Apnoea diving or freediving is diving with no air supply. It requires only basic equipment like mask, fins and snorkel. Freediving equipment is specifically designed to assist in apnoea diving. The lips of freediving masks usually have a single instead of a double sealing lip and a smaller air volume. This reduces the additional functional dead space caused by the mask. The fins have larger blades and made out of carbon. With long unbroken stokes and good efficiency due to the shape of the fins, energy consumption is reduced. Also, monofins are commonly used for freediving. With them divers have different movements compared to conventional fins and hence exert different muscles. In general, both fin types are equally suitable for freediving. The choice is more or less a personal preference. Snorkels should have a sufficient diameter (1.5–2.5 cm) and should not be too long (35 cm). With regular snorkels alone, normal respiration approximately is reduced by 70%. Therefore, snorkels are not used in freediving competitions. If the diameter is too small, CO2 elimination may be decreased as airflow resistance might be increased. This results in poor ventilation. In contrast, a too wide and long snorkel increases functional dead space and leads to CO2 retention. Diving with snorkels of 1 m or longer causes an increased pulmonary negative pressure in immersed divers. This can result in inability of respiratory muscles to expand the lungs against the ambient pressure. This might result in increased respiratory rate, as breathing reflexes are reinforced by chest compressions. Additionally, inhalation is significantly reduced by the increased ambient pressure. Moreover, on exhalation, the lower limit of the residual volume is exceeded. Hence, pulmonary hypobaric barotrauma may develop. The maximum length of the snorkel should not exceed 50 cm (◘ Fig. 27.1).
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