Repeated Hypoxic Syncope in a Helicopter Pilot at a Simulated Altitude of 18,000 Feet

Abstract

Military aircrew members undergo regular aviation physiology training, including hypobaric chamber flight to demonstrate the effects of hypoxia at altitude on human physiology and to strengthen hypoxia awareness. Premature near syncope under hypoxia will jeopardize flight safety. However, the causes of such abnormality may not be apparent. We present a case of a healthy army helicopter pilot who suffered repeated premature near syncope and bradycardia during chamber flight at 18,000 ft (5486 m). He had no previous record of fainting. A passive head-up tilt table study under normoxia revealed cardiovascular responses compatible with vasovagal syncope. After considering the nature of the abnormality, mission characteristics, and the operating altitude for the helicopter, the pilot was granted a restricted waiver, with missions limited to below 10,000 ft (3048 m) in multicrew helicopters. Evaluation of hypoxic syncope focuses on cardiovascular, neurologic, and psychiatric systems. The possibility of newly acquired dysautonomia as the cause of hypoxia syncope was supported by normal hypoxia tolerance in previous chamber flights and the results of the head-up tilt table study. The possible pathophysiology of hypoxic syncope and its association with a vasovagal constitution is presented. For a previously healthy helicopter pilot, impaired hypoxia tolerance may cause in-flight incapacitation. The cause should be investigated and the possibility of recurrence in flight should be evaluated before an adequate aeromedical decision can be made.