by Cameron Martz

Cardiovascular Conditioning

Some experienced divers believe that exercise will do little for their diving since they already have a good breathing rate and efficient water skills. This is a very limited view of what their heart and lungs do for them. Achieving a high level of cardiovascular fitness does a lot more than just improve your gas consumption. It may also increase the safety of your dives in several ways:

1. Increased physical reserves for dealing with problems.

2. Delayed/reduced panic response.

3. Increased rate of inert gas elimination.

4. Reduced cost of free phase gas formation.

Increased Physical Reserves:

This is a no-brainer. The fitter you are, the more physically demanding a task you can handle successfully. You can swim faster and farther. You can manage larger amounts of equipment. Your heartrate and respiration are lower, and gas consumption will increase less for a given increase in workload. There are no more obvious results of cardiovascular conditioning than these.

Imagine being able to upgrade your car from your current engine to one that is more powerful and gets better gas mileage. As your muscle cells adapt to exercise, they increase their number of mitochondria (the energy machines of the cell) and their quantity of aerobic enzymes (the oxygen-utilizing chemicals). With these adaptations, muscle cells become stronger while becoming much more efficient with the oxygen they receive. A fit diver will thus be able to perform more work with each breath of gas, or use less gas to perform the same amount of work as a less fit diver. Not only does this give you the capacity to do more during each dive, this also increases the chance that you can solve any problems that might arise.

The Panic Response:

An important side benefit of a reduction in heart rate and respiration relates to the panic response. The human brain responds to an increase in heart rate and respiration with an increased emotional response, whether it is love, anger, or panic. A feedback loop forms in a tense situation when a diver senses danger, then responds with an increase in physical activity. This causes the divers heartrate and respiration to increase, which results in the brain increasing its perception of danger, which then elevates the divers heartrate and respiration, which then further increases the brains perception of danger, and so on until the diver can no longer perform the appropriate response. Thus, the fitter you are, the further you will be from your panic threshold merely because your heartrate and respiration are not as affected by increasing physical demands.

Increased Rate of Inert Gas Elimination:

The rate at which inert gas is eliminated from the tissues of your body for a given pressure gradient depends upon the solubility and vascularity of the tissues and the efficiency of your lungs.

Fat tissue off-gasses much more slowly than lean tissue, partly because it holds a greater quantity of dissolved gasses than other tissues. This storage problem is further compounded by the tissues low vascularity. Cardiovascular training, when combined with a healthy diet, will result in an increased ratio of lean tissue to fat tissue in an athletes body. The body of a fit diver will off-gas as a system faster than that of an unfit diver.

Keep in mind that a reduction in adipose tissue, or body fat, reduces the amount of natural insulation a diver has. Thus, adequate protection from the water becomes even more important for the fit diver.

Cardiovascular training also increases the efficiency of the lungs through several mechanisms. As you overload your cardiovascular system through exercise, you stimulate your lungs to exchange carbon dioxide and oxygen, primarily, at a much faster rate. Your body adapts by increasing the vascularity of the lung tissue as well as increasing the surface area of the lungs at the alveoli. Not only is a greater quantity of blood present in the lungs of a fit diver, but the vascular changes also allow a faster rate of gas diffusion for each unit volume of blood. Fortunately, these adaptations are not specific to oxygen or carbon dioxide- a pressure gradient for any gas will result in an increased transfer of that gas from blood to lungs. We will return to the importance of these effects later.

Reduced Cost of Free Phase Gas Formation:

Every dive has the potential to produce bubbles, whether it be a thirty-minute shallow reef dive or a world record setting deep cave penetration. The size and amount of bubbles formed depend primarily upon the amount of dissolved gas and the rate of ascent. Mismatch the rate of ascent for the amount of dissolved gas, and the bubbles set in motion a series of problems, known to divers as Decompression Sickness (DCS).

Contrary to popular belief, bubbles are not the only cause of blockages in the circulation. The arterial capillaries are generally large enough to allow the passage of many free phase bubbles. Rather, it is believed to be the secondary effects of these bubbles that cause many of the blockages, or emboli, associated with DCS.

The emboli believed to be associated with DCS result from several sources. The body releases several types of chemicals in response to the vascular insult resulting from bubble formation, and these chemicals have been shown to reduce the blood supply to the tissues, even without the presence of gas emboli. Additionally, certain proteins involved in the bodys defense against illness may adhere to the bubbles themselves, causing blockages and decreasing the permeability of the bubbles. These proteins not only increase the size of the bubbles, but they also increase the time required to clear the bubbles out of the bloodstream.

Divers must always keep in mind that our lungs are our first defense against the effects of breathing compressed gas. The diffusing capacity of the lungs is much greater than needed at rest. This built-in safety factor is what allows the lungs to act as a very effective bubble filter in the event of free phase gas formation in the bloodstream. The alveoli are designed to trap both solid and gaseous emboli, preventing them from traveling further through the circulation. Gaseous emboli are eliminated through diffusion, which as described above, is improved through cardiovascular conditioning. Cardiovascular conditioning further increases this safety factor by allowing the lungs to trap a greater quantity of bubbles within their increased surface area and vascularity.

Not all emboli are filtered by the lungs, however. Small bubbles can pass through the pulmonary circulation only to collect and form emboli elsewhere in the body. Also, the accumulation of proteins and platelets occur throughout the circulatory system as a result of free phase gas formation. This is where the other vascular effects of cardiovascular conditioning may become so important.

The diameter of blood vessels varies based upon a number of factors, including vascular insult. However, cardiovascular conditioning increases the maximum possible diameter of many existing blood vessels. Thus, an embolus may travel further downstream before becoming lodged. The further down the circulation an embolus can pass, potentially fewer branches will be blocked and less tissue will be affected.

Cardiovascular conditioning increases collateral circulation, which means that a given mass of tissue may have more pathways from which to receive oxygen-rich blood. Thus, if an embolus becomes lodged in one pathway, the tissues of a fit diver may receive more blood than those of an unfit diver via other pathways.

Cardiovascular conditioning also increases the efficiency with which cells utilize the oxygen they receive. In other words, tissues of a fit diver require less oxygen to maintain their base metabolic rate than those of an unfit diver. This has to do with an increase in aerobic enzymes contained with in the cells, as well as a few other structural changes to the cells. Thus, tissues of a fit diver may better survive a reduction in blood supply compared to those of an unfit diver.