Human beings have escaped drowning by surviving extraordinary periods under water. The longest documented was a young child, submerged for 40 minutes beneath the icy waters of a frozen lake. Breath cannot be held for much more than 2-3 minutes, so how could a child possibly have survived all that time without brain damage? Dolphins, seals, and whales, all air breathers like us, manage to remain submerged on a single breath hold for anything up to 40 minutes, and yet they are just as dependent on oxygen as we are. To understand how they are able to do this, it is necessary to consider some of the physiological principles at play.
Certain tissues of the body are much less sensitive to oxygen starvation than others. For example, blood (which carries the oxygen) can be shut off from an arm or leg, and, provided the limb is being rested, the muscle can survive without its blood supply (and therefore without oxygen) for well over half an hour. It does so by drawing on its store of glucose, which it converts to lactic acid. This is known as anaerobic respiration, and muscle has a large anaerobic reserve. The brain, though, has no such reserve, and any interruption to blood running up the carotid arteries results in unconsciousness almost at once, with permanent damage occurring after 2-3 minutes. The heart is also vulnerable to oxygen starvation, because, unlike the muscles of the arms or legs, it can't be rested, and must go on beating, rapidly using up its anaerobic reserve.
So the strategy for a diving mammal is clear: its precious oxygen pool (carried by the red blood cells) must remain the sole preserve of the brain and heart, while the rest of the body can do without. This is achieved by reflex constriction, all blood vessels with the exception of the carotid and coronary arteries, stimulated by the effect of cold water touching the nose and face, and also by the very act of breath-holding. With only the brain and heart arteries to supply, the heart rate can slow right down, and drops to around 10 beats per minutes.
Evolution has robbed terrestrial mammals of the diving reflex, of which just a vestige remains, detectable only in the first few months of infancy. So, if, it is not the diving reflex influencing survival, then what? The answer lies with the temperature of the water. This is best explained by reflecting on the history of surgery. With the discovery of anaesthesia, surgeons found that they could operate on almost any part of the body, but they couldn't operate on the heart while it was beating. To stop the heart for anything more than a few minutes meant risking brain damage, so there was insufficient time for surgery.
However, animals appeared to recover fully from heart surgery with the circulation stopped, provided that their bodies had been cooled down a little first — the theory being that reduced body temperature meant a lower metabolic rate and so less oxygen consumption by the brain. The cooler the animal, the longer the circulation could be stopped without obvious harm. The first heart surgery to be performed on a human subject took place in 1954. The patient was a child with a hole in the heart. Circulation was stopped for 15 minutes while surgeons repaired the defect. The operation was a complete success.
Of course, now that the work of circulation and breathing can be done by heart/lung machines, the cooling process can be controlled much more easily, and it is possible to take body temperature as low as 18°C. At that temperature, blood flow to the brain can be stopped for up to 60 minutes before tissue damage (such as a stroke) begins to show itself, though re-warming must be taken very slowly.
The analogy between cerebral protection with deep hypothermia, and cold water submersion, now becomes obvious. The chance of survival is increased in those whose brains have become coolest before the heart stops beating. It would be expected that very small children or infants submerged in ice cold water should have the best chance of all, because their rate of cooling is highest: this is indeed the case. So the clinical approach to resuscitation of these very cold individuals might logically be to reverse the cooling procedure used in heart surgery, and use the heart/lung machine to support circulation while re-warming. And remarkable results have indeed been achieved at centres where these facilities are available.
To summarize; experience in the use of deep hypothermia in cardiac surgery has provided insight into the physiological principles governing survival after very long periods spent under water. Furthermore, these principles have been applied to the techniques used to resuscitate victims of near-drowning.
— Mark Harries
See also cold exposure; drowning; hypothermia; resuscitation; survival at sea.
The Oxford Companion to the Body. Copyright © 2001, 2003 by Oxford University Press. All rights reserved.
