Fear and Loathing Below 130 Feet
By Karl Shreeves
Deep—220. On air. A throb in my temples hints that perhaps I’m narked. Maybe. Wow, this is just like the stuff at the dentist’s office. There was something I needed to do. What was it? Air—check. Deco status—check. Oh, now I remember—breathe. Don’t want CO2 buildup. Where’s Ed? The world retreats down a tunnel as I swim over to an intriguing wreck. I alternately hug and push off the hull as the current buffets me against it. Oh, there’s Ed, pointing to his wrist. What? Check time. Up already? I just got here. Climbing the anchor line, passing 180 feet. First deco stop, 80 feet according to one computer. No prob. The other says 120. Yeah, like that’ll happen.
Passing 150. My pessimistic computer revises first stop to 100 feet and by the time I reach 80 any remaining narcotic veil streams away with the current as Ed and I stop to pay tribute to Haldane.
“Great dive!” Ed signals, jabbing his fist with gusto.
“Yeah? Wish I ’d been there.”
Definition of deep dive: A dive that makes you feel like the time your college buds agreed to chug shooters every time the hockey team scored, and they won, 98–zip.
Well, that’s not the definition any more. But as the above anecdote from my dark ages B.T. (Before Trimix) illustrates, my concerns with deep diving initially sprang from nitrogen narcosis. For those who’ve not experienced it, being narked (nitrogen narcosis) is an intoxicating sensation, not unpleasant in a Hunter S. Thompson sort of way, but hazardous in that being deep underwater while blitzoid is akin to slugging six jiggers of Jack Daniels before landing a 747. You should be terrified, but you’re too numb to feel the danger. Make that danger squared.
Narcosis results when gas under pressure dissolves into lipids in nerve tissue and retards nerve impulse transmission. The prevailing theory, the Meyer-Overton hypothesis, states that the more soluble a gas is in cell lipid, the more narcotic it is. Nitrogen is pretty soluble, which is why you get narked when you dive. Nitrous oxide is very soluble, which is why dentists use it to make drilling time a happy time, whereas helium is not very soluble in cell lipids, making it ideal for deep diving.
Although you don’t usually feel it, research shows that people have measurably slower responses and impaired learning as shallow as 66 feet, with most divers noticeably affected at 99 feet. But it’s not that cut and dry. Narcosis is a fickle disorder: at 130 feet you may be clear headed one day and three steps into Timothy Learysville the next. It’s also documented that with repeated exposure, most divers learn to compensate for the narcosis…to a degree.
Another point is that oxygen is virtually as soluble in lipids as nitrogen, so you can’t expect enriched-air nitrox to appreciably reduce narcogenesis. Drugs also interact with nitrogen much like combining sedatives. For instance, one beer may not affect you, nor would a dive to 60 feet. If you’re talking both at once, though, that’s a whole other ball game. Remember—cocktails at depth are a whole bunch stronger, hence the admonitions not to party before heading down.
Can You Swim A Straight Line?
Narcosis created one reason for the 130-foot recreational depth limit, and in technical diving, kicks up the controversy about whether deep-air diving is stupid or not—a debate stirred primarily by zealots who begin arguing before determining what they even mean by deep. But the dive community is reaching some consensus about depth limits for air diving based on narcosis and other considerations.
Sixty feet is an appropriate limit for novice divers since it keeps narcosis and short no-stop limits from being significant variables while developing experience and dive skills. However, 130 feet should be acknowledged as an absolute limit. Below this depth, narcosis begins to significantly impair coordination and cognitive skills. Additionally, this depth is a significant threshold considering the limitations of single tank/single regulator scuba configurations.
A 130-foot limit for air diving on complex technical dives, especially overhead environment diving (wrecks, caves, etc.) is advisable. Narcosis has no place on a dive where too many variables factor into your safety. Cave diving accident analysis shows diving deeper than 130 on air as the primary source of narcosis and its hazards.
Even simple dives in open water using redundant life support and following principles for air supply management (i.e. not single tank, recreational rigs) is risky at 185 feet. You’d definitely be narked at this depth, so emergency procedures had better be second nature. Deeper than 185 takes you above 1.4 ata PO2, making oxygen toxicity a concern.
Two-hundred twenty feet? Some people dive this deep on air (PO2 1.6), but more and more of the technical diving community regard air dives below 185 feet as a good way to get skimmed by the gene pool filter. Like driving drunk, you might get away with it if nothing goes wrong, but if something does, there’s a good chance you’ll be too blotto to avert disaster.
Deeper Than That
Technical dives below 185 feet or below 130 for cave/complex dives require trimix (helium, nitrogen and oxygen) or heliox (helium and oxygen). Helium doesn’t dissolve readily into cell lipids, so it’s not very narcotic. It also dilutes oxygen, which helps to offset oxygen toxicity. Unfortunately, helium dissolves very well into water (i.e. your body tissues), requiring special gases, special tables and special techniques for decompression. Your dive computer cannot handle it, so there’s more to helium diving than buying all of Boffo the Clown’s balloons and pumping them into a set of doubles. This is the most extreme form of deep diving, requiring special training and extensive experience.
Once upon a time trimix divers began their training with a narco deep-air run, the illogic being that until you’re stoned out of your gourd at 200-plus, you can’t appreciate the safety and advantages of diving sober—kinda like you won’t appreciate parachutes until you’ve been skydiving without one. Having made narco deep-air runs in times B.T., I can tell you that the comparison is clear, but I imagine I don’t have to run you over with my pickup for you to appreciate the benefits of looking both ways before crossing the street. At least, let’s hope so.
Karl Shreeves is vice president, technical development for PADI and DSAT, and an active technical diver. If you ever see him at 200 feet on air, rescue him.