Comparing the costs of inhaled anesthetics

The immediate cost of an inhaled anesthetic results from an interplay between four factors: (1) the cost per milliliter of liquid anesthetic, (2) the volume of vapor that results from each milliliter of liquid, (3) the effective potency of the anesthetic (what concentration must be delivered from a vaporizer to provide a clinically appropriate level of anesthesia), and (4) the background flow of the gases that is chosen. A background flow that supplies only the gases/vapors required (taken up) by the patient (a "closed circuit") produces the least cost but also the least control of anesthetic level, whereas a high flow prevents rebreathing (a non-rebreathing system) but produces the greatest cost and control. We define greater "control" as a smaller ratio of delivered to alveolar concentrations. A lower solubility of an anesthetic accords the same level of control at a lower background flow rate than is achieved at a higher background flow rate with a more soluble anesthetic. Thus, a poorly soluble anesthetic may be used with a lower background flow rate than a more soluble anesthetic and may offer greater control and/or decreased cost.

This report presents a method of determining the cost of inhaled anesthetic use. As an example, the cost of delivering a desflurane anesthetic is compared with that of delivering an isoflurane anesthetic, assuming both provide an alveolar concentration of 1 MAC. The comparison is based on the pharmacokinetic differences of the two anesthetics: taking into account that for a given therapeutic anesthetic concentration (MAC), for desflurane a lower flow rate of background gas is needed to produce similar control (relationship between delivered and alveolar gases) than is needed for isoflurane.

The analysis demonstrates that the relative cost of administering the newer and less soluble anesthetic, desflurane, can be less than, greater than, or the same as the cost of administering isoflurane, depending on the background gas inflow rate selected.

The manner in which inhaled anesthetics are used and their kinetic differences are important determinants of relative cost.