Diving Medicine

By Dr. David Sawatzky

In the last column we looked at epilepsy and seizures.  However, epilepsy and seizures are a relatively rare cause of loss of consciousness.  The most common cause of loss of consciousness is fainting (syncope) and it results in one of every 30 visits to an emergency room.

Syncope is the sudden loss of consciousness due to cerebral ischemia, followed by a spontaneous recovery.  Basically, not enough oxygen is delivered to the brain and it stops working.  This is usually due to a fall in blood pressure.  Gravity pulls the blood towards the feet (when we are standing) and if the blood pressure is too low, no blood (oxygen) is delivered to the brain.

When a person faints, they usually fall over.  This largely removes the effect of gravity and even a very low blood pressure will deliver blood to the brain when you are lying down.  Conversely, if you start to feel lightheaded you should immediately lie down.  If possible you should prop your legs up on something so that they’re higher than your body.  This allows gravity to pull the blood from your legs into your chest and make if very easy for your heart to pump blood to your head.  If you observe someone faint, lay them down immediately and prop up their legs. If a person who’s fainted is put in a sitting – or worse – standing position, the time it takes for oxygen to get to the brain will be dramatically prolonged, and the person may convulse.  This would be a hypoxic seizure and not epilepsy.

There are a very large number of reasons people faint.  Blood pressure is controlled by the output of the heart and the resistance to flow in the small arteries.  When we are sitting or lying down, the heart and small arteries can relax, as it’s very easy to maintain a normal blood pressure.  When we suddenly stand up, gravity pulls all of the blood towards our feet.  To counteract this immediate force, the heart has to beat harder and the small arteries must constrict.  This response takes up to 30 seconds and so you might feel light headed when you first stand up.  This is called postural or orthostatic hypotension and can be avoided by standing up more slowly.

Dehydration reduces the volume of blood circulating in the body and makes it more likely that orthostatic hypotension will result in fainting.  If a person takes too many medications to lower their blood pressure they might faint and some blood pressure medications interfere with the cardiovascular reflexes required to combat orthostatic hypotension.

Stimulation of the vagus nerve results in slowing of the heart and dilation of the blood vessels in the legs, resulting in a fall in blood pressure.  Many things can result in stimulation of the vagus nerve including situational stress such as having blood taken, starting a new job, etc.  It can also result from stimulation of the carotid sinus receptors.

The carotid sinus receptors are located in the carotid arteries in the neck and measure the blood pressure.  When the blood pressure rises, they send a signal to the heart via the vagus nerve to slow down so that blood pressure remains constant.  They can be stimulated by massage (used to control some forms of rapid heart rates) and can be stimulated by a wet or drysuit hood that is too tight!

Another mechanism to reduce blood flow to the brain is to increase the pressure in the chest.  This reduces the amount of blood returning to the heart.  If the heart does not have enough blood to pump, blood pressure falls.  Many recognized forms of syncope result from these two mechanisms.  Coughing or swallowing can stimulate the vagus nerve and increase the pressure in the chest (cough or swallow syncope).  Going for a pee (micturition syncope) or having a bowel movement (defecation syncope) can have similar effects.  Going for a pee – standing up in the middle of the night when blood pressure is already low – is particularly dangerous.

Any disease that interferes with the tone of the nerves in the legs (Diabetes, Parkinsonism) can result in syncope, as the small arteries will not contract normally when the person stands up.  High altitude (over 10,000 feet/3,000m) reduces the partial pressure of oxygen in the air we are breathing and can result in syncope (commercial jets fly with a cabin pressure equal to 6,000 to 8,000 feet – 1,800 to 2,400m).

Transient ischemic attacks are basically balls of platelets that block the circulation to part of the brain for a few minutes and then dissolve, restoring flow.  This can result in syncope, as can a migraine.

Finally, there are a number of heart problems that will result in reduced blood flow to the brain and syncope.  These include abnormal heart rhythms (too fast, too slow), heart valve problems (stenosis or major regurgitation), pulmonary artery hypertension, tears in the aorta (aortic dissection), and diseases of the heart muscles (cardiomyopathies).  There are many other problems that can result in longer losses of consciousness but this is not syncope.

In addition to the above, I have seen two divers develop syncope for another reason.  On the first night of a basic course, a 32 year old, fit and muscular student jumped into the pool to swim the required 16 lengths.  He set off at a brisk speed but after only six very fast lengths, he got out of the pool and sat on the deck, breathing heavily for a couple of minutes.  He then got up, walked over and sat down against the wall.  After another couple of minutes, other divers noticed that he was losing consciousness and caught him just as he was about to fall over.  They lowered him to the floor and at first could not detect a pulse or breathing.  He was also very pale.  After a few minutes they detected a very weak pulse and he slowly started to wake up.  By the time the ambulance arrived he had largely recovered but was convinced to go the hospital anyway.  A doctor examined him, his blood work was entirely normal, and he was released feeling completely fine.

When I was about 35 I had a few similar episodes.  I was not aerobically fit at the time but I had a job that allowed me to work out on a universal gym.  I usually finished my workout with 20 to 30 leg presses of 550 pounds (250kg) followed by stretching.  On several occasions I felt a bit dizzy while stretching and once my pulse was about 40 and I felt terrible.  I lay down on my back with my legs elevated for several minutes and completely recovered.  A few weeks later I pushed myself to my absolute limit with 50 leg presses.  During stretches five minutes later I suddenly felt as though I were going to die.  Again, I lay down on my back but this time I was paralyzed, barely breathing and my heart was barely beating.  Someone saw me and ran and got a medic but by the time they arrived I was starting to come around.  Ten minutes or so later I felt completely normal.

I had just completed a master’s degree in exercise physiology and was working with some of the best exercise physiologists in the world but it still took a while to figure out what had happened to me, and to the diving student.

When muscles contract they get energy from a molecule called ATP (adenosinetriphosphate) and from CP (creatine phosphate).  The quantity of ATP and CP in muscles will allow you to walk for about 60 seconds, run for 30 seconds or perform all out exercise for only six seconds!  Obviously the muscles need a way to replenish these molecules.  They do this by burning glucose (sugar).  One molecule of glucose can produce 36 molecules of ATP but the reactions involved require several molecules of oxygen (O2) and produce carbon dioxide (CO2) and water.

When a muscle is exercising at a low level, ATP can be replaced by this mechanism.  As the muscle is forced to work harder, it uses ATP up faster than it can be replaced and a second, less efficient mechanism comes into play.  Glucose is broken down into pyruvic acid, producing two molecules of ATP but using no oxygen.  The pyruvic acid is broken down into CO2 and water when O2 is available but it is converted into lactic acid when O2 is not available.  Lactic acid builds up in the muscle cell and is most likely the cause of the pain you feel when you work a muscle maximally.  The lactic acid leaks out of the muscle cell into the blood and is carried to the liver where it is broken down.

In an aerobically unfit individual, lactic acid is produced when the muscle is working at over 60 per cent of maximum capacity.  An aerobically fit individual can work up to 80 per cent of maximum before significant lactic acid is produced.  An aerobically fit individual will also metabolize the lactic acid faster, reducing the maximum level reached in the blood.  Finally, a person with a large muscle mass will produce more lactic acid than a smaller person.

Both the new dive student and I had large muscles and were not aerobically fit.  We worked our muscles to maximum and produced huge quantities of lactic acid.  The peak lactic acid level is reached in the blood approximately five minutes after the maximal exercise and that is when we experienced our most severe symptoms (both of our blood pressures would have been extremely low).  As the lactic acid is converted to CO2 and water by the liver, the symptoms resolve.  The problem can be prevented by maintaining a better level of aerobic fitness, and by being a bit more reasonable when exercising!

We have seen that fainting (syncope) is very common and can be avoided by immediately lying down with your legs elevated when you start to feel light headed.  It can be caused by a very large number of problems and situations.  Fortunately, it is virtually impossible while in the water as the water completely removes the effect of gravity.  However, be cautious when climbing up a ladder onto a boat or dock after a dive, especially if you’re wearing a lot of gear, working very hard, and are dehydrated and tired, as syncope is a definite possibility.