Chapter 4- Diving Physiology Diving Physiology 121 The Dive Reflex When cold water contacts your face and body, your heart rate slows (bradycardia) and blood circulation to your limbs decreases. The combined effect is called the dive reflex. The dive reflex does not normally reduce your oxygen demand as it does in marine mammals. As long as you are conscious, your oxygen requirement rises during immersion in cold water due to the extra work to keep yourself warm. Cold may have some protective value in a drowning situation, and helps explain why some people have been revived after prolonged cold water submersion. That is not the dive reflex, however. Several mechanisms contribute to the dive reflex. Cold receptors in your face and body send a direct signal to lower your heart rate. The effect of cold lowers heart rate by itself; even cold air works. This is only to a point, and extreme cold can raise your heart rate from pain. Breath-holding, by itself, can lower your heart rate. When you are immersed, blood shifts out of your limbs due to both buoyancy in the water and the effect of cold. This blood shift helps you retain body heat, which seems to be a main purpose of the dive reflex in humans. Blood transferring to your upper body from your limbs increases blood returning to your heart. In turn, that increases the amount of blood pumped back out of your heart. A reflex decrease in heart rate occurs to keep your output the same as before. That’s why the dive reflex is called a reflex. In extreme record breath-hold dives, pressure also contributes to the dive reflex. The dive reflex is pronounced in marine mammals, but is variable in humans, sometimes dramatic, sometimes minimal. The dive reflex is sometimes used in medicine for patients having an acute attack of rapid heart rate. Briefly putting their face in a pan of cool water slows the heart rate. Effects: Your heart rate can drop so low that abnormal heart beats occur. Irregular heart beats usually pose no threat if you are otherwise healthy, within limits, but can be a problem if you have underlying heart disease. Extremely low heart rate, and accompanying arrhythmias, can be a problem, previous cardiac problems or not. Prevention: Get in the water slowly, rather than plunging in. Pre-wet your face and hands. Get regular physical checkups with full cardiovascular screening. Be in good physical shape. Your physical condition is not known to affect your predisposition to the dive reflex, but to possible ill effects from it. BAROTRAUMA Barotrauma is a general term for injuries due to pressure (figure 4-10). The Greek word baros means weight or pressure. From “baros,” we get words like barometer (pressure measurer), hyperbaric (high pressure), and bariatrician (a physician who cares for the obese). Trauma means injury. Your body contains several air spaces that can be mechanically injured when pressure is very different between your air space and ambient pressure. The pressure differential usually occurs when openings to your air spaces are blocked, preventing you from equalizing the difference in pressure. BLOCK ASCENT Types of Blocks Sinuses Ears Teeth Stomach/Intestinal Lungs FIGURE 4-10. THE TERM REVERSE BLOCK IS USED TO DESCRIBE EXPANDING AIR THAT IS TRAPPED IN THE BODY.
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