A Scoping Review of the Equivalent Air Depth Concept.

Abstract

Surface-oriented diving with nitrogen-oxygen breathing gases (nitrox) other than air will commonly be planned according to the Equivalent Air Depth (EAD) principle. EAD is the depth at which a diver breathing air will inhale the same pN₂ as the nitrox-breathing diver. Using conventional air decompression tables based on the EAD depth will allow longer bottom times or shorter decompression times compared to air breathing at the same water depth. This scoping review aimed to investigate the validity of the EAD principle. A literature review identified 13 eligible studies that reported DCS incidence or vascular bubbles following air and nitrox exposures in humans and experimental animals. Four experimental (N=1,597 mandives) and two epidemiological (N=249,109 mandives) studies on humans did not disclose a higher DCS incidence or bubble scores following nitrox dives compared to air dives of similar EAD. Seven experimental animal studies consistently showed that a high pO₂ (>2 atm) increased DCS susceptibility in goats and rats. This can likely be explained by the reduction of the "oxygen window" due to the high tissue and venous pO₂. There is insufficient data from animal studies to conclude whether this can be reproduced within a pO₂ range applicable for human operational diving (<1.6 atm). There is presently no reason to modify the EAD principle for manned diving. Still, we would advise improving the monitoring of operational nitrox dives to confirm that nitrox dives have a DCS incidence similar to that of EAD- and bottom time-matched air dives.