Medical Q&A

DAN medical specialists and researchers answer your dive medicine questions

IS MY DIVE PROFILE SAFE?

Q: I am currently taking a Master Scuba Diver course. Our instructor wants us to dive an extended time at depth. Is that safe with regards to nitrogen build-up? I have an upcoming dive with the following profile:

40m: 20 minutes | 30m: 2 minutes 20m: 2 minutes | 12m: 4 minutes 9m: 5 minutes | 6m: 6 minutes 3m: 16 minutes

Every time above 40m includes a one-minute ascent. Is this a safe profile, or should it be more conservative? A: The profile you describe is within the limits of currently available dive tables and is slightly more conservative than the U.S. Navy dive tables. The dive profile is decompression (deco) diving, which requires decompression stops, and all these tables were developed for square profiles. I assume your instructor chose these times based on the dive tables of your training organisation or a commercially available dive planner.

If you are concerned about an increased risk of decompression sickness (DCS), you can consider options to make the dive more conservative, such as the following:

Add time to the shallower deco stops (e.g., another two minutes at 6m and 9m and follow the ascent rates that your instructor proposed. Ascent rates are one of the main keys to safe decompression. Add enriched air nitrox (EAN) as a decompression gas starting on the deco stops to add oxygen to your decompression phase and speed up nitrogen elimination. Keep the same times as for the air profile, even if you switch to EAN. Decrease your bottom time by a few minutes. It is always important to stay well hydrated before every dive, especially in deco diving.

AIR USAGE

Q: I am a scuba instructor at a resort that offers introductory scuba experiences. A student who made one dive to 6m for less than 20 minutes used a half tank of air and later told me that he started to feel awkward as if he were stoned. Was he experiencing nitrogen narcosis? A: At a depth of 6m, the partial pressure of nitrogen is not elevated to the levels that cause nitrogen narcosis, the effects of which usually appear at a depth of at least 33m but sometimes can occur in somewhat shallower water.

A variety of things — such as dive gear, underlying medical conditions, psychological conditions, or drugs and medications — could cause your student’s experience, but we need more details to provide a proper explanation. His gas consumption may indicate hyperventilation occurred during the dive.

Without further speculation, the student will need a dive medical exam, and he should discuss this incident with a dive medical physician if he wishes to pursue training. If the physician finds no psychological or medical contraindications, instructors should initially conduct his dive training slowly and with close observation to ensure no recurrence. DANAP.org