Since the publication of the "Risks of Ascent" article on 2nd December I have received a few e-mails questioning the risk of barotraumas during freediving. In the article I should have emphasized that under normal conditions the risk of barotrauma is slim to non-existent. Theoretically however, there is still a chance that it may occur.
The ratio of the freedivers total lung capacity to residual lung volume (TLC: RLV) at the surface generally determines the critical diving depth before lung squeeze and ultimately potential barotraumas/air embolism. This ratio typically averages 4:1 at the surface. For example, for a diver with a 6L TLC and a 1.5L RLV, Boyles’ law predicts that TLC would compress RLV at 30m. No danger from lung squeeze, barotrauma or air embolism exists if lung volume remains greater than RLV, because sufficient air remains in the lungs and rigid respiratory passages to equalize pressure and prevent damage from compression. If TLC during a dive decreases below RLV pulmonary air pressure becomes less than the external water pressure. The unequalized pressure then creates a relative vacuum within the lungs. In severe cases of lung squeeze, blood literally bursts from the pulmonary capillaries through the alveoli and into the lungs (air embolism/ pneumothorax). In this situation, freedivers literally drown in their own blood. Further increases in depth will cause compression fractures of the ribs as the chest cavity caves in from excessive external pressure.
A general note: In many instances, the ratio TLV: RLV at the surface significantly underestimates the actual depths achieved by freedivers. Part of the explanation may relate to a reduced RLV as immersion progresses because of a shift toward greater intrathoracic blood volume. Consequently, a smaller RLV underwater increases the TLV: RLV and increases the maximal depth before reaching the critical ratio.
