Forum Summary

Many gases have significant effects on human health.  Some gases are medically useful.  Carbon monoxide (CO), for example, can be used as an immunosuppressant, and nitrous oxide (N2O) is a widely used anesthetic.  Some other gases, however, can be extremely harmful to people and to the environment.  Carbon dioxide (CO2) and methane (CH4), for example, are greenhouse gases that have the potential to disrupt the earth’s climate.  Many gases are harmful at some concentrations and beneficial at others, so anesthesiologists are continually searching for new ways to use gases to improve human health.

Nitric oxide (NO) is a medically important gas that anesthesiologists have recently begun using as a selective pulmonary vasodilator.  Nitric oxide is a free radical that is naturally produced in the body and that does not travel through the bloodstream.  This gas gives anesthesiologists the ability to lower the blood pressure in a patient’s pulmonary arteries without lowering his or her systemic blood pressure.  Many infants are born with persistent pulmonary hypertension, and a sizeable fraction of these children respond very well to nitric oxide.  Before the introduction of nitric oxide, the only option for newborns having difficulty breathing was extracorporeal membrane oxygenation (ECMO), a risky mechanical system designed to oxygenate the blood.  Not all babies respond to treatment with nitric oxide, and the source of this variation is poorly understood.  Nitric oxide is now used to treat approximately 20,000 infants per year, and each year, it is also used to treat approximately 10,000 older patients with pulmonary hypertension. 

Hydrogen sulfide is another gas with potential medical applications, but its effects on the body have not been as well studied as those of nitric oxide.  Hydrogen sulfide is a flammable gas that smells like rotten eggs, and it is a metabolic poison that can cause death at high concentrations.  It is hoped that hydrogen sulfide will someday allow trauma victims to be placed in suspended animation until appropriate medical care is available.  When a mouse inhales a small dose of hydrogen sulfide, its body temperature, respiratory rate, and heart rate decrease, but its blood pressure remains constant.  The gas inhibits cytochrome c oxidase, a molecule that is vital to electron transport in mitochondria, and in mice, hydrogen sulfide has been shown to reduce oxygen consumption.  Reducing metabolism after an injury is known to limit organ damage.  The effectiveness of hydrogen sulfide in larger animals has not been demonstrated.     

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