Arterial blood gas changes in progressively deeper breath-hold dives

Studies involving breath-hold dives as deep as 60 m have identified a spike in arterial Po₂ ([Formula: see text]) and Pco₂ ([Formula: see text]) apparently associated with lung compression by water pressure as depth increases. Competitive breath-hold dives may exceed 100 m, raising the possibility that a spike in [Formula: see text] might contribute to narcosis-like symptoms sometimes experienced. To predict the magnitude of this spike, three elite breath-hold divers were instrumented with radial artery catheters and had arterial blood specimens taken at the bottom on separate dives to 20, 40, 60, and 80 m with another specimen either after beginning apnea but before leaving the surface (one participant) or on arrival back at the surface while still apneic (two participants). Contrary to previous expectations, the greatest spike in [Formula: see text] was at 40 m (mean: 29.3 kPa, 220 mmHg) and the trajectory of rise in [Formula: see text] also plateaued at 40 m [mean: 5.61 kPa, 42.1 mmHg (from a mean baseline of 2.53 kPa after predive hyperventilation) and 5.54 kPa, 41.6 mmHg at 60 m] before rising again after 60 m (80 m mean: 6.9 kPa, 51.5 mmHg). This suggests that a spike in arterial gases associated with lung compression was detectable until the greatest proportional changes in lung volume with increasing pressure had occurred. Thereafter, [Formula: see text] decreased through metabolism, and [Formula: see text] increased through simple metabolic accumulation. The compression effect seemed maximal at 40 m (five atmospheres absolute pressure) which may relate to the simple Boyle's law prediction that the lung may be compressed to close to residual volume (20%-30% of total lung capacity) at this pressure.NEW & NOTEWORTHY In descending deep breath-hold divers, spikes in [Formula: see text] and [Formula: see text] associated with compression of the lungs by increasing water pressure have been reported. This study, which reports the deepest arterial blood gas specimens taken to date, raises the possibility that these spikes are detectable until the greatest proportional changes in lung volume with increasing pressure have occurred, after which [Formula: see text] will decrease through metabolism and [Formula: see text] will increase through simple metabolic accumulation.