(see Table 17-1: Physiochemical Properties of Volatile Anesthetics and
Fig. 17-1: Chemical structure of inhaled anesthetics
)
- Isoflurane
- Isoflurane is a halogenated methyl ethyl ether that has a high degree of stability and has become the “gold standard” anesthetic since its introduction in the 1970s.
- Coronary vasodilation is a characteristic of isoflurane, and in patients with coronary artery disease, there has been concern that coronary steal could occur (rare occurrence).
- Desflurane
- Desflurane is a completely fluorinated methyl ethyl ether that differs from isoflurane only by replacement of a chlorine with a fluorine atom.
- Compared with isoflurane, fluorination of desflurane results in low tissue and blood solubility (similar to nitrous oxide), greater stability (near-absent metabolism to trifluoroacetate), loss of potency, and a high vapor pressure (decreased intermolecular attraction). A heated and pressurized vaporizer requiring electrical power is necessary to deliver desflurane.
- Disadvantages of desflurane include its pungency (it cannot be administered by face mask to an awake patient), transient sympathetic nervous system stimulation when FI is abruptly increased, and degradation to carbon monoxide when exposed to dry carbon dioxide absorbents (more so than isoflurane).
- Desflurane has the lowest blood:gas solubility of the potent volatile anesthetics permitting rapid emergence even with prolonged surgical procedures and in obese patients.
- Sevoflurane
- Sevoflurane is completely fluorinated methyl isopropyl ether with a vapor pressure similar to that of isoflurane. It can be used in a conventional vaporizer.
- Compared with isoflurane, sevoflurane is less soluble in blood and tissues (it resembles desflurane), is less potent, and lacks coronary artery vasodilating properties.
- Sevoflurane has minimal odor and pungency (it is useful for mask induction of anesthesia) and is a potent bronchodilator.
- Similar to enflurane, the metabolism of sevoflurane results in fluoride, but unlike enflurane, this has not been associated with renal concentrating defects.
- Unlike other volatile anesthetics, sevoflurane is not metabolized to trifluoroacetate but rather to hexafluoroisopropanol, which does not stimulate formation of antibodies and immune-mediated hepatitis.
- Sevoflurane does not decompose to carbon monoxide or to dry carbon dioxide absorbents but rather is degraded to a vinyl halide (compound A), which is a dose-dependent nephrotoxin in rats. Renal injury has not been shown to occur in patients even when fresh gas flows are ≤1 L/min.
- Xenon
- This inert gas has many characteristics of an “ideal” inhaled anesthetic (blood gas partition coefficient of 0.14, provides some analgesia, nonpungent, does not produce myocardial depression or alter coronary blood flow).
- The principal disadvantages of xenon are its expense (difficult to obtain) and high minimum alveolar concentration (MAC) (71%).
- Nitrous Oxide
- Nitrous oxide is a sweet-smelling, nonflammable gas of low potency and limited blood and tissue solubility that is most often administered as an adjuvant in combination with other volatile anesthetics or opioids.
- Controversy surrounding the use of nitrous oxide is related to its unclear role in postoperative nausea and vomiting, potential toxicity related to inactivation of vitamin B12
, effects on embryonic development, and adverse effects related to its absorption into air-filled cavities and bubbles. (Compliant spaces such as a pneumothorax expand, and noncompliant spaces such as the middle ear experience increased pressure.)
- Inhalation of 75% nitrous oxide may expand a pneumothorax to double its size in 10 minutes.
- Accumulation of nitrous oxide in the middle ear may diminish hearing after surgery.