Partial liquid ventilation--turning back a PAGE on evolution.

On an evolutionary time scale, liquid ventilation is not new. Land-dwelling vertebrates derived from fish 350 million years ago, and fish continue to breathe through a liquid medium. However, the ability to sustain mammalian respiration artificially by a liquid ventilation technique is generally accredited to work in the early 1960s, where mice were kept alive breathing hyperbaric salt solutions. 1 The hyperbaric conditions were required in order that sufficient oxygen could be dissolved in solution to sustain cell respiration. This was taken further by Clark and Gollan in 1966 when they described the use of silicone oils and fluorocarbons at atmospheric pressure in supporting the life of mice and cats receiving a liquid ventilation technique. 2 Silicone oils were found to be too toxic to use in living creatures but the perfluorocarbons showed more promising characteristics. Perfluoro-carbons (PFC) are a group of stable, inert, colourless and odourless liquids which are not soluble in either lipids or water. Oxygen, carbon dioxide and most other gases, however, are dissolved. The solubility of oxygen in PFC is of the order of 50 ml per 100 ml of solvent, and carbon dioxide 200 ml per 100 ml of solvent at atmospheric pressure (i.e. 20 and three times the solubility of each gas in plasma, respectively). They have a low predisposition to react with biological tissue. PFC may be derived from organic compounds (e.g. benzene) by replacement of carbon bound hydrogen atoms with fluorine, by heating the organic compound with cobalt trifluoride or by electrochemical fluorination techniques. 3 Throughout the 1970s and 1980s, further work was done in animals, both with normal lungs 4 and in models of prematurity and lung injury. 5 6 Two milestones have been reached in the past 10 yr. First, until recently liquid ventilation required relatively complex extracorporeal equipment to oxygenate and remove carbon dioxide from the liquid medium. This involved filling the functional residual capacity with PFC and using tidal volumes consisting entirely of liquid, and was known as (total) liquid ventilation. Fuhrman, Paczan and DeFrancisis 7 described how it was possible to sustain ventilation in pigs with a hybrid technique, that is a medium of perfluorocarbon but using a standard ventilator (so called perfluorocarbon associated gas exchange (PAGE) and also known as partial liquid ventilation (PLV)), while Greenspan and colleagues described the first human use of this respiratory medium (in preterm infants with respiratory distress syndrome). Within the …