Inhalation anesthetic agents

Published on 07/02/2015 by admin

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Last modified 07/02/2015

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CHAPTER 64

Inhalation anesthetic agents

Bradley Anderson, MD and Michael J. Murray, MD, PhD

The inhaled anesthetic agents form a cornerstone for modern anesthetic delivery. Their roots run deep into medical history as they have helped pave the way toward modern-day surgical procedures. Today, four main inhaled anesthetic agents are in use in the United States: N₂O, isoflurane, sevoflurane, and desflurane. Each possesses differing characteristics in their relative pharmacokinetic and pharmacodynamic profiles, making them uniquely suitable for use in different situations.

Originally used in nineteenth-century dentistry, N₂O, a colorless gas at room temperature, is commonly referred to as laughing gas. Although N₂O is not often used today, when it is used, it is in combination with other inhalation anesthetic agents rather than as a sole agent.

Desflurane, sevoflurane, and isoflurane are all fluorinated inhalation anesthetic agents. Compared with ether, inhalation anesthetic agents that have been halogenated with fluorine have reduced flammability as well as greater molecular stability. The use of desflurane and sevoflurane is gradually replacing that of older inhalation anesthetic agents because desflurane and sevoflurane result in the fastest and second-fastest respectively, induction and emergence times (because of their low blood solubility).

Pharmacokinetics

The pharmacokinetics of a drug refers to its absorption, distribution, excretion, and metabolism inside the body. For inhaled anesthetic agents, absorption is accomplished through the alveoli in the lungs. As these drugs are inhaled, they are exposed to the rich vascular supply of the lungs (i.e. the pulmonary capillary beds), where they are absorbed and distributed systemically. The partial pressure of the inhalation anesthetic agent in the central nervous system (PCNS) is proportional to the arterial partial pressure (Pa), which, in turn, is proportional to the alveolar pressure (PA) at equilibrium. Therefore, at equilibrium, the PCNS is directly proportional to the PA.

The uptake of an anesthetic agent from the lung into the bloodstream is dependent on three main factors (excluding the concentration and second-gas effects) (Box 64-1). The first is the alveolar–mixed venous partial pressure difference P(A − ). The next is the solubility of the anesthetic agent in the blood, defined as the blood-gas partition coefficient (λ), and the last is cardiac output (CO). Using these factors, a simple calculation can be used to help determine the uptake of any given inhaled anesthetic agent. From this equation, it is apparent that, if any of the three factors is increased, the result will be a larger uptake of the anesthetic agent: