Breath-hold diving: Running on reserve -Part 5

By Dr Rob Schneider
Rob Schneider completes his five-part series on breath-hold diving by considering the kinds of problems associated with breath-hold diving.
Part 1 of this series introduced the concepts and overview of breath-hold diving, the second part considered the equipment available for use and Part 3 focused on the physics of breath-hold diving. Part 4 added the physiological aspects and so we now close the series with the fifth instalment, which considers the damage, death and drowning associated with breath-hold diving.
Our human design was not specifically intended for the aquatic environment. Still, we flaunt this idea regularly and impetuously for our own gain or enjoyment. During breath-hold diving, the alien environment we invade presents us with many challenges and obstacles. If we do not succeed in overcoming or mitigating them, we run a very clear and present risk of injury and/or death. In the words of a record-holding free diver, “You’re running on a reserve tank and there’s no warning before you hit empty!”
So, let us consider the kinds of problems that can occur as we apnoea and immerse ourselves.
General concerns
As with many other forms of diving, there are several dangers that lurk in, around or on the water. Floating objects, either propelled or not, can cause injury to an unsuspecting breath-hold diver. Conversely submerged objects pose a similar risk for injury and poor visibility compounds these hazards.
Various environmental factors can contribute to the chances of injury or difficulties in breath-hold diving. Tides, currents, the temperature of the water and air, wind, choppy seas, big swells, sudden changes in weather, and sea states may all contribute their fair share to the risk pool.
 Lest we forget, we share the underwater environment with many of the resident organisms. Many of these are harmless, though some are certainly not! Snapping (bites), stinging (tentacles or nematocysts), sticking (barbs), shocking (electric rays and eels) and snacking (i.e. poisoning through shellfish or fish meat) are all potential sources of injury or harm.
Arguably, one of the most dangerous agents in the water may be another human doing something foolish. By their decisions and actions (or lack thereof) many injuries result from human-mediated events.
The topic of drowning falls outside the scope of this article, but is obviously the final common pathway of death in breath-hold diving.
Injury to gas-filled spaces in the human body (as a result of environmental pressure changes) is called a barotrauma. “Baro” means pressure and “trauma” means injury. Sometimes barotraumas are called “blocks” or “squeezes” in the diving vernacular. Blocks, by convention, are assigned to problems encountered during a reduction in pressure, such as during an ascent. Squeezes occur on the way down. Breath-hold diving may precipitate both subtypes of barotrauma. The process of pressure-volume change is governed by Boyle’s law. All gasfilled spaces are potentially vulnerable. Some, like the gastrointestinal tract (GIT), have a release valve on either end which usually solves the problem of a GIT block. Others, like the middle ear space, usually require intentional manoeuvres, like the Valsalva or its derivatives.
Gas-filled spaces in the human body include the lungs, the middle ear space, mastoid air cells, sinuses, GIT, the trachea oral cavity and the nasal cavity.
Squeeze injuries occur when gas volumes become compressed to the point where their walls offer resistance and result in distortion, swelling, bleeding or ruptures of membranes. Some organs are very resistant to these changes, while others are not. Lung squeeze is a relatively rare and unique form of a barotrauma in breath-hold diving. In breath-hold diving, unlike scuba diving, lung volumes become compressed with depth. As such, a point may eventually be reached where the lungs are compressed below the minimum volume at which point structural damage starts to occur. The body has several methods to compensate for reductions in lung volume. Partial collapse of the diaphragm and central pooling of blood in the chest both help to mitigate the decrease in lung volume and protect against the development of a lung squeeze. The physical manifestations of lung squeeze include breathlessness, coughing up blood and chest pain. It is not only associated with extremely deep dives and this keeps the condition partially shrouded in mystery awaiting more research.
Blocks result from barriers to the escape of the expanding gas. Unlike scuba diving, in which additional compressed gas may be added to the body’s gas cavities, breath-hold diving can only redistribute the gas that was contained in the body at the surface. Some gas is moved to the middle ear and sinuses to avoid squeeze in these organs. However, the total gas volume in the diver upon return to the surface is actually slightly less than the volume at the beginning (due to oxygen consumption and some nitrogen absorption). This generally protects organs like the lungs and GIT from blocks, but the ears and sinuses need to vent the redistributed gas back into the nasal cavity. If this fails to occur, due to the blockage of the communicating tubes, pain and damage may occur. Barotrauma on ascent is virtually unheard of in breath-hold diving.
The signs and symptoms of blocks and squeezes generally include pain, bleeding, dizziness, nausea, hearing loss and shortness of breath. The severity of injury ranges from mild to terminal (which rarely occurs).
Functional disturbances
The unique shifts of fluids and gas, together with the physiological reflexes associated with immersion and diving, may precipitate significant heart rate and rhythm disturbances during breath-hold diving. The centralised pooling of blood into the chest and, specifically, the heart during descent causes a higher preload on the heart. In other words, more blood needs to be pumped than usual. This distension of the heart chambers is arrhythmogenic and may precipitate abnormal heart rhythms. The diving response itself increases the vagal tone with a lowering of the heart rate in relaxed individuals. This mayalso produce an abnormal electrical condition and contraction of the heart.Young, healthy divers are usually unaffected. However, older individuals orthose with heart or lung problems, may suffer harmful disruptions and even
cardiac arrest.
Because breath-hold diving already implies a limited supply of oxygen, it is not surprising that a lack of oxygen, hypoxemia (hypoxia), is the most obvious concern. Life goes on during breath-hold diving and oxygen supplies dwindle as they are consumed in the body’s metabolism. In lieu of significant oxygen stores, a deficit develops fairly quickly (usually within two to three minutes at most) depending on the level of exertion. If the blood oxygen content drops below a critical threshold, consciousness is lost. This loss is incipient and rarely gives any warning.
Ascent hypoxia (sometimes called shallow-water blackout) is the term used to describe the unique type of hypoxia most commonly associated with breathhold diving. Although oxygen pressures are initially raised during descent as a function of lung volume reduction, the reverse happens during the ascent. A precipitous drop usually occurs in the last 5-10 m and this is where blackouts usually occur. Carbon dioxide (CO2) build-up (the usual signal to resume breathing) usually forces a person to break breath-holding before oxygen levels reach critical values. A sophisticated interaction with receptors in the aortic arch control the warning system of elevated CO2. However, extensive training and conditioning, in combination with deliberate hyperventilation (i.e. the process of rapidly inhaling and exhaling to lower CO2 levels artificially) may disable this warning system: the CO2 levels start off low and take a longer time to build up to the breaking point. Oxygen supplies are not increased appreciably through hyperventilation. As a result, the disabled alarm system does not avoid the impending problem of hypoxia. Once the hypoxic threshold is reached, the diver loses consciousness and, unless they happen to be positively buoyant and float to the surface, invariably drowns unless he/she is rescued. Sometimes a diver will surface at the point of reaching the hypoxic threshold and experience what is called a “samba” (i.e. a convulsion similar to what is seen in human centrifuge blackout recovery). Cerebral function recovers quickly once the hypoxia is corrected. Obviously, diving alone or without attentive, competent, in-water observers (buddies) is unwise.
Decompression illness
An arterial gas embolism (AGE) and decompression sickness (DCS) fall under the umbrella term of decompression illness (DCI). In breath-hold diving, AGE is highly unlikely for all practical purposes. DCS, on the other hand, may actually occur with extensive, repetitive, deep breath-hold diving. In fact, this has been well described in native sea harvesting breath-hold divers like the Ama divers in Japan. Fortunately, it is uncommon otherwise. Some nitrogen is absorbed from the lungs into the blood during each breath-hold dive. This is released on ascent, but at a slower rate. As such, accumulation is possible and with extreme depth excursions or explosive ascent rates, gas bubble evolution may occur.
Nitrogen narcosis
Nitrogen narcosis is thought to occur in breath-hold diving. Many experienced breath-hold divers are able to exceed 30 m where nitrogen narcosis is known to occur. The symptoms of nitrogen narcosis are rarely reported and may even have been forgotten. Fortunately, they do not seem to be incapacitating; breath-hold diving is usually not associated with significant mental task-loading.
Concluding remarks
In spite of all the potential grief that may befall a breath-hold diver, it is an immensely pleasurable activity that may be enjoyed safely as long as the risks are understood and avoided.
This concludes the five-part series on breath-hold diving. I hope you all enjoyed reading it as much as I did writing it. So, please do not hold your breath for another instalment as there is none!
  • Brubakk, A.O. & Neumann, T.S. Bennett and Elliot’s Physiology and
  • Medicine of Diving, 5th edition.
  • Edmonds, C., Lowry, C., Pennefather, J. & Walker, R. Diving and Subaquatic
  • Medicine. 4th edition.


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