Hypotension induced by central hypovolaemia and hypoxaemia.

Our question was whether the reduced orthostatic tolerance that accompanies hypoxaemia in some (not all) subjects might be associated with an abnormally large release of adrenaline. Eight normal young men were exposed to lower body negative pressure (LBNP) at -30 to -40 mmHg while breathing air or 10% O2 in N2. Four subjects developed hypotension and bradycardia whenever LBNP was applied during hypoxaemia; four showed a rise in heart rate and stable blood pressure. During normoxia plasma adrenaline concentration did not rise during LBNP in any subject, nor during hypoxaemia plus LBNP in the subjects who remained normotensive. In the four men whose heart rates and blood pressures fell during LBNP with hypoxaemia, adrenaline rose markedly, reaching 200-1600 pg ml-1. All subjects showed similar elevations in noradrenaline concentration during LBNP in both normoxia and hypoxaemia. The results suggest that reduced tolerance to central hypovolaemia during hypoxaemia could stem from known vasomotor and cardiac effects attending high plasma concentrations of adrenaline.