in t he same proportion after HDT when compared with before. Conclusion: Re sults indicate that the carotid baroreflex and the peripheral sympathe tic system were not deficient after 48 h HDT.