The Risks Of Ascent
Posted By Erik Seedhouse on 2 December 2002
The greatest danger to the free diver during an ascent is from pulmonary barotraumas, the consequence of which may be fatal.
A free diver not exhaling during an ascent or making a fast ascent will cause their lung volume to expand rapidly. If the ascent is too fast then the musculoskeletal cage may exceed its expansion limits. Once this situation is reached, injuries such as pulmonary barotrauma, pneumothorax, and rupture of the pulmonary vein causing an air embolism may occur. The most common of these injuries is pulmonary barotraumas, sometimes referred to as pulmonary overpressure syndrome.
For an equivalent change in depth the risk of barotrauma is greatest near the surface, a fact explained by Boyle's law. A breath-holding free diver rising from 33 feet to the surface experiences a change in ambient pressure from two to one atmospheres. If the lungs fully expand within the chest cavity the lung volume will double. By contrast, a 33-foot rise from 99 to 66 feet depth (i.e., from 4 to 3 atmospheres) would maximally increase a free diver's lung volume only 33 percent, posing less risk of a barotrauma.
Using the same example, a breath-hold ascent from 33 feet would result in the lung volume doubling, almost guaranteeing barotrauma if breath were held at or near the diver's total lung capacity. If the lungs could not vent expanding air they would be subjected to a distending pressure of nine times the barotrauma threshold! Experiments in dogs undergoing rapid ascent in a decompression chamber demonstrated that the lungs withstand much higher pressures (before barotrauma occurs) if 'over stretching' of the thoracic cavity is prevented.
Although both over-stretching of lung tissue and the pressure of expanding air are factors that may predispose the free diver to lung trauma, pressure seems to be the major problem. The pressure-difference across the lungs (from inside to outside) that is the threshold for experimental barotrauma is about 80 mm Hg, a change that may occur with a breath-hold ascent from only four feet! At this pressure the alveoli are prone to tearing and may vent air into the interstitial space. From here, the air may take one of three routes. If it travels to between the lungs or around the lungs a pneumorathorax will result (discussed later), and if it escapes into the bloodstream an air embolism will occur. Neither is a pleasant experience.
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