1. Long-term oxygen therapy in appropriate patients prolongs survival and corrects neuropsychological function. Some tests of mental function paradoxically improve during short periods of oxygen withdrawal in patients on long-term oxygen therapy, although the mechanism of this response is unknown.

2. To evaluate the effects of transient hypoxaemia on plasma adrenaline and noradrenaline levels, we studied eight oxygen-dependent patients who underwent either a 4 h period of oxygen withdrawal or their routine therapy, in a randomized, blinded fashion, on 2 separate days.

3. Plasma noradrenaline did not differ at baseline between study days. During normoxic conditions, plasma noradrenaline levels did not increase significantly with time. By contrast, under hypoxic conditions, plasma levels of noradrenaline rose significantly from 0 to 4 h (P < 0.05). The magnitude of the noradrenaline response was correlated with baseline noradrenaline such that subjects with the highest resting levels had the largest increase during hypoxia (r = 0.95, P < 0.001).

4. Plasma adrenaline did not differ at baseline between study days and there were no significant effects of hypoxia on plasma adrenaline levels.

5. We conclude that the sympathetic nervous system, but not the adrenal medulla, is stimulated in chronically oxygen-dependent subjects made acutely hypoxaemic. The magnitude of this stimulation appears to be related to the state of sympathetic nervous system activity at baseline. Improvement in some tests of mental function during transient periods of oxygen withdrawal may be due to non-specific arousal caused by a catecholamine surge.

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