Chapter 4- Diving Physiology Diving Physiology 113 hard hats and large snorkels, which have large dead-air spaces where exhaled CO2 can collect and be rebreathed. Tendency to retain CO2 may increase with chronic exposure to high CO2 environments, such as hard hat or heavy working diving situations. Your body gets accustomed to higher levels and allows these abnormal levels to occur without the usual autocorrection by increasing ventilation. This can also affect chronic bronchitis patients and those with severe chronic asthma. However, some CO2 retainers lack any history of acclimatization. Effects: Initial response is to increase your breathing rate. Higher concentrations produce respiratory discomfort, dizziness, faulty mental processes, and stupor. Unconsciousness can occur with no warning, and you could drown. Aftereffects include headache, nausea, and dizziness. CO2 toxicity increases risk and effects of nitrogen narcosis and oxygen toxicity. First Aid: Effects reverse quickly with exposure to fresh air. Prevention: Avoid shallow or slow breathing during exertion. Be in shape for the exertion demanded by your planned dive. Maintain a normal breathing pattern (figure 4-5). Research studies have found that retention during working dives at moderate depth is a reality, although there is some debate on just how common it is. Only when the breathing gas is a helium-oxygen mixture, rather than a nitrogen-oxygen mixture, is an increase in body carbon dioxide small or absent. Hyperventilation and Shallow Water Blackout Hyperventilation can mean several things that have the end result of lowering your blood CO2 levels through overbreathing (figure 4-6). In diving, it refers to shortterm, rapid, unusually deep breathing above the rate needed for your level of activity. Divers may hyperventilate without meaning to during high stress situations, from various health problems, or deliberately to extend breath-holding time. Hyperventilation lowers CO2 in your body (hypocapnia), the reverse of CO2 buildup. CO2 signals your body to breathe. Below a certain level of CO2, you do not feel like breathing. If you hyperventilate to extend the time you can do a free dive (breath-hold dive), several things can happen. Lowered CO2 makes you feel that you do not need to breathe, so you continue your descent. Exertion uses oxygen quickly. On ascent, partial pressure of carbon dioxide in your lungs drops, as do all gas pressures on ascent, continuing the false impression that you do not need to breathe. Your oxygen level also falls, and can fall too low before you realize you must return to the surface and breathe. You may lose consciousness and drown. Loss of consciousness during breath-hold diving is currently, though mistakenly, named Shallow Water Blackout because it usually happens on ascent in shallow water. But since 1944, the term already had the established meaning of blackout from CO2 retention. During World War II, British Royal Navy torpedo divers were passing out without warning while using oxygen rebreathers with worn-out carbon dioxide scrubbers. They were breathing the CO2 that should have been removed. The phenomenon was named “shallow water blackout” because oxygen rebreathers could not be used in deep water because of oxygen toxicity (explained below). Effects: Hyperventilation produces lightheadedness, faintness, blurred vision, rapid pulse, and tingling around your mouth and extremities. Loss of consciousness from blunted “need to breathe” signals during activity following hyperventilation can lead to drowning. First Aid: Stop hyperventilating. If hyperventilation is emotional, find out what is frightening you and correct FIGURE 4-5. A NORMAL BREATHING PATTERN IS ESSENTIAL FOR DIVING.
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