A CASE OF BREATH-HOLDING AND ASCENT-INDUCED CIRCULATORY HYPOTENSION

We report a case of transient circulatory depression due to inadverten t apnea of a subject during decompression from a simulated dive. The d ive consisted of exposure to air at 5 bar and subsequent decompression stops. Arterial blood pressure and a lead II ECG were recorded contin uously. During decompression from 1.6 to 1.3 bar, the subject inadvert ently held his breath. Arterial pressure fell rapidly from 120/80 to 6 0/53 mmHg within 20 s. Recognizing that the subject held his breath, o ne of the supervisors ordered him to resume breathing, and arterial bl ood pressure was restored rapidly. This circulatory depression was pro bably due to reduced stroke volume such as described for the syncope o f ascent: with the subject retaining his breath, the expanding lung vo lume increased intrathoracic pressure resulting in impaired venous ret urn.