Hypoxia and Breath-Hold Diving

In a simple breath-hold, with no immersion or preceding hyperventilation, the breaking point (the irresistible urge to breathe) is initiated mainly by a rise in CO2 level, and to a lesser extent by a fall in arterial O2.

In breath-hold diving, hypoxic blackout is sometimes called breath-hold syncope or shallow water blackout. Because ‘shallow water blackout’ was first used in 1944 to describe loss of consciousness from the use of closed-circuit diving suits, it is best avoided in the breath-holding context. ‘Hypoxic blackout’ is a reasonable alternative.

There are two causes of this disorder – hyperventilation and ascent – and because they may occur concurrently, they are often confused. The hyperventilation effect is independent of depth and may be encountered in 1-meter-deep swimming pools, often by children trying to swim greater distances underwater.

Breath-hold divers who train to extend their breath-hold and also dive deep (e.g. free diving competitors, spear fishing) risk hypoxia of ascent, with loss of consciousness and subsequent drowning.

With hypoxia there is little or no warning of impending unconsciousness. With increased experience the breath-hold diver can delay the need to inhale by various techniques, without improving the O2 status. Breath-hold time can be extended (but not with increased safety) by, for example, feet-first descent, training (adaptation), swallowing, inhaling against a closed glottis and diaphragmatic contractions.

One way of avoiding this hypoxia is to inhale 100 per cent O2 before the breath-hold.


In 1961, Craig observed that swimmers who hyperventilated could stay longer underwater but then lose consciousness with little or no warning1. They were often competing, against others or themselves, and often exercising. The hyperventilation extended their breath-holding time because it washed out a large amount of CO2 from the lungs, often to half the normal levels.

The build-up of CO2 is the main stimulus forcing the swimmer to surface and breathe. After hyperventilating it takes much longer for this level (the ‘breaking point’) to be reached. Under these conditions, the diver may extend the breath-hold to the point that PaO2 drops to a level inadequate to sustain consciousness. Increased exercise exacerbates this effect by increasing O2 consumption.

The combination of these two effects (hyperventilation and exercise) can be deadly. One can demonstrate this dangerous combination in the following experiment: When the swimmer is concentrating on some purposeful goal, such as trying to spear a fish or retrieve a catch, he or she is more likely to ignore the physiological warning symptoms of an urge to breathe (resulting from the rise in CO2 level in the blood) and delay the breaking point.

The dangers of hyperventilation and breath-hold diving are diagrammatically illustrated in Figure 16.3. It illustrates that, with earlier hyperventilation, the time to reach the irresistible urge to breathe (the breaking point) is prolonged. This extra time may allow the PaO2 to fall to dangerous levels (hypoxic danger zone).

A diagrammatic representation of changes in arterial oxygen and carbon dioxide levels with breath-holding.
Figure 16.3 A diagrammatic representation of changes in arterial oxygen and carbon dioxide levels with breath-holding. Point A, without preceding hyperventilation; point B, with preceding hyperventilation. PaCO2, arterial partial pressure of carbon dioxide; PaO2, arterial partial pressure of oxygen.


Ascent hypoxia was first described in military divers losing consciousness as they surfaced with low O2 levels in their rebreathing equipment.

In breath-hold divers, with descent the pressure rises proportionately in the alveolar gases, thus increasing the available O2, CO2 and nitrogen. Some O2 can be absorbed and used, some CO2 absorbed and buffered, and some nitrogen absorbed and deposited in tissues.

Thus, if a diver having 100 mm Hg O2 and 40 mm Hg CO2 in the alveolar gases was immediately transported to 2 ATA, the lungs would halve their volume; the O2 would be 200 mm Hg, and the CO2 80 mm Hg. Both would pass into the pulmonary capillary blood, the O2 to be used and the CO2 to be buffered. The alveolar PO2 and PCO2 therefore would decrease rapidly. By the time these values were both back to ‘normal’ levels, with O2 at 100 mm Hg and CO2 at 40 mm Hg, the diver would appear to be in a satisfactory respiratory status – until he or she ascended. With an expansion of the lungs to twice their size at depth, the pressures in both gases would halve; i.e. the O2 would drop to 50 mm Hg (approaching a potentially dangerous hypoxia level) and the CO2 to 20 mm Hg – if the ascent was immediate.

Because ascents do take time, more O2 will be consumed, extracted from the lungs during the ascent, and the CO2 will increase toward normal as a result of the gradient between the pulmonary blood and alveoli.

The drop in O2 is then able to produce the loss of consciousness, the ‘syncope’ or ‘blackout’, commonly noted among spear fishers. This condition is referred to as hypoxia of ascent. In deeper dives it becomes more likely, and with some very deep dives, the loss of consciousness may occur on the way to the surface in the top 10 metres (probably an explanation for the ‘7-metre syncope’ described by French workers).

Other causes of hypoxia in breath-hold diving include salt water aspiration and the drowning syndromes (see Chapters 21, 22 and 24).

Snorkeling/Breath-Hold Diving Equipment

The simplest assembly of diving equipment is that used by snorkelers – a mask, snorkel and a pair of fins. In colder climates, a wetsuit may be added for thermal insulation and a weight belt to compensate for the buoyancy of the suit. In tropical waters, a ‘stinger suit’ provides not only a little thermal comfort but also some protection from box jellyfish and other stings.


A mask is needed to give the diver adequate vision underwater. The mask usually covers the eyes and nose. Traditionally, masks were made from rubber, although now most are made from silicone. The mask seals by pressing on the cheeks, forehead and under the nose with a soft silicone edge to prevent entry of water. Swimming goggles, which do not cover the nose, are not suitable for diving. The nose must be enclosed in the mask so that the diver can exhale into it to allow equalization of the pressure between the face and mask with the water environment. It should be possible to block the nostrils without disturbing the mask seal to enable the wearer to perform a Valsalva manoeuvre. Full-face masks that cover the mouth as well as the eyes and nose, or helmets that cover the entire head, are more commonly used by professional divers and are considered in the section on professional diving equipment.

The faceplate of the mask should be made from hardened glass. A diver with visual problems can choose from a selection of corrective lenses that are commercially available. These are designed to attach directly to certain masks.

Alternatively, prescription lenses can be ground and glued to a variety of masks. Ocular damage can occur if hard corneal lenses are used for diving (see Chapter 42). Certain contact lenses may be lost if the mask floods and the diver fails to, or is unable to, take preventive action. Some people with allergy problems react to the rubber of the mask, although this is rarely an issue with silicone.

All masks cause a restriction in vision. With most masks, the diver can see about one third of his or her normal visual field. The restriction is most marked when the diver tries to look down toward the feet. This restriction can be a danger if the diver becomes entangled. However, there are some masks available with a tilted lens to provide a better downward field of vision.

The more nervous beginner may find the visual restriction worrying and may possibly fear that there is a lurking predator just outside the field of vision. The visual field varies with the style of mask. Experimentation is also needed to find which mask gives a good seal, to minimize water entry. The diver needs to master a technique to expel water from the mask. If it is not learned and mastered, a leaking mask can become a major problem, sometimes leading to panic.


The typical snorkel is a tube, about 40 cm long and 2 cm in diameter, with a pre-moulded or creatable U-bend near the mouth end. A mouthpiece is fitted to allow the diver to grip the tube with the teeth and lips. The tube is positioned to pass upward near the wearer’s ear to enable him or her to breathe through the tube while floating on the surface and looking down. Any water in the snorkel should be expelled by forceful exhalation before the diver inhales through the snorkel.

Many attempts have been made to ‘improve’ the snorkel by lengthening it, adding valves, modifying its shape and some other means. There is little evidence of the success of most of these attempts.

All snorkels impose a restriction to breathing. A typical snorkel restricts the maximum breathing capacity to about 70 per cent of normal. The volume of the snorkel also increases the diver’s anatomical dead space. Because of this, increasing the diameter substantially to reduce the resistance is not a viable option. These problems add to the difficulties of a diver who may be struggling to cope with waves breaking over him or her (and into the snorkel) and a current that may force the diver to swim hard. There have also been anecdotal reports of divers inhaling foreign bodies that have previously lodged in the snorkel.


Fins (or flippers) are mechanical extensions of the feet. Fins allow the diver to swim faster and more efficiently, and they free the diver’s arms for other tasks. The fins are normally secured to the feet by straps or are moulded to fit the feet. Various attempts have been made to develop fins that give greater thrust with special shapes, valves, controlled flex, springs and materials, all competing for the diver’s dollar. Some of these fins can improve the thrust, but the wearer needs to become accustomed to them. Others have little effect.

Divers often get cramps, either in the foot or calf, if fins are the wrong size, if the diver has poor technique or if the diver has not used fins for an extended period. The loss of a fin may also cause problems for a diver, especially if he or she has to a swim against a current, or fails to attain appropriate orientation underwater or buoyancy on the surface.


Even without the buoyancy of a wetsuit, some divers require extra weights to submerge easily. The weights are made from lead, and most are moulded to thread onto a belt. Some weights are designed to fit into pouches, either on a belt or, for scuba divers, attached to a buoyancy compensator device (BCD). Whatever weighting mechanism is used needs to be fitted with a quick-release buckle or other mechanism to allow a diver to drop the weights quickly and so aid his or her return to, or enable the diver to remain on, the surface. The situations in which a quick-release buckle may not be fitted (or may be de-activated) are those where it would be dangerous to ascend, such as in caves where there is no air space above the water.

In some circumstances, it is necessary for a diver to ditch the weight belt to reach, or remain on, the surface in an emergency. Such situations include an emergency in which the scuba diver cannot inflate the BCD, for example, if the diver is out of breathing gas. Unfortunately, divers often fail to release the belt if they are in difficulty. The reason for this omission is not clear, but it is likely often the result of stress or panic. Adequate initial training and practice help to reinforce the skill so that it will become more automatic when required. It also needs to be reinforced periodically. Unfortunately, much of the current training fails to focus adequately on this important emergency drill.

An alternative drill of taking the belt off and holding it in one hand (preferably away from the body) is useful in some situations in which the diver is likely to become unconscious and inflating the BCD is not an option or may not be sufficient (e.g. when deep). In the event of unconsciousness, the belt will hopefully fall away, causing the diver to rise to the surface. Holding the belt away from the body should reduce the chance of entanglement with the diver if it is dropped.

In many fatal diving accidents the diver did not release his or her weights.

This basic free diving equipment is adequate for diving in shallow, relatively warm water. Experience with this gear is excellent training for a potential scuba diver. The diver can gain the basic skills without the extra complications caused by scuba gear. It allows a more realistic self-assessment of the desire to scuba dive and the subsequent rewards. With the confidence gained in snorkeling and breath-hold diving and the associated aquatic skills, the diver is also less likely to become as dependent on the breathing apparatus. In cold climates, a snorkel diver needs a suit to keep warm. Suits are discussed in Chapter 27.


Breath-Hold Diving

The origins of breath-hold diving are lost in time. Archaeologists claim that the Neanderthal human, an extinct primitive human, dived for food, likely in the first instance gathering shellfish by wading at low tide before diving from canoes. By 4500 BC, underwater exploration had advanced from the first timid dive to an industry that supplied the community with shells, food and pearls.

From the ancient Greek civilization until today, fishers have dived for sponges, which, in earlier days, were used by soldiers as water canteens and wound dressings, as well as for washing.

Breath-hold diving for sponges continued until the nineteenth century when helmet diving equipment was introduced, allowing the intrepid to gamble their lives in order to reach the deeper sponge beds. Greek divers still search the waters of the Mediterranean Sea as far afield as northern Africa for sponges.

The ancient Greeks laid down the first rules on the legal rights of divers in relation to salvaged goods. The diver’s share of the cargo was increased with depth. Many divers would prefer this arrangement to that offered by modern governments and diving companies.

In other parts of the world, industries involving breath-hold diving persist, to some extent, to this time. Notable examples include the Ama, or diving women of Japan and Korea, and the pearl divers of the Tuamoto Archipelago.

The Ama has existed as a group for more than 2000 years. Originally the male divers were fishermen, and the women collected shells and plants. The shells and seaweed are a prized part of Korean and Japanese cuisine. In more recent times, diving has been restricted to the women, with the men serving as tenders. Some attribute the change in pattern to better endurance of the women in cold water. Others pay homage to the folklore that diving reduces the virility of men, a point many divers seem keen to disprove.

There is a long history of the use of divers for strategic purposes. Divers were involved in operations during the Trojan Wars from 1194 to 1184 BC. They sabotaged enemy ships by boring holes in the hull or cutting the anchor ropes. Divers were also used to construct underwater defences designed to protect ports from the attacking fleets. The attackers in their turn used divers to remove the obstructions.

By Roman times, precautions were being taken against divers. Anchor cables were made of iron chain to make them difficult to cut, and special guards with diving experience were used to protect the fleet against underwater attackers.

An interesting early report indicated that some Roman divers were also involved in Mark Anthony’s attempt to capture the heart of Cleopatra. Mark Antony participated in a fishing contest held in Cleopatra’s presence and attempted to improve his standing by having his divers ensure a constant supply of fish on his line. The Queen showed her displeasure by having one of her divers fasten a salted fish to his hook.
Marco Polo and other travellers to India and Sri Lanka observed pearl diving on the Coromandel Coast. They reported that the most diving was to depths of 10 to 15 metres, but that the divers could reach 27 metres by using a weight on a rope to assist descent. They carried a net to put the oysters in and, when they wished to surface, were assisted by an attendant who hauled on a rope attached to the net. The divers were noted to hold their nose during descent.

The most skilled of the American native divers came from Margarita Island. Travellers who observed them during the sixteenth, seventeenth and eighteenth centuries reported that these divers could descend to 30 metres and remain submerged for 15 minutes. They could dive from sunrise to sunset, 7 days a week and attributed their endurance to tobacco! They also claimed to possess a secret chemical that they rubbed over their bodies to repel sharks. The Spaniards exploited these native divers for pearling, salvage and smuggling goods past customs. The demand for divers was indicated by their value on the slave market, fetching prices up to 150 gold pieces.

Free diving appears to have evolved as a modern sport in the mid-1940s, initially as a competition among Italian spearfishers. Currently the sport, which is steadily gaining popularity, encompasses a variety of disciplines. These include the following:

In ‘no limits’, a diver can use any means to travel down and up the line, as long as the line is used to measure the distance. Most divers descend down a line using a weighted sled and return to the surface aided by an inflatable balloon. Officially recorded depths in excess of 210 metres have been achieved using this method.

‘Constant weight apnoea’ diving is where descent and ascent occur along a line, although the diver is not permitted to pull on this line to assist movement. No weights can be removed during the dive. Mono-fins or bi-fins can be used.

‘Constant weight without fins’ is the same as constant weight apnoea but without the use of fins.

With ‘variable weights’, the diver again descends with the aid of a weighted sled, but this weight is limited. Ascent is achieved by finning or pulling up the cable, or both.
‘Free immersion’, which emerged in places where equipment was difficult to obtain, involves a finless diver (with optional suit, mask or weights) who pulls himself or herself down and then up a weighted line.

‘Static apnoea’ involves resting breath-holding (usually lying in a pool) with the face submerged. Officially recorded times in excess of 11 minutes have been achieved using this method.

‘Dynamic apnoea’ measures the distance covered in a pool during a single breath-hold.