Nitrox workshop dings - Divers Alert Network

Rubicon Foundation Archive (http://rubicon-foundation.org)Lang (ed.): DANNitrox Workshop, Divers Alert Network, November 2000allegedly inherent and expected increased risk and to tailor-make them individual PO2 limits, fortheir own safety as well as for possible liberalization of the global limits.The inherent risk alluded to is that of CO2 retention resulting from exercise hypoventilation, aterm that describes a situation where the increase in ventilation does not match that of CO2production. In this case, CO2 is temporarily "retained" until a new steady state is reached wherethe product of (the lower) alveolar ventilation and (the higher) alveolar CO2 again equals CO2production.Diving hypoventilation can ensue from either one or any combination of the following, all ofwhich pertain to nitrox diving:1. A depth-related increase in gas density and work of breathing,2. A PO2 -related blunting of ventilatory drive,3. A dive-related individual trait, which could be one or both of:• Inborn - the diving vocation selects natural exercise hypoventilators, who feel morecomfortable under water.• Acquired - being the only aerobic sport limited by the quantity of breathing gas, agas-saving breathing pattern is adopted with time.There is individual variability of the ventilatory response to all 3 factors and the actualdegree of hypoventilation resulting from their combination is subject-specific. However,compared to the sensitivity to CNS O2 toxicity, it is much more consistent and reproducible in agiven subject.The rationale for CO2 retention exacerbating CNS O2 toxicity is given below:HypoventilationIArterial hypercapnia (t P a CO 2 )ICerebral vasodilationItCerebral tissue PO 2I1 CNS oxygen toxicity thresholdThe weakest link in the above chain, where individual traits are concerned, is that thecerebro-vascular response to CO2 has not been measured during exercise in known "normalchronic hypoventilators", alias "CO2 retainers", and not shown to behave in accord with thegeneral population. Frequent complaints of immediate and transient post-dive headaches inestablished retainers (who usually "learn how to live with it") may hint that vasodilation indeedexists, at least in some subjects.82