Enriched-air/oxygen for pre-flight surface intervals

Abstract

Introduction: Current guidelines for pre-flight surface intervals (PFSI) of 12-18 hours represent a delay if urgent medevac is required. This study seeks to investigate the potential use of enriched air mixtures and oxygen to decrease PFSI.

Methods: Simulated dive profiles assumed a descent rate of 30m/min and an ascent rate of 10m/min. Dive times included descent plus time at maximum depth. Six single dive profiles were selected, at three popular recreational diver certification depths; three profiles at or near the no-decompression time limits for those depths, and three with shorter times. Inert gas pressures at the end of the dives and at 6, 12, 18 and 24 hrs breathing air post-dive were estimated for ZH-L16B compartments using the R package ‘scuba’. Then, still assuming the dives were made on air, surface interval times were estimated to reach equivalent (or less) pressures while breathing EANx32, EANx36, or 100% oxygen.

Results: Oxygen-rich gases accelerated removal of inert gas, at least halving the 6h times with EANx32 (mean 2:59 vs 6:00 hours), and reducing the 24 hours PFSI across all tested dives to a mean of 1:07 hours with 100% oxygen (Table 1).

Table 1. Pre-flight surface interval (hr:mins) required while breathing either EANx32, EANx36 or O2 before reaching equivalent tissue pressures as either 6, 12, 18 or 24 hours breathing normobaric air Dive Profile D’(~m)/ mins EANx32 EANx36 100% Oxygen 6 hr 12 hr 18 hr 24 hr 6 hr 12 hr 18 hr 24 hr 6 hr 12 hr 18 hr 24 hr 40(12) / 60 2:54 4:15 4:51 5:21 2:28 3:20 3:54 4:15 0:45 0:55 0:59 1:02 40(12) / 120 3:41 5:47 7:12 8:15 3:15 4:55 6:00 6:45 1:11 1:31 1:42 1:48 60(18) / 20 2:05 2:42 3:01 3:14 1:43 2:09 2:22 2:30 0:28 0:31 0:33 0:33 60(18) / 40 3:07 4:11 4:55 5:25 2:31 3:29 2:58 4:19 0:48 0:56 1:00 1:03 60(18) / 55 3:21 4:57 5:57 6:40 2:55 4:09 4:53 5:23 1:00 1:12 1:19 1:22 100(30) / 20 2:46 3:49 4:24 4:48 2:11 3:06 3:31 3:47 0:43 0.50 0:53 0:54 D’=depth in feet of seawater, ~m=approximate depth in metres of seawater, mins=total dive time in minutes from descent to start of ascent, EANx32=Enriched air nitrox containing 32% oxygen, EANx36=Enriched air nitrox containing 36% oxygen, hr=hours of surface interval breathing normobaric air

Conclusions: The estimated PFSI from breathing EANx36 decreased to a mean of 42% the equivalent of 6 hours breathing air, and to a mean of 19% the equivalent time for 24 hours of post-dive breathing air. There was some variance with respect to the dive profile, with longer profiles showing less improvement due to greater saturation in slower tissues. This study presents required PFSI time estimates for breathing various gases to reach equivalent estimated compartment pressures after 6-24 hours of breathing normobaric air. These estimated times do not take into account physiological factors such oxygen-induced vasoconstriction, exercise, hydration, etc, and the results will need confirmation by actual human trial(s) before practical adoption.