Introduction: Syncope at altitude of otherwise healthy individuals is a well-known phenomenon (22). We report on the cardiovascular effects observed in subjects exposed to hypoxia to illustrate the role of the sympathetic-adrenergic system in hypoxic syncope. This study describes unexpected episodes of (near) syncope during two crossover trials at simulated altitude in a low pressure chamber.
Methods: In study A, 30 healthy male volunteers underwent 4 exposures to short-term (20 min) acute severe hypoxia (20,000 ft or 6096 m) to assess psychological performance. In study B, five volunteers were studied during prolonged exposure (1 h) to moderate hypoxia (13,500 ft or 4115 m) with and without concomitant low dose infusion with atrial natriuretic peptide to investigate the effects on pulmonary gas exchange.
Results: In study A (acute severe hypoxia), 6 out of 120 exposures (5%), in 5 subjects, were accompanied by lightheadedness, pallor, sweating, and bradycardia. Two subjects (2%) had syncope with cardiac asystole. In study B during moderate hypoxia without atrial natriuretic peptide, adverse reactions were absent and the (nor)epinephrine levels remained unchanged. Concomitant infusion with atrial natriuretic peptide resulted in near syncope (recumbent in 3, standing in 2) at an oxygen saturation of 82%. While the epinephrine level had eightfold increased, mean arterial pressure fell from 94 to 40 mm Hg and heart rate from 79 to 44 bpm. The norepinephrine level remained unchanged illustrating a dissociated sympathetic-adrenergic response. All subjects with syncope recovered spontaneously within few minutes in Trendelenburg's position with oxygen supplied. None suffered from prolonged side effects.
Conclusion: It is concluded that exposure to acute severe hypoxia is a sufficient cause for syncope in healthy individuals. Enhanced vasodilatation to epinephrine may contribute to the withdrawal of sympathetic and enhancement of parasympatic activity, leading to vascular collapse, bradycardia or asystole (Bezold-Jarisch reflex). Patients fully recover in Trendelenburg's position with supplemental oxygen and further clinical examinations are not necessary.