By Dr Simon Mitchell
Note: Dive New Zealand’s full interview with Robert Hewitt appears in the April/May 2006 issue 93
Mr Hewitt’s survival over 75 hours of immersion in 16 degree water was a truly extraordinary feat. Here, we will briefly review the physiological and psychological problems he had to overcome.
If we exclude drowning because of poor water skills (which was clearly not an issue in Mr Hewitt’s case), perhaps the greatest peril for the victim of prolonged immersion is hypothermia. Water conducts heat away from the body more than 20 times as fast as air and, in the unprotected victim in cold water, this is more likely to cause early death than any other cause. There is no doubt that survival would have been impossible without the protection of a wetsuit in the 16oC water. Even with a wetsuit, there would be a significant temperature drop to dangerous levels over such a period of immersion. Mr Hewitt reported feeling delirious and disorientated toward the end of his third day in the water. This is consistent with a core body temperature drop to 32 â 33oC (down from the normal 37oC). A further loss of 2 or 3 degrees would have resulted in unconsciousness and drowning. This would probably have happened quickly because Mr Hewitt actually removed his wetsuit top in his confused state.
Assuming the victim has nothing to drink (as was the case with Mr Hewitt), a second major problem with such a prolonged immersion is dehydration. There are many stories of longer survival without water, in non-immersed situations, but when immersed in water, aspects of your normal physiology actually work against you. The cold (which causes constriction of blood vessels in the limbs) and the loss of a gravity effect (which normally causes blood to pool in the legs) mean that the majority of the blood volume is ‘squeezed’ into the ‘central’ circulation. One means by which the body regulates its water balance is by sensing a stretch of central blood vessels and the heart chambers. With all this blood shifting into the central circulation, these stretch reflexes would have been activated, effectively telling the brain that the body has an ‘excess’ of fluid in the blood vessels. This results in the brain providing less stimulus to the kidneys to conserve water and more urine is produced, and it explains why you always want to pee during a dive! It follows that the victim of immersion dehydrates faster than someone who simply does not have access to drinking water.
In regards to dehydration, Mr Hewitt probably helped himself by eating the crayfish he had caught earlier in his dive. This would have supplied him with a small amount of water. I understand he also attempted to catch rainwater in his mouth. Anything would have helped. Finally, possibly as a result of his Navy training, he did not make the classic error of drinking sea water. The problem with this is that sea water contains salt at a very high concentration.
The body actively defends the amount of salt it carries, so any excess salt has to be excreted by the kidneys. This would be fine if our kidneys were able to produce a very concentrated salty urine, because we would just drink the salt water and excrete the salt while retaining the water. Unfortunately however, our kidneys are not very good at concentrating salt in the urine, and so in order to excrete the salt ingested with sea water, we actually have to add water from our own body reserves to produce urine dilute enough for our kidneys to handle. The net result is that by drinking sea water we potentially lose more water than we gain!
A related issue is the potential for collapse and death in a dehydrated immersion victim who is rescued from the water incorrectly. Because an immersion victim becomes progressively dehydrated by the mechanisms described above, they are very prone to developing catastrophically low blood pressure and cardiac arrest if gravity is suddenly reapplied and large amounts of blood suddenly redistribute to the legs. This can occur if the victim is pulled from the water by their shoulders in the upright position. Luckily, Mr Hewitt was rescued by his professional Naval colleagues who recognized this possibility and rescued him in the horizontal position.
Other important issues over such a long immersion include exhaustion and psychological problems. Shipwreck survivor stories include tales of others who simply became exhausted and more or less ‘allowed’ themselves to drown. To a degree, Mr Hewitt would have been protected by his own fitness and the buoyancy of his wetsuit, but he must have been well and truly exhausted by the time of his rescue. Finally, we can only speculate at the psychological battles one must fight when drifting at sea for days with no prospect of certain rescue. That is an issue best addressed by Mr Hewitt himself.
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