Using volatile inhalational anesthetic agents as part of a balanced anesthetic regimen remains the simplest and most economical approach for most facilities. But what do inhalational agents do? What are their similarities and differences? Does the choice of agent make a clinical difference? Read on for the answers.

What do they do?
Although volatile inhalation agents (VIAs) come as liquids, they're readily vaporizable at a slightly higher temperature, which allows administration into the lungs. Besides producing unconsciousness, volatile inhalation agents prevent movement to noxious stimuli (such as surgical skin incision), produce amnesia and analgesia and prevent heart rate increases from surgical manipulation.

Inhalational anesthetics are different from most drugs given to a patient during surgery. First, they're administered into the lungs using an anesthesia machine (see "Welcome to the Machine"), making it easy to increase or decrease the level of the agent in the body.

Second, unlike IV meds, for which you might have difficulty determining the amount of drug at the site of action, anesthesia providers are better able to estimate the amount of volatile inhalation agent in the brain, allowing for better control of anesthesia depth, even in lieu of a consciousness monitor.

There are currently four volatile inhalation agents available: desflurane (Suprane), sevoflurane (Ultane), isoflurane (Forane) and halothane (Fluothane). Only the first three are routinely used in today's clinical practice. Desflurane and sevoflurane are the newest agents on the market and differ from the others by their low solubility in blood and, in the case of desflurane, tissue.

While volatile inhalation agents have been around for years, we still don't fully understand how they produce their effects. We do know the brain is the site of action; it's believed the agents disrupt nerve transmission in select areas throughout the central nervous system.

How are they used?
The most common use of volatile inhalation agents is maintenance of general anesthesia in patients once they've been induced with an IV anesthetic such as propofol. Desflurane and sevoflurane are both routinely used in ambulatory surgery because their low blood solubility gives them fast recovery profiles.

Sevoflurane and halothane, due to their low pungency, can also be used to induce general anesthesia via facemask induction. Desflurane and isoflurane, because of their smell, can cause coughing, breath holding, salivation and laryngospasm. Mask induction is especially useful in pediatric cases because it allows you to place any IV lines after the child is unconscious, which many children find less frightening than IV induction. Sevoflurane is the agent of choice for pediatric mask induction.

How potent are they?
A key concept for comparing volatile inhalation agents is minimum alveolar concentration (MAC). MAC is defined as the alveolar (lung) concentration of anesthetic at one atmosphere that prevents movement in 50 percent of subjects in response to a painful stimulus (such as surgical skin incision) it's an indication of an agent's potency. The lower the MAC, the greater the potency.

Isoflurane is the most potent (MAC 1.15 percent), followed by sevoflurane (MAC 2.05 percent), then desflurane (MAC 6 percent). Providers routinely take advantage of the additive effects of inhalational agents on MAC - adding a second agent reduces the required concentration of the first.

For example, administering 67 percent nitrous oxide reduces desflurane's MAC value from 6 percent to about 2.17 percent. This has at least two advantages: It reduces the amount of volatile inhalation agent needed, lessening the potential of adverse effects; and it reduces the cost per minute of the volatile inhalation agent. Nitrous oxide is much less expensive than the volatile inhalation agents.

How do they differ?
The primary differences between the agents relate to their chemical stability, metabolism, effects on the heart and solubility. Let's look at each.

• Chemical stability. Much has been written about sevoflurane's chemical stability and its degradation to Compound A when it passes through the anesthesia machine's CO2 absorber. The reason for the concern is that Compound A causes kidney damage in rats. Compound A formation is enhanced as the CO2 absorbent's temperature increases, which occurs with low background gas-flow rates.

To date, sevoflurane has been widely used worldwide with no evidence of human consequences from Compound A exposure. However, sevoflurane's package insert carries a warning that exposure should not exceed 2 MAC hours at flow rates of 1L/min to

< 2L/min, with flow rates of