Model-based administration of inhalation anaesthesia. 4. Applying the system model

Area of Science:

• Anesthesiology
• Pharmacokinetics
• Medical Engineering

Background:

• Developed a simple dosing strategy for rapid isoflurane induction and minimal-flow maintenance (0.5 L/min total fresh gas flow).
• Aimed to achieve and maintain end-expired isoflurane concentrations within a therapeutic window (0.8-1.1 vol%) within 5 minutes for at least 30 minutes.

Purpose of the Study:

• To test a novel, simple dosing strategy for rapid isoflurane induction and maintenance under minimal fresh gas flow conditions.
• To evaluate the reliability and efficacy of the proposed regimen in adult surgical patients.

Main Methods:

• Computed a three-stage isoflurane regimen using a fixed vaporizer setting (3 vol%) and varying fresh gas flows (3 L/min, 1.5 L/min, 0.5 L/min) and nitrous oxide:oxygen ratios.
• Evaluated the scheme in 58 adult patients undergoing surgery, primarily for eye, ear, nose, and throat procedures.

Main Results:

• End-expired isoflurane concentration reached the therapeutic window within 2 minutes and stabilized at a mean of 0.96 vol%.
• The mean concentration remained within the desired 0.8-1.1 vol% window from 3 to 60 minutes post-induction.
• Oxygen and nitrous oxide concentrations evolved as computed, though inspired oxygen decreased below 30% in some patients; the scheme required adaptation in six patients due to hypotension.

Conclusions:

• The developed isoflurane dosing regimen is easily remembered and reliable for anesthesia maintenance.
• The strategy is adaptable but necessitates vigilant monitoring of gas/vapor concentrations and hemodynamic variables.