Liquid ventilation.

For 350 million years, ®sh have breathed liquid through gills. Mammals evolved lungs to breathe air. Rarely, circumstances can occur when a mammal needs to `turn back the clock' to breathe through a special liquid medium. This is particularly true if surface tension at the air±liquid interface of the lung is increased, as in acute lung injury. In this condition, surface tension increases because the pulmonary surfactant system is damaged, causing alveolar collapse, atelectasis, increased right-to-left shunt and hypoxaemia. The aims of treatment are: (i) to offset increased forces causing lung collapse by applying mechanical ventilation with PEEP; (ii) to decrease alveolar surface tension with exogenous surfactant; (iii) to eliminate the air±liquid interface by ®lling the lung with a ̄uid in which both oxygen and carbon dioxide are highly soluble to serve as a respiratory medium. This third concept, liquid ventilation, recalls the early work of von Neergaard, who showed that the pressure necessary to expand a lung ®lled with air is almost three times that required to distend a lung ®lled with liquid.