Freediving its Physiology and potential hazards

By Fred Bove, M.D., Ph.D.

Most of us don't find sport diving equipment excessive but others find the simplicity of freediving a significant advantage. In many of the tropical islands of the Caribbean, freediving to reefs 20 to 30 feet deep will provide many of the same visual pleasures obtained by scuba divers. When I watch a group of divers (myself included) donning equipment for a dive, I am often struck by the contrast between this routine and the simplicity of breath-hold diving. Clearly, the advantage of scuba is increased bottom time. Yet, many freedivers see and enjoy the underwater world without the burden of a compressed air supply.

Human freedivers cannot match the performance of marine mammals such as whales, who can dive to 1,000 feet for an hour or more and return to the surface without decompression. We are land mammals, poorly adapted to life in the sea. Whales, porpoises and seals, along with other diving species, have adapted physiologically to survive prolonged underwater exposure. Similar physiologic adaptations can be found in people but only to a minimal degree. In diving animals, adaptive mechanisms have developed to allow more oxygen storage before a dive and less oxygen use during a dive. The ability of muscles to work with limited oxygen is well developed in the diving mammals and in humans, and comes into play with exercise of any kind. Diving mammals are able to hold large stores of oxygen in their muscles via high concentrations of the protein myoglobin, which is similar to the hemoglobin in the blood. This internal reservoir of oxygen can be used U/W.

The hemoglobin in the blood of diving mammals also seems to let go of the oxygen easier, thus allowing the oxygen stores to be used efficiently. Reduced oxygen use is accomplished by a unique control system that shuts off blood to organs and tissues not needed during a dive, then restores flow when the animal surfaces. The stomach, liver, intestines and kidneys can tolerate a reduced blood supply during a dive since their functions can be temporarily suspended. Muscles can function without externally supplied oxygen for a short time, so they are also deprived of blood during a dive.

The metabolic debt accrued during the dive is stored in the form of lactic acid, which is metabolized after oxygen is restored to the muscle. If these tissues are deprived of blood and oxygen, where does the oxygen go? To perform properly, a diving animal must have a fully functional brain and heart. These two organs have little tolerance for reduced oxygen. Oxygen needs for these two organs is about 25 percent of the total body need. The combination of extra stored oxygen plus redirection of blood from nonvital organs to the brain and heart gives the animal the needed bottom time.

The reflex controls that shift blood to the brain and heart are only partially understood. They are collectively called the diving reflex. It results in a marked reduction of heart rate and of the amount of blood pumped per minute by the heart. In a seal for example, the heart rate can fall from 50 beats per minute on the surface to 8 beats per minute underwater. In humans, we find a rudimentary diving reflex that can lower heart rate significantly during diving but which is of minor importance in prolonging bottom time on a freedive. In a study several years ago, we found the heart rate could be slowed even more by physical conditioning; one wonders whether we could adapt better by prolonged training.

Potential Hazards

Shallow water blackout occurs when a freediver hyperventilates to prolong bottom time. The excess breathing does little to increase oxygen stores; its main effect is to lower carbon dioxide in the blood. Since the urge to breathe comes from the combination of high C02 and low O2 in the blood, a low oxygen level can develop before the urge to breathe is strong enough to force it to resume. With a rapid ascent to the surface from even a moderate depth, the sudden fall in ambient pressure will cause oxygen partial pressure to fall even farther and cause a blackout either at or near the surface. The situation is perfect for a drowning accident. Prevention is accomplished by avoiding prolonged hyperventilation before a freedive and not pushing bottom time to the limits of endurance.

Decompression sickness is possible while freediving. However, it only occurs with repeated deep dives and short surface intervals. Usually fatigue will develop before DCS. There have been several excellent studies of this phenomenon, known as taravana to the pearl divers of the South Pacific.

Chest squeeze is unlikely on a freedive because blood shifts into the lung blood vessels to fill up the space left behind by the reduced air volume. Chest squeeze can occur when breath-hold diving to depths of 150 feet or greater, however, deeper freedives (of more than 250 feet) have been done by individuals with large chest volumes that can withstand greater degrees of compression. Chest squeeze results in bleeding into the lungs and broken ribs.

In spite of current knowledge about freediving, it has severe limitations. So I will reluctantly continue to don wetsuit, weightbelt, BC, tank, regulator and everything else to do what some animals can do with a single gulp of air.