[Anesthesia with low fresh gas flow in clinical routine use]

Comparison of the Effects of Minimal and High-Flow Anaesthesia on Cerebral Perfusion During Septorhinoplasty

Objective

The aim of this study was to analyse the effects of minimal-and high-flow anaesthesia on cerebral oxygenation during septorhinoplasty with controlled hypotension using near-infrared spectroscopy.

Methods

Eighty patients scheduled for septorhinoplasty under general anaesthesia with controlled hypotension were randomised into two groups: minimal-flow (MF) or high-flow (HF). Both groups received desflurane anaesthesia to maintain bispectral index values at 40%-50% and 0.25-0.5 μg kg^-1 min^-1 i.v. remifentanyl infusion to maintain mean arterial blood pressure between 55 and 65 mmHg. The MF group received 5 L min^-1 of fresh gas flow for the first 10 mins then the gas flow was reduced 0.4 L min^-1. The HF group received 2 L min^-1 of fresh gas flow throughout. Haemodynamic parameters and cerebral oxygen saturation were measured.

Results

There were no statistical differences in demographic variables, duration of anaesthesia and surgery, time to extubation and proceeding to an Aldrete score of 9. There were no statistical differences in haemodynamic parameters, end-tidal CO₂ and cerebral oxygen saturation. The amount of desflurane used in the MF group was significantly lower than that used in the HF group (30.5±9.8 mL vs. 48.5±12.1 mL; p<0.05).

Conclusion

MF and HF anaesthesia did not lead to any difference in cerebral oxygen saturation in patients undergoing septorhinoplasty with controlled hypotension. MF anaesthesia may thus be used as safely as HF anaesthesia is.

Brief review: theory and practice of minimal fresh gas flow anesthesia

PURPOSE

The aim of this brief review is to provide an update on the theory regarding minimal fresh gas flow techniques for inhaled general anesthesia. The article also includes an update and discussion of the practical aspects associated with minimal-flow anesthesia, including the advantages, potential limitations, and safety considerations of this important anesthetic technique.

PRINCIPAL FINDINGS

Reducing the fresh gas flow to < 1 L·min(-1) during maintenance of anesthesia is associated with several benefits. Enhanced preservation of temperature and humidity, cost savings through more efficient utilization of inhaled anesthetics, and environmental considerations are three key reasons to implement minimal-flow and closed-circuit anesthesia, although potential risks are hypoxic gas mixtures and inadequate depth of anesthesia. The basic elements of the related pharmacology need to be considered, especially pharmacokinetics of the inhaled anesthetics. The third-generation inhaled anesthetics, sevoflurane and desflurane, have low blood and low tissue solubility, which facilitates rapid equilibration between the alveolar and effect site (brain) concentrations and makes them ideally suited for low-flow techniques. The use of modern anesthetic machines designed for minimal-flow techniques, leak-free circle systems, highly efficient CO(2) absorbers, and the common practice of utilizing on-line real-time multi-gas monitor, including essential alarm systems, allow for safe and cost-effective minimal-flow techniques during maintenance of anesthesia. The introduction of new anesthetic machines with built-in closed-loop algorithms for the automatic control of inspired oxygen and end-tidal anesthetic concentration will further enhance the feasibility of minimal-flow techniques.

CONCLUSIONS

With our modern anesthesia machines, reducing the fresh gas flow of oxygen to 0.3-0.5 L·min(-1) and using third-generation inhaled anesthetics provide a reassuringly safe anesthetic technique. This environmentally friendly practice can easily be implemented for elective anesthesia; furthermore, it will facilitate cost savings and improve temperature homeostasis.