Gases are usually administered in the context of general anesthesia, either alone or in combination with a variety of infusional agents. In most situations, the gas is made by vaporizing a liquid (usually a halogenated hydrocarbon or ether). Validated vaporizers, usually designated for use with a single liquid, are required. In addition, many patients are paralyzed during surgery and mechanical ventilators (including a hand-squeezed bag) must consider drug economy, occupational exposure of the staff, carbon dioxide scrubbing, and other pharmacokinetic features that are rarely encountered elsewhere. This is quite apart from the usual considerations of tidal and minute volumes, oxygen supply, blood pressure management, etc. Gas flow can be measured with various devices, and exhaled gas concentrations (including for carbon dioxide) can now be measured real-time. Malignant hyperthermia is an adverse event that is almost always associated with the inhalation of a halogenated hydrocarbon, and which can be treated with dantrolene (Strazis and Fox, 1993).

The theory relating physicochemical properties of gases and the partial pressure at which they can achieve anesthesia is beyond the scope of this chapter. Indeed, this question hinges on how the state of anesthesia can itself be measured, one of the more difficult pharmacodynamic endpoints in pharmaceutical medicine. One wit, also a famous cardi-othoracic anesthesiologist, has commented: 'If you can tell me what consciousness is, then I will tell you what anesthesia is!'.

There are some uses for gaseous drugs outside of surgery. Nitrous oxide and oxygen mixtures are sometimes used as analgesics during labour, or when transferring patients in pain by road or helicopter. In very cold weather, nitrous oxide can liquify, reducing the delivered dose; shaking the container helps.

Helium/oxygen mixtures are used to improve oxygenation in patients with subtotal airways obstruction, exploiting the superior flow characteristics of the lighter gas. The use of this mixture as prophylaxis against nitrogen narcosis in the deep sea, while minimizing fire hazard, is also well-described. Fire hazard from oxygen (arguably a gaseous drug under some circumstances) is important; the disastrous fire inside the command capsule of Apollo 3, during a lift-off rehearsal on the pad at Cape Kennedy, took place within a pure oxygen atmosphere. Reduction in total atmospheric pressure, to reduce fire hazard, has since been employed in all pressurized American space vehicles, but they still contain supra-atmospheric partial pressures of oxygen.

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