Hyperventilation is the term applied to breathing more than is necessary to keep the body's carbon dioxide tensions at proper level. Hyperventilation (whether voluntary or involuntary) has little effect on the body's oxygen levels, but abnormally lowers the partial pressure of carbon dioxide in the blood and delays the normal urge to breathe. If the carbon dioxide stores are ventilated below the stimulus level, there will be little urge to breathe until late in the breathhold. The oxygen partial pressure falls progressively as oxygen is consumed continuously. Exertion causes oxygen to be consumed faster, and decreases sensitivity of the carbon dioxide breakpoint mechanism. This permits the oxygen level to go lower than it would otherwise. When the diver ascends, the drop in oxygen partial pressure in the lungs may be sufficient to stop further uptake of oxygen completely. At the same time, the partial pressure of carbon dioxide in the lungs also drops, giving the diver the false impression that he need not breathe. Low levels of oxygen do not cause a powerful demand to resume breathing; thus, the level of oxygen in the blood may reach the point at which the diver loses consciousness before he feels a demand to breathe.

WARNING Hyperventilation is dangerous and can lead to unconsciousness and death.

3-7.1 Unintentional Hyperventilation. Unintentional hyperventilation can be triggered by fear experienced during stressful situations. It can be partly initiated by the slight "smothering sensation" that accompanies an increase in dead space, abnormal static loading, and increased breathing resistance. Cold water exposure can add to the sensation of needing to breathe faster and deeper. Divers using scuba equipment for the first few times are likely to hyperventilate to some extent because of anxiety.

3-7.2 Voluntary Hyperventilation. Voluntary hyperventilation (taking a number of deep breaths in a short period of time) can produce symptoms of abnormally low carbon dioxide tension (hypocapnia). Under these circumstances, one may develop a lightheadedness and tingling sensations. Hyperventilating over a long period, produces additional symptoms such as weakness, headaches, numbness, faintness, and blurring of vision. The anxiety caused by the sensation of suffocation that often initiates hyperventilation and continues in spite of adequate ventilation, may lead to a further increase in breathing and a vicious cycle develops. Severe hypoc-apnia with muscular spasms, loss of consciousness and shock may be the end result. The diver must pay attention to his breathing rate and, in the event of fear-induced hyperventilation, take steps to remain calm and control his breathing.

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