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. 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 been established yet.
Liquid breathing is sometimes called fluid breathing; however, this usage can be confusing as both liquids and gases are fluid substances.
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 breatheable liquid. Many prototypes are used for animal experimentations, but experts recommend continued development of a liquid ventilator toward clinical applications.
Gas pressure increases with depth, rising 1 bar every 10 meters to over 1,000 bar at the bottom of the Mariana Trench. Diving becomes more dangerous as depth increases, and deep diving presents many hazards. All surface-breathing animals are subject to decompression sickness, including aquatic mammals and free-diving humans (see taravana). Breathing at depth can cause nitrogen narcosis and oxygen toxicity. Ascending after breathing at depth can cause air embolisms, burst lung, and collapsed lung.
Special breathing gas mixes such as trimix or heliox ameliorate the risk of decompression illness but do not eliminate it. Heliox further eliminates the risk of nitrogen narcosis but introduces the risk of helium tremors below 500 feet. Atmospheric diving suits maintain body and breathing pressure at 1 bar, eliminating most of the hazards of descending, ascending, and breathing at depth. However, the rigid suits are bulky, clumsy, and very expensive.
Liquid breathing offers a third option, promising the mobility available with flexible dive suits and the reduced risks of rigid suits. With liquid in the lungs, the pressure within the diver's lungs could accommodate changes in the pressure of the surrounding water without the huge gas partial pressure exposures required when the lungs are filled with gas. Liquid breathing would not result in the saturation of body tissues with high pressure nitrogen or helium that occurs with the use of non-liquids, thus would reduce or remove the need for slow decompression. (This technology was dramatized in James Cameron's 1989 film The Abyss.)
A significant problem, however, arises from the high viscosity of the liquid and the corresponding reduction in its ability to remove CO2. All uses of liquid breathing for diving must involve total liquid ventilation (see above). Total liquid ventilation, however, has difficulty moving enough liquid to carry away CO2, because no matter how great the total pressure is, the amount of partial CO2 gas pressure available to dissolve CO2 into the breathing liquid can never be much more than the pressure at which CO2 exists in the blood (about 40 mm of mercury (Torr)).
At these pressures, most fluorocarbon liquids require about 70 mL/kg minute-ventilation volumes of liquid (about 5 L/min for a 70 kg adult) to remove enough CO2 for normal resting metabolism. This is a great deal of fluid to move, particularly as liquids are generally more viscous than gases, (for example water is about 850 times the viscosity of air). Any increase in the diver's metabolic activity also increases CO2 production and the breathing rate, which is already at the limits of realistic flow rates in liquid breathing. It seems unlikely that a person would move 10 liters/min of fluorocarbon liquid without assistance from a mechanical ventilator, so "free breathing" may be unlikely.