Improving Air Consumption
By Fred Bove, M.D., Ph.D.
I recently received a query from a diver who is concerned because he uses his air supply faster than his diving partners. Exploring the causes of rapid air consumption and the factors that control breathing while diving can be a helpful exercise for all divers.
Breathing gas is consumed to supply oxygen to the body and to remove carbon dioxide. The consumption of oxygen is directly related to the intensity of physical activity. When workload increases, oxygen consumption increases and the ventilation rate (the amount of air moved in and out of the lungs each minute) increases. In diving, oxygen consumption is also increased by shivering, which is the body’s method of generating heat from involuntary muscle activity. Although oxygen constitutes about 21 percent of air, in a single breath on the surface less than a quarter of the oxygen is removed. On the surface, air enters the lungs carrying about 21 percent oxygen, and leaves the lungs carrying about 16 percent oxygen. At depth, because of the increased partial pressure, the same consumption of oxygen will extract a smaller percent from the compressed air, so that the exhaled air will contain more than16 percent oxygen. The missing oxygen is replaced by carbon dioxide.
If the breathing gas contains a higher percentage of oxygen, one might think that the oxygen could be delivered with fewer breaths, ventilation rate could be reduced, and the gas supply would last longer. However, this is not the case because a ventilation rate that prevents accumulation of carbon dioxide in the blood must be maintained.
Carbon Dioxide Production
Body metabolism follows two pathways depending on the availability of oxygen. Tissues and organs require oxygen for steady state performance and function but can withstand periods of inadequate oxygen by changing metabolism. Our metabolic fuels consist of sugar and fat. Fat metabolism is preferred during exercise but requires oxygen. If adequate oxygen is not available, fat is not metabolized, and the tissues are obligated to metabolize sugar. The burning of sugars and fats in the body when oxygen is available (aerobic metabolism) results in production of carbon dioxide. When needed oxygen is not available, tissues revert to metabolism without oxygen (anaerobic metabolism).
Sugar metabolism without oxygen produces lactic acid. As lactic acid builds up in muscles and in the bloodstream, the muscles become more acidic, and muscle performance degrades.
After exercise is finished and oxygen is again available, lactic acid is metabolized to carbon dioxide. Because carbon dioxide is eliminated by respiration, breathing does not return to resting level after exercise until all the lactic acid is metabolized. Oxygen and carbon dioxide levels in the blood and the acidity of the blood all contribute to the control of ventilation.
Respiratory rate underwater therefore depends not only on oxygen need, but also on the need to remove carbon dioxide, and the need to control the acidity of the blood. Oxygen-rich breathing gas offers no advantage in reducing gas consumption because when oxygen is abundant, ventilation rate is controlled by the need to eliminate carbon dioxide and to maintain normal acidity of the blood. Thus a breathing gas mixture rich in oxygen cannot be conserved by reducing the breathing rate. This practice (skip breathing) leads to increased carbon dioxide levels in the blood and carbon dioxide toxicity.
Under any diving condition, gas consumption depends on the intensity of physical activity. Exercise intensity depends on how fast a diver swims, the efficiency of swimming and, in some cases, the need to generate heat to combat hypothermia. A large diver will consume more oxygen and produce more carbon dioxide because of the increased amount of muscle tissue. A small diver has an advantage in gas consumption because of a smaller muscle mass.
A diver with severe heart or lung disease consumes air at a very high rate during even minimal exercise. These individuals are so poorly conditioned that they produce lactic acid at a low level of exercise. If an individual has a low maximum oxygen consumption, anaerobic exercise will begin at a low workload and increased ventilation will be required to balance the acid production in the blood. These individuals should not be diving due to limitations caused by illness.
Another cause of high gas consumption is hyperventilation related to excitement or anxiety. This is a common cause of increased gas consumption in novice divers. When a healthy diver, who is not exercising heavily underwater, uses gas at a rapid rate, the cause is usually anxiety. Often gas consumption improves with experience. A diver who breathes rapidly underwater for no physiological reason requires more training to develop an improved comfort level when diving.
For most divers, a single 80 cubic foot tank compressed to 3,000 PSI with air or nitrox should last 50 to 60 minutes on a typical multilevel dive ranging from 40 to 100 feet. If you are using gas at a faster rate or if you consume your air supply well before other members of your diving group, you should have a health checkup to rule out heart or lung disease. If no health problems are present, seek advice about your diving technique and breathing from a diving instructor. Lowering gas consumption is accomplished by avoiding heavy exercise, avoiding hypothermia, learning to be comfortable underwater and maintaining good health. This will be the case whether you breathe air or oxygen-enriched mixed gas. Finally, be sure air is not leaking from your tank fittings, hoses or regulators.
For more information on diving medicine, visit www.scubamed.com