Liquid breathing is a form of respiration in which a normally air-breathing organism breathes an oxygen-rich liquid (such as a perfluorocarbon), rather than breathing air. By selecting a liquid that is capable of holding large amounts of oxygen and CO2, gas exchange can occur. This requires certain physical properties such as respiratory gas solubility, density, viscosity, vapor pressure, and lipid solubility which some perfluorochemicals (PFCs) have. Thus, it is critical to choose the appropriate PFC for a specific biomedical application, such as liquid ventilation, drug delivery or blood substitutes. The physical properties of PFC liquids vary substantially; however, the one common property is their high solubility for respiratory gases. In fact, these liquids carry more oxygen and carbon dioxide than blood. In theory, liquid breathing could assist in the treatment of patients with severe pulmonary or cardiac trauma, especially in pediatric cases. Liquid breathing has also been proposed for use in deep diving and space travel. Despite some recent advances in liquid ventilation, a standard mode of application has not yet been established. Approaches Because liquid breathing is still a highly experimental technique, there are several proposed approaches. Total liquid ventilation Although total liquid ventilation (TLV) with completely liquid-filled lungs can be beneficial, the complex liquid-filled tube system required is a disadvantage compared to gas ventilation—the system must incorporate a membrane oxygenator, heater, and pumps to deliver to, and remove from the lungs tidal volume aliquots of conditioned perfluorocarbon (PFC). One research group led by Thomas H. Shaffer has maintained that with the use of microprocessors and new technology, it is possible to maintain better control of respiratory variables such as liquid functional residual capacity and tidal volume during TLV than with gas ventilation. Consequently, the total liquid ventilation necessitates a dedicated liquid ventilator similar to a medical ventilator except that it uses a breathable liquid. Many prototypes are used for animal experimentation, but experts recommend continued development of a liquid ventilator toward clinical applications. Specific preclinical liquid ventilator (Inolivent) is currently under joint development in Canada and France. The main application of this liquid ventilator is the ultra-fast induction of therapeutic hypothermia after cardiac arrest. This has been demonstrated to be more protective than slower cooling method after experimental cardiac arrest. Partial liquid ventilation In contrast, partial liquid ventilation (PLV) is a technique in which a PFC is instilled into the lung to a volume approximating functional residual capacity