Breath holding in divers and non-divers--a reappraisal.

The two phases of breath holding, the voluntary inactive and involuntary active phases, were identified by non-invasive methods using the induction plethysmograph. Eight trained divers and 7 non-diving control subjects familiar with respiratory apparatus were studied. During breath holding from normocapnia and total lung capacity it was not possible to distinguish between the two groups in respect of the pattern or duration of breath holding or alveolar gas tensions at the breakpoint. Divers could, however, hold their breath much longer after hyperventilation (165 +/- 40.0 and 121 +/- 31.4 seconds; P less than 0.01). This was associated with a longer second phase than occurred in non-divers (78.0 +/- 29.7 and 17.6 +/- 13.1 seconds; P less than 0.01) and more severe alveolar hypoxia (percentage oxygen 7.6 +/- 1.8 and 10.9 +/- 1.7%; P less than 0.01). It is concluded that these divers had a hyperventilation-dependent attenuated hypoxic ventilatory response. Subjects could also be identified who have either a very short (less than 10 seconds) or very long (greater than 45 seconds) second phase. They were considered to be at risk of developing underwater hypoxia and unexpected loss of consciousness. It is further suggested that analysis of the phases of breath holding holds promise as a screening test of both novice and experienced divers.