Breath-hold diving - physiology and pathophysiology

Apnea diving, i.e. diving with breath-holding, is the oldest known form of diving and practised in two forms: during their daily work shifts professional apnea divers perform 150 - 250 dives of 1 - 2 minutes duration to depths of 5 - 20 m. By contrast a depth of 154 m and an apnea duration of 8:06 min represent the current world records for apnea diving as extreme sport. The short time period beneath the surface induces profound cardiovascular and respiratory effects which are substantially different from surface apneas. The variations of the blood gas tensions are due to the interaction of metabolism and the rapid sequence of compression and decompression and thereby help to explain most of the apnea-related accidents. Decompression sickness is possible with repetitive apnea dives provided depths of more than 15 - 20 m are reached. Apnea-divers can reach depths beyond the physiologic limit theoretically given by the quotient of total lung capacity and residual volume because of the immersion-related blood shift into the thorax together with the increased total lung capacity resulting from deliberate pulmonary overinflation using the "lung packing" manoeuvre. The latter, however, increases the risk of pulmonary barotrauma. Similar to diving mammals and birds human apnea divers exhibit a fall in heart rate, in particular at low water temperature. This phenomenon called the "diving reflex", can be trained and serves as an "oxygen sparing effect", but increases the incidence of ventricular arrhythmia.