by Tim Peever
Some Physiological Factors and Possible Controls of Oxygen Toxicity
Oxygen Toxicity manifests itself in two ways, pulmonary oxygen toxicity and central nervous system (CNS) oxygen toxicity. Of these, CNS oxygen toxicity is the most serious concern to scuba divers.
Pulmonary oxygen toxicity involves attack on the gas transfer membranes in the lungs by oxygen free radicals from long-term exposure to high partial pressures of oxygen. The time involved to manifest symptoms of pulmonary oxygen toxicity is many hours and would not affect a diver except in cases of recompression therapy.
According to a DAN article, “Oxygen toxicity in the lungs (pulmonary oxygen toxicity) is like getting a bad case of the flu, but it will rarely cause permanent damage. The most common situation in which pulmonary oxygen toxicity might occur is during very long recompression treatments.” More can be read here:
Oxygen toxicity of the brain, CNS oxygen toxicity, is much more serious as it almost always occurs while the diver is under water. CNS oxygen toxocity can culminate in grand mal convulsions. This in and of itself is not harmful and results in no permanent damage but can result in impact injuries if the victim strikes something hard or sharp in the thrashes of the convulsions. The most serious risks are that the stricken diver will lose his / her mouthpiece underwater and drown or lose buoyancy control, rising rapidly to the surface causing an embolism or DCS complications.
Avoiding oxygen toxicity seems simple. The occurrence and type of oxygen toxicity correlate with the O2 concentration, the ambient pressure, the length of time supplemental O2 is inhaled, and the diver’s level of activity. Tables have been published indicating partial pressure and time limitations, an example is provided below. Algorithms have also been written, one is discussed here: http://www.maths.leeds.ac.uk/~read/scuba/baker3.pdf. Knowing the partial pressure, tracking exposure time and staying within the limits will prevent oxygen toxicity issues.
Oxygen Partial Pressure and Exposure Time Limits for Nitrogen-Oxygen Mixed-Gas Working Dives (from NOAA 1991 Diving Manual)
Oxygen toxicity of both types (CNS and pulmonary) is caused by oxygen free radical attack. CNS oxygen is the most serious for divers. Can a diver reduce susceptibility to CNS oxygen toxicity through diet? It is known that many foods and supplements contain antioxidants. Vitamins C and E are well known antioxidants. Can these reduce the likelihood of a CNS oxygen toxicity episode?
This is of interest not only to divers but also to the medical community where patients are kept on oxygen at atmospheric pressure or under hyperbaric conditions for therapy. According to this article (http://medind.nic.in/jac/t03/i3/jact03i3p234.pdf ) research has been tried successfully on animals and is under evaluation in humans. Vitamins E and C are administered to high-risk infants being administered oxygen. These vitamins are commonly available as supplements or can be obtained naturally. Vitamin C is present in many fruits. Vitamin E can be obtained from eating meat, fish, nuts or soy.
Another antioxidant is beta-carotene. Beta-carotene is a precursor to vitamin A (retinol) and is present in liver, egg yolk, milk, butter, spinach, carrots, squash, broccoli, yams, tomato, cantaloupe, peaches, and grains.
Flavonoids are natural chemicals found abundantly in plants. They are present in fruit and vegetables especially in apples and onions. You also get them in red wine, beer, ale, stout and tea. Chemically they are known as polyphenols. Research suggests that flavonoids have potent antioxidant properties.
Co-enzyme Q-1O is a natural vitamin-like substance that acts as an antioxidant. Co Q10 exerts its antioxidant effect by both scavenging singlet oxygen and by inhibiting the decomposition of hydrogen peroxide and release of free radicals. We are able to combine lower forms of Co Q (6, 7 and 9) derived from plant-source foods, fish and fowl into Co QlO and eating animal protein (which supplies us with pre-formed Co QlO). The highest dietary sources of CoQ10 come from (in descending order according to content): fresh sardines and mackerel; the heart and liver of beef, pork and lamb; and the meats of beef and pork, along with eggs. There are plenty of vegetable sources of CoQ10, the richest currently known being spinach, broccoli, peanuts, wheat germ and whole grains -in that order.
This article (http://www.vspn.org/vspnsearch/aow/oxygentoxicity.htm) quotes enzymes that block the reactions that cause CNS toxicity. “Superoxide dismutase (SOD), catalase, and glutathione peroxidase (GPO). These enzymes prevent free radical chain reactions. They are available as supplements and occur naturally in the livers of fish and mammals.
For any of the dietary sources of vitamins, enzymes and other nutrients, the emphasis is on fresh food with minimal processing. The carrot picked from your garden today will do you much more good than the canned variety.
Studies have been conducted which show that regular physical exercise enhances the antioxidant defense system and protects against free radicals. These changes occur slowly over time and appear to parallel other adaptations to exercise. On the other hand, intense exercise in untrained individuals overwhelms defenses resulting in increased free radical damage. Thus, the “weekend warrior” who is predominantly sedentary during the week but engages in vigorous bouts of exercise during the weekend may be doing more harm than good.
Almost any article on oxygen toxicity includes a discussion of the divers level of exertion relative to oxygen toxicity risks. It is recommended to not exceed 1.4 ata PO2 in dives involving high levels of exertion. It is known that a lot of energy is used in the process of digesting food. Respiratory rates rise and pulse is elevated after a meal by as much as 20% above the resting baseline. People feel tired after eating a large meal; digestion is a lot of work!
Linking heavy food intake prior to a dive with oxygen toxicity risk seems logical. Eating a small light meal prior to making a dive involving high PO2 exposure would make sense. Certainly avoiding large heavy meals would not only make the dive more comfortable but also should reduce the level of exertion on the dive and reduce the chances of oxygen toxicity.
Physical stress is a well known factor in CNS. What about mental stress? Pulse and respiratory rates soar with anxiety as much or more than they do with exertion. It would seem that this should be considered a factor equal to cold or current in determining the maximum operating depth of a breathing mix. The diver should consider this in planning dives which push his or her experience boundaries. Staying well above the MOD in the event of anxiousness would also be prudent.
In conclusion therefore, vitamins, enzymes and other nutrients that cause resistance to oxygen free radicals can be ingested to reduce the risk of oxygen toxicity. Foods containing these substances would be good to include in the diet of a practicing diver exposing themselves to elevated PO2 levels. Supplements can also be taken which contain these substances. Regular exercise is important to enhance the antioxidant defense system. It is also logical that avoiding large, heavy meals before a dive involving high PO2 exposure would be sound advice. Factoring in and being aware of anxiety as a potential factor in CNS is also advised. In no case should any of this be considered an excuse to violate the published tables, but rather good practices to follow in order to provide a greater safety margin against a CNS episode.