Pharmacokinetics

Nitrous oxide is very insoluble in blood and other tissues (Table 13-1). This results in rapid equilibration between delivered and alveolar anesthetic concentrations and provides for rapid induction of anesthesia and rapid emergence following discontinuation of administration. The rapid uptake of N2O from alveolar gas serves to concentrate coadministered halogenated anesthetics; this effect (the "second gas effect") speeds induction of anesthesia. On discontinuation of N2O administration, nitrous oxide gas can diffuse from blood to the alveoli, diluting O2 in the lung. This can produce an effect called diffusional hypoxia. To avoid hypoxia, 100% O2 rather than air should be administered when N2O is discontinued.

99.9% of absorbed nitrous oxide is eliminated unchanged via the lungs. Nitrous oxide can interact with vitamin Bn resulting in vitamin Bn deficiency (megaloblastic anemia and peripheral neuropathy) following long-term nitrous oxide administration. Thus, N2O is not used as a chronic analgesic or as a sedative in critical care settings.

CLINICAL USE Nitrous oxide is a weak anesthetic agent and produces reliable surgical anesthesia only under hyperbaric conditions. It does produce significant analgesia at concentrations as low as 20% and usually produces sedation in concentrations between 30% and 80%. It frequently is used in concentrations of -50% to provide analgesia and sedation in outpatient dentistry. Nitrous oxide cannot be used at concentrations >80% because this limits the delivery of adequate O2. Consequently, N2O is used primarily as an adjunct to other anesthetics. Nitrous oxide substantially reduces the requirement for inhalational anesthetics. For example, at 70% nitrous oxide, the MAC for other inhalational agents is reduced by about 60%, allowing for lower concentrations of halo-genated anesthetics and a lesser degree of side effects.

One major problem with N2O is that it will exchange with N2 in any air-containing cavity in the body. Moreover, because of their differential blood:gas partition coefficients, nitrous oxide will enter the cavity faster than nitrogen escapes, thereby increasing the volume and/or pressure in this cavity. Examples of air collections that can be expanded by nitrous oxide include a pneumothorax, an obstructed middle ear, an air embolus, an obstructed loop of bowel, an intraocular air bubble, a pulmonary bulla, and intracranial air. Nitrous oxide should be avoided in these clinical settings.

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