Uptake of Desflurane and Isoflurane During Closed-Circuit Anesthesia with Spontaneous and Controlled Mechanical Ventilation

Abstract

Although theoretical models predict uptake of inhaled anesthetics during closed-circuit anesthesia (CCA), clinical data for most anesthetics are conflicting or non-existent. In addition, the effects of patient characteristics and mode of ventilation on anesthetic uptake are unclear. Forty-one ASA physical status I or II adult patients undergoing a variety of 1-1.5 h surgical procedures were randomly allocated to receive CCA with desflurane or isoflurane with ventilation being either spontaneous or controlled. An end-expired anesthetic concentration of 1.3 minimum alveolar anesthetic concentration (MAC) was maintained by continuous injection of the liquid anesthetic into the circuit using a syringe pump. After an initial 4-min wash-in period, uptake during the first hour of CCA was nearly constant. Uptake was the same whether ventilation was spontaneous or controlled. Patient characteristics (age, height, weight, weight3/4, and body surface area) were comparable between groups and did not correlate with uptake. The virtually constant uptake after wash-in of desflurane and isoflurane contrasts with the square root of time model of Lowe and Ernst. These findings may greatly simplify CCA.

A simple method for evaluation of the uptake of isoflurane and its comparison with the square root of time model

“…Inter-individual variation is also a factor affecting uptake kinetics. [478] On plotting the infusion requirements over time, we found that it varied for similar Et-Iso in different patients [Figure 4]. This can be due to individual differences in the factors determining uptake i.e., CO, blood/gas partition coefficient (λ B/G ).…”

“…[19] Hence, it may be difficult to predict completely what is happening in vivo . [7] Brody's equation has been derived from a large ‘normal’ sample population under resting conditions. It does not necessary predict the output for the individual patient and might be different from the measured CO in clinical situations.…”

Closed-form solutions for the optimum equivalence of first-order compartmental models and their implications for classical models of closed-circuit anesthesia

“…The rationale behind the practice is illustrated in figure 3. However, it has been noted (Westenskow et al 1983, Hendrickx et al 1997 that the unit dose method tends to overestimate the amount of anesthetic agent needed and so lead to the administration of more agent than is desired. The analysis here would suggest that this occurs due to use of the Severinghaus relationship from time zero, and from the misconception of the underlying compartmental pharmacokinetics that we have demonstrated arises from that.…”

The Severinghaus square root of time relationship for anesthetic uptake and its implications for the stability of compartmental pharmacokinetics

“…It is conceivable that if this analysis were performed with high-potency volatile inhalational anesthetics with low fat solubility and therefore short fat time constant, further discrimination might be possible during anesthetics of 100 min duration. Such analysis would have to be based upon empirical studies of the uptake of these high-potency agents (Hendrickx et al 1997, 2003, Yasuda et al 1991a, 1991b rather than using the Severinghaus data for nitrous oxide.…”

“…they tested the model, they compared its behavior with Lowe's square root of time (Lowe andErnst 1981, da Silva et al 1997), implying that this simpler empirical relationship is the standard against which models should be measured. Much of this was reviewed by Hendrickx et al (1997Hendrickx et al ( , 2003Hendrickx et al ( , 2004. That all the compartmental models discussed above have rapid uptake in the early phase and slow uptake in later phases suggests that they have a behavior similar to the original multi-exponential model as well as the inverse square root of time model.…”