Extreme scuba diving medicine

Abstract

The vast majority of recreational dives are performed by divers using single cylinders of compressed air and open-circuit breathing systems, typically to a maximum depth of 40 m of seawater (msw). More extreme dives (in terms of depth and duration) are performed by a small subgroup of recreational divers who refer to their activity as ‘technical diving’. Technical divers substitute helium for nitrogen in gas mixes for deep diving to reduce the narcotic effect of nitrogen respired at high partial pressures and to reduce the density of the gas. These mixes also contain less oxygen than air in order to manage the risk of cerebral oxygen toxicity. Adequate gas supplies for long dives are carried in multiple cylinders, or divers may utilise ‘rebreather’ devices that recycle expired gas through a carbon dioxide (CO2) absorbent and which include a system for maintaining a safe inspired pressure of oxygen (PO2) as oxygen is consumed. These devices are complex and error-prone, and there is some evidence for relatively high accident rates in their use. Using these techniques, compressed gas dives between 40 and 100 msw are now relatively ‘routine’, and more extreme dives to depths in excess of 300 msw have been completed. A challenge of deep technical diving is the effect on respiratory physiology. There are multiple factors (including increased density of the respired gas) that increase the work of breathing during deep dives. This, in turn, may cause significant perturbation of normal respiratory control. In particular, there is a tendency for divers to hypoventilate and retain CO2 which can produce a number of dangerous secondary effects. Another significant challenge is uncertainty over the optimal protocol for decompression from deep dives. Technical divers utilise progressively more oxygen-rich breathing mixes during ascent in order to accelerate inert gas elimination, but there is uncertainty over how to plan the duration and depths of the ‘decompression stops’ that are completed during the ascent to allow time for this elimination to occur.