Effect of the physical properties of isoflurane, sevoflurane, and desflurane on pulmonary resistance in a laboratory lung model.

Background: Airway resistance depends not only on an airway's geometry but also on flow rate, and gas density and viscosity. A recent study showed that at clinically relevant concentrations, the mixtures of volatile agents with air and oxygen and oxygen-nitrogen affected the density of the mixture. The goal of the current study was to investigate the effect of different minimum alveolar concentrations (MACs) of three commonly used volatile agents, isoflurane, sevoflurane, and desflurane, on the measurements of airway resistance.

Methods: A two-chamber fixed-resistance test lung was connected to an anesthesia machine using the volume control mode of ventilation. Pulmonary resistance was calculated at baseline (25% oxygen in air); at 1.0, 1.5, and 2.0 MAC; and also at the same concentrations, 1.2% and 4%, of isoflurane, sevoflurane, and desflurane mixtures with 25% oxygen in air. The analysis of variance test for repeated measures and probabilities for post hoc Tukey and least significant difference tests were used.

Results: Isoflurane affected pulmonary resistance only at 2 MAC. Sevoflurane caused a significant increase of pulmonary resistance at 1.5 and 2 MAC, whereas desflurane caused the greatest increase in pulmonary resistance at all MAC values used. At 1.2% concentration, no difference from the baseline resistance was observed, whereas at 4%, the three agents produced similar increases of pulmonary resistance.

Conclusion: High concentrations of volatile agents in 25% oxygen in air increased the density of the gas mixture and the calculated resistance of a test lung model with fixed resistance.