[JNUS]

CLINICAL BACKGROUND

A 22-year-old male recreational diver was performing an open-ocean dive using a closed-circuit rebreathing apparatus with a group of fellow recreational divers. When he surfaced, he was unconscious; observers do not recall if his mouthpiece was in place. He was immediately placed into a nearby dive boat.

On-site divers trained in basic life support noted that the diver was initially unconscious and apneic, though he had a pulse. They delivered 2 rescue breaths, after which the diver began breathing spontaneously. He quickly regained consciousness and was immediately given oxygen 10 L/min by face mask. Within about 30 seconds, he began coughing pink, frothy sputum. The diver reported feeling normal except for fullness in his chest and difficulty in taking a deep breath. Focused neurologic examinations and assessments of his vital signs were performed at 5-minute intervals. The findings were reported as normal and stable during the trip to shore, which was approximately 30 minutes.

On presentation, the patient states that he had difficulty maintaining a proper partial pressure of oxygen (PO2) throughout his dive. He believes that his PO2 was too low and that his vision was graying out as he ascended to the surface. According to the dive master, the group’s diving profile was consistent with a no-decompression dive, with a maximum depth of 75 feet of seawater. The divers are uncertain about how long the patient was unconscious, though they remember seeing him conscious at approximately 10 feet of seawater during the ascent. If correct, this would have meant that the patient was unconscious for about 20 seconds underwater at a standard ascent rate of 30 ft/min.

On physical examination, the patient’s face-mask reservoir contains approximately 20 mL of pink, frothy sputum. The patient is a healthy-appearing man in no acute distress, but his work of breathing is increased. He has a pulse of 80 bpm, a blood pressure of 104/89 mm Hg, and an oxygen saturation of 98% while breathing oxygen 10 L/min by face mask and 92% while breathing room air. The neurologic examination reveals no deficits. His speech is normal, and there is no fullness of the neck, jugular venous distention, or tracheal deviation. Auscultation of the left lower lung field reveals diminished breath sounds. The remainder of the physical examination yields no abnormalities.

A chest radiograph is unrevealing. A computed tomography (CT) scan of his chest is obtained.

What was the cause of the loss of consciousness on ascent?

HINT

The cause is similar to the cause of shallow-water blackout. By definition, the final diagnosis requires that the patient survive — at least temporarily — after the initial accident.