DDM
I think what I underlined is exactly the point and I am very happy you said this. So what are the facts? Can anyone answer these simple questions to help him evaluate the risk? What is the percentage of seizures in the normal diver at a 1.6 ppo2 or cns tox <100%? How much does it go up when said diver has had a prior head injury? How much does it increase when said diver is on ultram? What is Lakerpride's risk of seizure diving at a 1.6 ppo2?
The last relevant question is what is the survival rate of a seizure at depth?
The facts are that we do not know what his risk is for a seizure is. We all agree that his risk is increased. Beyond that is a guess, even if a highly educated guess of highly trained people. It is also safe to say (as you pointed out) the survival of a seizure is very very low without a helmet/full face mask and a lot of luck.
Telling him NO, is not educating him and letting him decide. We can not and should not make decisions for other people.
Jimmy
Jimmy,
Your last question is probably the most important one because the survival rate for recreational divers who seize underwater is essentially zero. Of course seizure is not the only symptom of CNS oxygen toxicity, but it's very frequently the first to appear. As you pointed out, a full face mask can help eliminate the possibility of losing the gas supply, but those are awkward to use in technical diving unless you're on a rebreather or have a fancy gas-switching manifold.
The questions about probability of seizure are difficult to answer. The risk varies among divers, and changes from day to day in individuals. Work level and blood CO2 level are known risk factors and, of course, can vary from dive to dive. Immersion is a risk factor in and of itself. Most of the human experimentation was done in the 1930s and 1940s by the the British and U.S. Navies and was published by Dr. Kenneth Donald in 1992 in a book called
Oxygen and the Diver. Vann et al summarized this work in the report on the 2008 technical diving conference (reference: Vann RD, Mitchell SJ, Denoble PJ, Anthony TG, eds. Technical Diving Conference Proceedings. Durham, NC: Divers Alert Network; 2009; 394 pages.) It's available on the Rubicon website
here. Page 53 of this publication has a graph that contains some figures, but again keep in mind that these can vary widely. In short, statistical modeling shows a risk of CNS oxygen toxicity of about 1% after about 30 minutes on 1.6 ATA, and about 2% after about 45 minutes.
One of the end results of the early work on CNS O2 toxicity was the promulgation of the U.S. Navy's operational limits on closed-circuit oxygen rebreather diving. Chapter 19 of the U.S. Navy Diving Manual, available for download
here, has the details, but essentially, 100% O2 CCR divers are limited to a transit depth of 20 FSW, or about 1.6 ATA, with limited excursions below 20 FSW permissible. The manual also states,
"
Although the limits described in this section have been thoroughly tested and are safe for the vast majority of individuals, occasional episodes of CNS oxygen toxicity may occur. This is the basis for requiring buddy lines on closed-circuit oxygen diving operations."
Keep in mind that the guys using 100% O2 CCRs in the Navy are not quite as susceptible to hypercapnia and are much more exercise tolerant than most of us.
How much more at risk of O2 toxicity is a diver who uses tramadol? I don't know. It's never been studied, and nowadays institutional review boards frown on making test subjects have seizures unless they're four-legged and like cheese. Hopefully though, after all this writing, I've done better than simply tell LakerPride, "NO". Throughout my ramblings I think I've provided a pretty good rationale, which is exactly what we'd do for any diver seen in clinic here. I hope he digests all this then goes and educates himself even further. VooDoo, bless his little pea-picking heart, provided a great resource in Dr. Kay.
Diving involves a lot of risk mitigation, and when things like medications put a diver at increased risk for adverse events, we try to make sure that divers understand those risks completely. Risk mitigation equals survival. That's why guys like Ed Viesturs can keep going up and down Everest, and folks like the WKPP can keep plunging in and out of dark scary holes and come back and tell us all about it.
Best regards,
DDM
