Uptake of enflurane and isoflurane during spontaneous and controlled ventilation

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

BACKGROUND

The square root of time (SqRT) model had been used to predict the uptake of volatile agents.

METHODS

We studied the rate of uptake of isoflurane in 10 patients using liquid isoflurane infusion through syringe pump into the closed circuit. The infusion rates were titrated to maintain a constant end tidal concentration of isoflurane of 1.5%. The predicted uptake values were also calculated from the square root principle and compared with the derived uptake.

RESULTS

The observed rate of uptake was higher than predicted from the Lowe and Ernst equation (P<0.001). There exists considerable inter-individual variability in uptake pharmacokinetics and it showed statistically significant correlation with ideal body weight, body weight (P<0.01), body surface area, and body weight¾ from 30 min of start of isoflurane infusion (P<0.05).

CONCLUSION

SqRT model is inaccurate in predicting isoflurane uptake and underestimates it during closed circuit anaesthesia.

Closed system anaesthesia in dogs using liquid sevoflurane injection; evaluation of the square-root-of-time model and the influence of CO 2 absorbent

OBJECTIVE

To determine whether predictable alveolar concentrations of sevoflurane are reliably produced in dogs when liquid sevoflurane is injected into closed circuit breathing systems, as calculated by Lowe's square-root-of-time anaesthetic uptake model, and to confirm the validity of the model using soda lime and calcium hydroxide lime.

STUDY DESIGN

Prospective clinical study.

ANIMALS

Eleven healthy dogs with a mean body mass of 34 +/- 9 kg scheduled for pelvic limb orthopaedic surgery.

MATERIALS AND METHODS

Following pre-anaesthetic medication, anaesthesia was induced with propofol and maintained with sevoflurane in a closed circle system. Epidural anaesthesia was performed with morphine and bupivacaine. Liquid sevoflurane was injected into the circuit by syringe, using dosages and time intervals derived from Lowe's square-root-of-time anaesthetic uptake model. The target alveolar concentration chosen was 1.1 x MAC (2.6% end-tidal sevoflurane). Either soda lime (group S; n = 6) or calcium hydroxide lime (Amsorb; group A; n = 5) were used for CO(2) absorption. Sevoflurane concentration and the respiratory gas composition were measured with an infrared gas analyser.

RESULTS

End-tidal sevoflurane concentrations were close to the predicted value of 2.6% at 9 minutes (2.53 +/- 0.1% group S; 2.60 +/- 0.26% group A) and 16 minutes (2.55 +/- 0.30 group S; 2.52 +/- 0.28% group A) but declined thereafter to reach 50% (group S) and 64% (group A) of the predicted value at 121 minutes. There was a constant trend towards higher end-tidal sevoflurane concentrations in group A but the difference was not statistically significant.

CONCLUSIONS

The square-root-of-time model leads to significantly lower alveolar concentrations than expected, suggesting that the rate of sevoflurane uptake in dogs declines less rapidly than predicted. The use of Amsorb tends to reduce the deviation from predicted concentrations.

CLINICAL RELEVANCE

The model used in this study provided only an approximate guide to the volume of liquid sevoflurane required. Consequently, the definitive dose schedule must be based on measured anaesthetic concentrations and clinical monitoring.

Uptake of desflurane and isoflurane during closed-circuit anesthesia with spontaneous and controlled mechanical ventilation

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.

The uptake of enflurane during anaesthesia

The uptake of enflurane at a constant end-expired concentration of 2% in oxygen was studied in 38 patients, ASA 1 or 2, undergoing elective orthopaedic procedures. The anaesthetic was administered using a computer-controlled closed circle system. Following an initial 4 min period during which the expired concentration of enflurane was established, the rate of uptake of enflurane showed a triexponential decline. The mean cumulative use of enflurane after 60 min was 10.7 ml, and after 120 min was 18.4 ml.