ABC of oxygen: Diving and oxygen (2024)

ABC of oxygen

All organisms require oxygen for metabolism, but the oxygen in water is unavailable to mammals. Divers (and diving mammals such as whales and seals) are entirely dependent on the oxygen carried in the air in their lungs or their gas supply. Divers also have a paradoxical problem with oxygen. At higher partial pressures oxygen causes acute toxicity leading to convulsions. To understand the diver’s narrow knife edge between fatal hypoxia and fatal hyperoxia we need to recall some of the physical properties of gases.

Physics

At sea level atmospheric pressure is 1 bar absolute (1 standard atmosphere =101 kPa=1.013 bars). The weight of the atmosphere exerts a pressure which will support a column of water 10 m high; 10 m under water the pressure on a diver is 200 kPa. The volume of gas in an early diving bell full of air at sea level is halved at 10 m according to Boyle’s law; at 20 m pressure is 300 kPa absolute and the gas is compressed into one third the volume.

The pressure on a diver increases by 100 kPa for every 10 m he or she descends

Dry air is composed of roughly 21% oxygen, 78% nitrogen, and 1% other gases. According to Dalton’s law the partial pressure of oxygen at any depth will be 21% of the total pressure exerted by the air and the partial pressure of nitrogen will be 78% of total pressure.

Gases dissolve in the liquid with which they are in contact. Nitrogen is fat soluble and at sea level we have several litres dissolved in our bodies. If the partial pressure of nitrogen is doubled (by breathing air at 10 m depth) for long enough for equilibration to take place we will contain twice as many dissolved nitrogen molecules as at sea level.

The effect of the increased partial pressures of oxygen is more complex. Doubling our inspired partial pressure of oxygen doubles the amount of oxygen in solution but does not double the amount of oxygen in the body since a large part of our oxygen content is bound to oxygen carrying pigments. The haemoglobin in arterial blood is virtually saturated at an inspired partial pressure of oxygen (Fio₂) of 21 kPa, and increasing the partial pressure of oxygen has little effect on the amount of oxygen bound to haemoglobin.

Breath hold diving

An average healthy person with no special training can hold his (or her) breath for about half a minute. During the breath hold the oxygen content of tissues decreases, but the breath hold is broken as a result of carbon dioxide production and resulting acidosis, which stimulates the respiratory centre. With practice you can resist the stimulus to breathe for longer but it remains carbon dioxide accumulation that causes release of the breath hold.

The breath hold can be extended further by hyperventilation immediately beforehand. Hyperventilation has little effect on the oxygen content of the body but blows off carbon dioxide so that you start with a higher cerebrospinal fluid pH. Hyperventilation does not alter the rates of oxygen consumption and carbon dioxide production, but the lower initial carbon dioxide content means that the hypoxic stimulus triggers respiration before the pH of the cerebrospinal fluid falls enough to do so. It may be possible to hold a breath for over 5 minutes by hyperventilation on 100% oxygen. The hyperventilation reduces the body’s carbon dioxide content but does not affect oxygen content much, but the Fio₂ of 100 kPa considerably increases the total oxygen content.

Hyperventilation before diving enables breath hold divers to stay down longer but is very dangerous. The diver starts with a low carbon dioxide content, a high pH, and a normal oxygen tension. During descent to, say, 30 m, the pressure increases fourfold, compressing the airspaces to one quarter their surface volume (from total lung capacity of 6 l to 1.5 l, near residual volume). The partial pressures of oxygen and nitrogen in the alveoli also increase fourfold and produce corresponding increases in arterial and tissue gas tensions. The alveolar carbon dioxide pressure does not change much because there is little carbon dioxide in the lungs at this point and the body has considerable buffering capacity. During the dive oxygen is consumed and carbon dioxide is produced. Because of the hyperventilation the diver does not feel the need to breathe until the arterial oxygen tension has fallen to levels which stimulate the carotid chemoreceptors. As the diver ascends hydrostatic pressure is reduced fourfold with a fourfold reduction in oxygen tensions in alveolar gas, arterial blood, and tissues. The rapidly falling cerebral oxygen pressure may be inadequate for consciousness to be maintained and the diver could drown during ascent.

The danger of hyperventilation applies to all breath hold divers, including snorkel divers and people swimming lengths underwater in pools. The reduction in oxygen pressure when coming to the surface from the bottom of a 2 m deep pool can be enough to cause unconsciousness, and some children have died this way.

Scuba and surface supplied diving

Amateur sport diving

Non-specialist doctors are unlikely to have much involvement with commercial divers, but most general practitioners will have amateur divers among their patients. Each year in Britain there are about 12 deaths and 100 cases of serious decompression illness requiring recompression. Most occur because divers failed to follow accepted safety precautions, equipment failed, or disease placed the diver at risk. Several organisations train sport divers in clubs and commercial schools. Instructors take new divers through basic theory and pool training to progressively more challenging and deeper open water dives. The trainee should be certified as competent before being allowed to undertake dives in the company of another diver without an instructor. Further training is needed before the qualified diver can progress to more adventurous diving.

Before anyone is allowed to start diving, and periodically when diving, they have to pass a diving medical examination to ensure freedom from diseases which might predispose to incapacity in the water or to diving related illnesses. The requirements for amateurs in all diving clubs in the United Kingdom are laid out in a common medical form.

Lung disease in divers is a particular problem. Significant lung disease which impairs exercise performance and the ability to cope with physically demanding conditions is obviously a contraindication to diving. Asymptomatic lung disease which does not affect exercise capacity is also a problem. Any lung disease which causes generalised or localised gas trapping (such as emphysema, bullae, cavities) may predispose to pulmonary barotrauma during ascent, even when the ascent rate is less than 10-15 m/min. During ascent from a dive the gas in a bulla increases as ambient pressure is reduced. If the bulla cannot empty adequately during the ascent it will burst causing local lung damage, pneumothorax, surgical emphysema, or arterial gas embolism. Gas in a pneumothorax will expand as pressure is reduced, causing a tension pneumothorax.

In the United Kingdom people with mild asthma who satisfy criteria laid down by the United Kingdom Sport Diving Medical Committee may be approved to dive by a medical referee. In some countries anyone with a history of asthma, even childhood asthma decades before, is not permitted to dive. Ironically those countries allow smokers to dive, yet a long term heavy smoker with evidence of small airways disease on flow-volume loops is probably at greater risk of pulmonary barotrauma than a patient with mild asthma who has never smoked.

There are also medical standards for non-respiratory diseases. People are advised not to dive if they have a condition which may cause incapacity in the water—for example, epilepsy or cardiac arrythmias—or predispose to diving related diseases. Intracardiac shunts predispose to decompression illness and hypertension predisposes to diving induced pulmonary oedema.

Footnotes