Here I am going to describe some of the main medical aspects related to freediving:


   Middle Ear Barotrauma
   Swallow Water Blackout
   Pool Blackout


There are several cavities in the human body (e.g. sinus of the nose, frontal sinus, middle ear). Because the water pressure is increasing during the descend of a diver those cavities have to be equalized to adjust the inside-pressure to the water-pressure. Usually there are no problems and this happens automaticly, except middle ear (see Middle ear barotrauma). Problems may occur while diving with headcold or influenza. Then, due to a stuffed up nose or sinus the air cannot reach the sinus to equalize the pressure. The diver then may feel a sucking in the sinus and frontal sinus and blood issues from the mucous membrane and. nosebleeds or even sinusitis can be the consequences.

Prevention of Barotrauma:

Middle Ear Barotrauma

Middle Ear Barotrauma is one of the most frequent diving injuries. It is the direct result of improper middle ear equalization. There is a connection between Middle Ear and throat - the Eustachian tube. When a diver is descending he has to equalize the middle ear through the Eustacian tube to adjust the middle ear pressure to the water pressure to prevent the eardrum from being injured.
The tube is approximately 1.5" long and is located in the back of the nasopharynx at approximately nostril level. The tube is normally closed and has a highly variable patency. This means that some individuals will virtually never have problems with middle ear equalization while diving. Others with narrow or partially obstructed Eustachian Tubes may have trouble equalizing their middle ears. These later individuals can dive safely, but for them middle ear pressurization requires meticulous attention to detail and much practice.
Fortunately, improper equalization draws attention to itself causing strong eardrum-ache. Pressurization of the middle ear provides a pillow of air behind the tympanic membrane, protecting the ear drum from barotrauma. As descent occurs, more air can easily enter an inflated Eustachian tube and pass into the middle ear, if pressurization begins early in the dive. If the Eustachian tube is allowed to collapse at any time during descent due to squeeze, the pressure to re-inflate it becomes greater. There are different techniques to equalize the middle ear. There is a good summary at:

Prevention of Middle Ear Barotrauma

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Prevention of Middle Ear Barotrauma:


While freediving, there is the possibility of a slight vakuum in the lungs and therefor theoreticaly of a lungs-barotrauma. In contrast to diving mammals the thorax-compression is limited due to the immovable breastbone. When descending the volume of the lungs decreases because of the increasing ambient-pressure. The maximum depth one can reach is thus given by the ratio of total lungs-volume to residualvolume. The residualvolume is the size after total exhaling. If a diver goes deeper, the lungs cannot be compressed further and the result would be a vakuum causing lymph to issue from the pulmonary alveolus. There are no known cases of a lungs-barotrauma. The reason is that the vakuum initiates strong oppressions and forces the diver to ascend.

Lungs Volumina in different depths

The illustration shows the pushed in thorax, the compressed lungs an the strong bent diaphragm (white).

It is known that there are freedivers who went deeper than the theoretical maximum. This is possible due to a phenomenon called Bloodshift. Here, blood shifts from the body into the blood vessels of the lungs to compensate the pressure.

Swallow Water Blackout:

Swallow Water Blackout (SWB) is a sudden loss of consciousness. It occurs quickly, insidiously, and without warning caused by oxygen starvation. The typical depth is about 15 ft (5 meters) during the ascend. The reason for this are the expanding lungs which results in an dramatically decrease of oxygen partial pressure. Beetwen 30 feet (10 meters) and the surface the lungs double their volume, thus, resulting in a decrease of oxygen partial pressure to a half. This also leads to a decrease of the oxygern content in the diver's blood because the oxygen-hungry lungs literally suck oxygen from the blood. Under certain circumstances the oxygen content then drops under the blackout level. Because the diver hasn't reached the surface yet the resulting unconsciousness often leads to death.
Due to their lack of adaptation beginning breath-hold divers aren't generally subject of this condition.
As mentioned above the dangerous phase is neither the descend nor the stay at great depths but the ascend. Because of increasing water pressure during the descend the oxygen level in the lungs increases too. Therefore divers may feel comfortable or reach a state of euphoria at remarkable depths and stay there too long. Now, the problem is the ascend. The reexpanding lungs increase in volume leading to a rapid decrease of oxygen to critical levels. Then, there is a net flow of oxygen from the diver's blood into the lungs. Once, a critical level of oxygen in the blood is reached it switches off the brain.

Prevention of SWB:

Pool Blackout

The typical situation for Pool Blackout is static breathholding or dynamic underwater practise after extensive hyperventilation. Hypervantilation doesn't increase the level of oxygen in the diver's blood but decreases the level of carbon dioxide. Now, the problem is that the breathing center is stimulated by the level of carbon dioxide if it reaches a certain value. While breathholding the level of oxygen in blood and lungs constantly decreases, the level of carbon dioxide constantly increases until it reaches the value stimulating the breathing center causing the diver to feel the need to breathe. But because the level of carbon dioxide is decreased due to hyperventilation the diver may feel no need to breathe, even if the oxygen level drops under the blackout limit. This results in a sudden unconsciousness.

Fig. 1 (just in quality)

Fig.1: In the first case (no hyperventilation) the diver feels the need to breathe cause the carbon dioxide level reaches the breathing reflex level (Time 1) before the unconsciousness occurs due to the oxygen level reaching the blackout limit (Time 2). In the second case the diver practised hyperventilation. Because of the lesser carbon dioxide concentration he will loose consciousness (Time 2) before the carbon dioxide level can reach the breathing reflex limit (Time 3) telling him that there is something wrong.

Prevention of Pool Blackout: