Cardiac asystole during an hypoxic drive study in a normal subject

A healthy 31-year-old man who had previously sojourned to an altitude of 5,000 meters with no detrimental effect developed sudden cardiac asystole during a progressive hypoxic ventilatory response (HVR) test. At the moment of asystole, his alveolar PO₂ (PAO₂) was 41 mm Hg and his arterial oxygen saturation (SaO₂) was 81 percent. Cardiopulmonary resuscitation was initiated, and after 20 seconds of asystole and apnea, he recovered normal sinus rhythm and spontaneous respiration. A subsequent ECG and cardiac enzyme levels were normal. During testing, he demonstrated depressed ventilation in response to hypoxia and a slowing of the heart-rate. Careful observation of heart rate and breath-by-breath ventilation during HVR tests may predict this potentially fatal complication.