The most commonly used volatile anesthetics are desflurane, sevoflurane, and isoflurane. The chemical structures can be classified as substituted halogenated ethers. In addition, halothane is a substituted halogenated alkane, a derivative of ethane. Isoflurane and enflurane are isomers that are methyl ethyl ethers. Desflurane differs from isoflurane in the substitution of fluorine for a chlorine atom, and sevoflurane is a methyl isopropyl ether.
The mechanism of action of inhalational anesthetics has not been completely elucidated. Broadly, they are postulated to enhance inhibitory receptors (GABAA and glycine) while dampening excitatory pathways (nicotinic and glutamate). Unspecified mechanisms also include suppression of nociceptive motor responses within the spinal cord, as well as supraspinal suppression causing amnesia and hypnotic state.
The end goal of administering inhaled gases is to create an anesthetic state by reaching effective concentrations within the central nervous system (Table 48-1). To arrive at this end point, effective partial pressures must be established within the lung’s alveoli, allowing the gases to equilibrate in the pulmonary vasculature and ultimately within the CNS. At equilibrium, the partial pressure of the gases in the alveoli will be equivalent to the partial pressures in the patient’s blood and brain. Inhaled anesthetics reach equilibrium due to the following: rapid bidirectional transfer of gases between alveoli, blood, and CNS; the low capacity of tissue and plasma to absorb inhaled anesthetics; and the low metabolism, excretion, and redistribution of volatile agents relative to the rate at which they are removed or added to the lungs. Simply put, inhaled agent levels in the brain are heavily dependent on the anesthetic gas concentrations in the alveoli.
Palveoli = Pblood = Pbrain
TABLE 48-1Physiochemical Properties of Volatile Anesthetics ||Download (.pdf) TABLE 48-1 Physiochemical Properties of Volatile Anesthetics
|Property ||Sevoflurane ||Desflurane ||Isoflurane ||Enflurane ||Halothane ||N2O |
|Boiling point (°C) ||59 ||24 ||49 ||57 ||50 ||−88 |
|Vapor pressure at 20°C (mm Hg) ||157 ||669 ||238 ||172 ||243 ||38770 |
|Molecular weight (g) ||200 ||168 ||184 ||184 ||197 ||44 |
|Oil:gas partition coefficient ||47 ||19 ||91 ||97 ||224 ||1.4 |
|Blood:gas partition coefficient ||0.65 ||0.42 ||1.46 ||1.9 ||2.50 ||0.46 |
|Brain:blood solubility ||1.7 ||1.3 ||1.6 ||1.4 ||1.9 ||1.1 |
|Fat:blood solubility ||47.5 ||27.2 ||44.9 ||36 ||51.1 ||2.3 |
|Muscle:blood solubility ||3.1 ||2.0 ||2.9 ||1.7 ||3.4 ||1.2 |
|MAC in O2 30–60 y, at 37°C PB760 (%) ||1.8 ||6.6 ||1.17 ||1.63 ||0.75 ||104 |
|MAC in 60-70% N2O (%) ||0.66 ||2.38 ||0.56 ||0.57 ||0.29 || |
|MAC, >65 y (%) ||1.45 ||5.17 ||1.0 ||1.55 ||0.64 ||— |
|Preservative ||No ||No ||No ||No ||Thymol ||No |
|Stable in moist CO2 absorber ||No ||Yes ||Yes ||Yes ||No ||Yes |
|Flammability (%) (in 70% N2O/30% O2) ||10 ||17 ||7 ||5.8 ||4.8 || |
|Recovered as metabolites ...|