An Unusual Cause of Fainting

Fainting is called "syncope" in medical circles and syncope is defined as "a fainting or swooning; a sudden fall of blood pressure or failure of the cardiac systole, resulting in cerebral ischemia and subsequent loss of consciousness". In more common English, a person passes out because the blood pressure falls and not enough blood is going to the brain to deliver the oxygen it needs to function normally. The body is well designed and when you faint, you usually fall over. This puts the head at the same level as the heart and makes it much easier for the blood to get to the brain. Therefore, when you feel weak and dizzy, if you immediately lie down or even better, lie down and prop your legs up on something so that gravity pulls the blood in your legs back to your heart, you will never faint.

Stedman's medical dictionary defines six specific types of syncope although there are several dozen reasons that blood pressure can fall. Carotid sinus syncope results when overactivity of the carotid sinus sends a signal to the heart and the heart slows down so much that it fails to maintain a normal blood pressure. The carotid sinus is a receptor in the large arteries in the neck and it normally measures the blood pressure in the carotid artery and controls the heart so that the pressure remains constant. The carotid sinus can be stimulated by pressure, a tight collar (wet or dry suit), or a massage of the neck can stimulate it and cause an abnormal drop in blood pressure. Hysterical syncope is an emotional response, often to avoid stress, that results in a drop in blood pressure. Laryngeal syncope is an unusual form of fainting caused by coughing. Micturation syncope occurs when a person is urinating and postural syncope results when the normal mechanisms that maintain blood pressure when we stand up, suddenly fail. The final specifically defined form of syncope is vasovagal syncope. The vagus nerve runs to the heart and when it is stimulated it causes the heart rate to slow down.

We have come full circle as stimulation of the carotid sinus receptors and many other events such as coughing result in stimulation of the vagus nerve and thereby a drop in heart rate and blood pressure and failure of adequate delivery of oxygen to the brain. The following two stories are of young and healthy divers who most likely had a loss of consciousness for the same, uncommon but important reason.

A few weeks ago I was asked to review a student on a basic diving course to see if they were fit to continue the course and dive. The student was 32 years old, very muscular and appeared to be very fit. On the first night of the course he jumped into the pool to swim the required 16 lengths and set off at a great speed. After 6 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 down the side of the pool but felt so weak that he 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 get a pulse or detect 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 was largely recovered but was convinced to go the hospital anyway. He was examined by the doctor there, his blood work was entirely normal, he was told he "fainted", and he was released. He felt completely fine.

Several years ago, when I was approximately 35, I had several very similar experiences. I was working at DCIEM and often had to stay in the building to provide medical coverage for experiments that were running over lunch. Therefore I could not go for a run and instead often worked out on the universal gym in the diving section. I would go through my program and near the end would sit down to do leg presses. I would do one set of leg presses with 250 kg usually for 20 to 30 repetitions and then do one more exercise and stretch. On several occasions I would get a bit dizzy while I was stretching and on two occasions came very close to passing out. The first time I felt very weak and lay down on my back. My heart slowed down to less than 40 beats per minute, I was breathing very slow and shallow, and felt absolutely terrible. The second time someone wandered by and saw me lying on the floor and went and got the medics. They were very concerned as I was unable to move or answer them for several minutes and my pulse was almost undetectable. On both occasions, I had pushed myself to my limit and had done 50 leg presses with 250 kgs. Of interest, I felt okay when I finished the leg presses, it was about five minutes later that the problems started. Both times it took about 30 minutes for me to recover fully.

I had just finished a Masters Degree in exercise physiology, had ready access to some of the top exercise physiologists in the world (at DCIEM) and of course had my medical background to draw upon. Nevertheless, it was very difficult to figure out what was happening. The following explanation is what I believe happened to me several years ago and to the diving student a couple of weeks ago.

When muscles contract they get the 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 be used to produce 36 molecules of ATP but the reactions involved require several molecules of oxygen and produce CO2 and H2O as end products.
When a muscle is exercising at a low level, the ATP can be replaced by this mechanism. As the muscle is forced to work harder it uses the ATP up faster than it can be replaced and a second, less efficient mechanism comes into play. The problem is that the blood can not deliver enough oxygen to the working muscle for some of the necessary reactions to occur. When one molecule of glucose is broken down, the first few reactions divide it into two molecules of pyruvic acid. These reactions do not require oxygen and they produce two molecules of ATP. When oxygen is available, the pyruvic acid enters the "Krebs cycle" to produce a total of 32 more molecules of ATP per molecule of glucose. When oxygen is not available, the pyruvic acid is converted to lactic acid. The lactic acid builds up in the working muscle and is probably the cause of the muscle pain that results when you work a muscle to its maximum. Although the production of only two molecules of ATP per molecule of glucose is not nearly as efficient as the production of 36 molecules, these reactions allow the muscle to do significantly more work than would be possible if all the ATP had to be produced by reactions involving oxygen. Now, what does all this mean?

The level of work at which these reactions become important depends upon many factors including the genetic makeup of the muscles, the type of training that the muscles have been exposed to in the recent past, and the type of training that the person has been doing. In the untrained individual, this "anaerobic metabolism" (not requiring oxygen) becomes important at about 60% of maximum effort. In the aerobically trained individual however, anaerobic metabolism does not really get going until 70% or even 80% of maximum! In addition, aerobic training increases the ability of the body to metabolize the lactic acid and therefore the levels in the blood are kept lower.
There are basically two kinds of muscle cells. "Slow twitch" muscles cells contract slowly, have a good blood supply, and can be thought of as endurance cells. "Fast twitch" muscle cells on the other hand contact very quickly, have a limited blood supply, and can be thought of as powerful but short duration cells. The percentage of each type of cell in your muscles is determined genetically and can probably not be changed. When you do endurance muscle training, you enlarge the endurance cells while intense strength training tends to enlarge the fast twitch cells. The only way to determine the percentage of each cell type in a muscle is to cut out a piece of the muscle and study it under a microscope with a special stain (I have had many pieces of muscle cut out as part of experiments at DCIEM and know that I have a high percentage of fast twitch cells). A good guess can be made, however, without the requirement for a muscle biopsy.

A person who is very strong for their size but who has limited endurance will most likely have a high percentage of fast twitch cells (eg. competitive weight lifters, not surprisingly I used to compete). Conversely, a person who is fairly strong but who is mostly noted for the ability to continue working for a very long time (endurance athletes) will most likely have more slow twitch cells. Most people will be somewhere in the middle.

With this background we can start to understand what happened to the diving student and to me. The situation where the person will generate the maximum amount of lactic acid is as follows. They will have a high percentage of fast twitch muscles cells, they will be very muscular, and they will have done little or no aerobic training in the past several months. They will then engage in an activity where they are working most of the muscles in their body to their absolute maximum. The lactic acid will build up in the muscles and after they stop exercising the maximum level will be reached in the bloodstream about five minutes later. We know from various medical diseases that a high lactic acid level causes a fall in blood pressure, shock, very limited breathing, stupor and/or coma. The student diver and I both fit the picture of the high risk individual and we both had symptoms consistent with high blood lactic acid levels and a time course that matches the known rise in levels after exercise.

So how does the high risk individual prevent this very scary and potentially serious problem from occurring? The first step is to get into shape aerobically. The student looked very fit and had been a highly competitive athlete in his younger years but he had not worked out for the past two years. I had not been doing much aerobic training either. Becoming aerobically fit will increase the percentage of maximal exercise required to generate significant lactic acid and will increase the ability of the body to metabolise it. Both of these effects will reduce the maximum lactic acid levels after exercise. A second technique is to take it a little easier, especially when performing total body exercise. Finally, if you make a mistake and push yourself too hard, when you feel faint, lie down on you back and prop your legs up on something. I have had to do this on several occasions and have never again experienced the extreme physiological symptoms described above.
In conclusion, these two stories are examples of an unusual cause of loss of consciousness. They are not common but the instructor or active diver will occasionally run into them and should be aware that they exist.