Mathematical method to build an empirical model for inhaled anesthetic agent wash-in

Minimal flow anesthesia can be initiated early with the use of higher fresh gas flow to facilitate desflurane "Wash-in"

Background and Aims:

More than 80% of delivered anesthetic gases get wasted at high fresh gas flows as they are vented out unused. Use of minimal flow anesthesia is associated with less waste anesthetic gas emission and environmental pollution. There is no approved or validated technique to initiate minimal flow anesthesia and simultaneously achieve denitrogenation of the breathing circuit. We studied the wash-in characteristics of desflurane, when delivered with 50% nitrous oxide, to reach a target end-tidal concentration at two different gas flow rates.

Material and Methods:

Patients were allocated randomly to two groups of 25 adults each. In Group A, with the vaporizer dial fixed at 4 vol %, after an initial fresh gas flow of 4 L/min was administered to wash-in of desflurane using the closed-circuit, with 50% N₂O in O₂, and in group B, 6 L/min was used. Minimal flow anesthesia, with 0.5 L/min, was initiated in both groups on attaining a target end-tidal desflurane concentration of 3.5 vol %. After initiation of desflurane delivery, the inspired/expired gas concentrations were noted every minute for 15 min.

Results:

In Group A, the target desflurane end-tidal concentration was reached in 499.2 ± 68.6 s±, and in the Group B (P < 0.001), it was reached significantly faster in 314.4 ± 69.89 s. Denitrogenation of the circuit was adequate in both groups.

Conclusion:

Minimal flow anesthesia can be initiated, without any gas-volume deficit, in about 5 min with an initial fresh gas flow rate of 6 L/min and the vaporizer set at 4 vol%.

1-1-12 one-step wash-in scheme for desflurane low flow anesthesia: performance without nitrous oxide

Background

We reported a 1-1-12 wash-in scheme for desflurane-nitrous oxide (N₂O) low flow anesthesia that is simple, rapid, and predictable. There remain some situations where N₂O should be avoided, which limits the generalizability of this wash-in scheme. The objective of our study was to determine the performance of this scheme in contexts where N₂O is not used.

Methods

We recruited 106 patients scheduled for elective surgery under general anesthesia. After induction and intubation, wash-in was started with a fresh gas flow of air:O₂ 1:1 L/min and a vaporizer concentration of desflurane of 12%. Controlled ventilation was then adjusted to maintain P_ACO₂ at 30–35 mmHg.

Results

The alveolar concentration of desflurane (F_AD) rose rapidly from 0% to 6% in 4 minutes in the same pattern as observed in our previous study in which N₂O was used. An F_AD of 7% was achieved in 6 minutes. An F_AD of 1% to 7% occurred at 0.6, 1, 1.5, 2, 3, 4, and 6 minutes. The rise in heart rate during wash-in was statistically significant, although not clinically so. There was a slight but statistically significant decrease in blood pressure, but this had no clinical significance.

Conclusion

Performance of the 1-1-12 wash-in scheme is independent of the use of N₂O. Respective F_ADs of 1%, 2%, 3%, 4%, 5%, 6%, and 7% can be expected at 0.6, 1, 1.5, 2, 3, 4, and 6 minutes.

1-1-12 one-step wash-in scheme for desflurane-nitrous oxide low-flow anesthesia: rapid and predictable induction

Background. We propose a 1-1-12 wash-in scheme for desflurane-nitrous oxide (N₂O) low-flow anesthesia. The objective of our study was to determine the time to achieve alveolar concentration of desflurane (F_AD) at 1, 2, 3, 4, 5, and 6%. Methods. We enrolled 106 patients scheduled for elective surgery under general anesthesia. After induction and intubation, wash-in was started with a fresh gas flow (FGF) of N₂O : O₂ 1 : 1 L min⁻¹ and vaporizer concentration of desflurane (FD) of 12%. Ventilation was controlled to maintain P_ACO₂ at 30–35 mmHg. Results. The F_AD rose rapidly from 0 to 4% in 2 min in a linear manner in 0.5 min increments. An F_AD of 6% was achieved in 4 min in a linear fashion from F_AD of 4% but in 1 min increments. An F_AD of 1 to 6% occurred at 0.6, 1, 1.5, 2, 3, and 4 min. Heart rate during wash-in showed a statistically, albeit not clinically, significant pattern of increase. By contrast, blood pressure slightly decreased during this period. Conclusions. We developed a 1-1-12 wash-in scheme using a FGF of N₂O : O₂ 1 : 1 L min⁻¹ and FD of 12% for desflurane-nitrous oxide low-flow anesthesia. A respective F_AD of 1, 2, 3, 4, 5, and 6% can be expected at 0.6, 1, 1.5, 2, 3, and 4 min.