Discussion

The closed-circuit anesthetic inductions you have just performed are similar to those described by Lowe²², and Lowe and Ernst²³, but use smaller, more frequent injections. Lowe's technique requires standard unit dose liquid injection at predetermined times based upon a model of the closed system, and on the observation that nitrous oxide requirement decreases approximately in proportion to the square root of times³⁶:This model was validated to be a good approximation by Connor and Philip.^{47, 48}

(1/^{ t} = 1/t^1/2 = t ^-1/2).

In the next exercise, you will administer closed-circuit anesthesia "by the book".

Exercise 10-3: Classic Closed-circuit Approach

Liquid injection following the "t-1/2" regimen is a classic closed-circuit approach.

SETUP

Parameter Selection Agent Isoflurane DEL (%) 0 Circuit Closed FGF (L/min) 0.25 To simulate closed-circuit liquid injection anesthesia following the t-1/2 model with VA (L/min) 4 isoflurane, adjust the parameters as shown. CO (L/min) 5 View (min) 15 Speed 5x Special none Unit Dose 0.5 mL

Before clicking the Begin button, administer a priming dose of 0.5 mL isoflurane by clicking on the syringe icon, and then adjust Anesthesia / Unit Dose back to its default value, 0.7 mL. Click Begin. Click on the liquid syringe at 1 minute, 4 minutes, and 9 minutes. You can use Special / Set Bookmark or the toolbar Bookmark button to pause the exercise at these precise times.

Figure 10-3a displays the Picture and Graph of Exercise 10-3, showing a completed 15-minute course of closed-circuit anesthesia with liquid isoflurane, using predetermined times of administration following the square root of time regimen.

Repeat the closed-circuit liquid injection technique with desflurane and halothane. For desflurane, choose a priming dose of 2.0 mL and a unit dose of 1.5 mL. For halothane, choose a priming dose of 0.5 mL and a unit dose of 0.7 mL. Again, make injections at 1, 4, and 9 minutes. These are examples of the classic closed-circuit approach introduced by Lowe²² and refined by Ernst²³. Priming doses used here are smaller.

Figure 103b displays the Picture and Graph of Exercise 10-3, showing a completed 15-minute course of dosed-circuit anesthesia with liquid desflurane using predetermined times of administration following the t^-1/2

regimen.

Note that when using desflurane, the volume of the priming dose exceeded the volume of the subsequent unit doses, while in isoflurane and halothane, the opposite is true. This is because desflurane's low blood/gas solubility decreases uptake into blood and tissues, but not into the breathing circuit.

Figure 10-3c displays the Picture and Graph of Exercise 10-3, showing a completed 15-minute course of closed-circuit anesthesia with liquid halothane using predetermined times of administration following the t^--1/2 regimen.

Discussion

The simulations in Exercises 10-2 and 10-3 have shown that liquid anesthetic injection into the breathing-circuit can produce satisfactory anesthesia. Viewing the vessel-rich group has shown that anesthetic tension in the location of interest (the brain) follows a relatively smooth time course. This is evident even when alveolar and especially inspired tension changes dramatically between injections.