Effects of inhalation anesthetics on airway dynamics determined by phasor analysis of respiratory impedance using the forced oscillation method.

By expressing the respiratory system as a two-compartment model and assigning a phasor in the complex plane to each impedance element in the model, the phasor of the respiratory impedance could be constructed graphically, the frequency characteristics determined from the locus of the latter and the effects of variations in the model elements on the frequency characteristics could also be expressed as the locus of the latter. Conversely, it is possible to detect small changes in mechanical properties on the respiratory system by plotting its frequency characteristics on the complex plane. We studied the effects of typical inhalation anesthetics, halothane, enflurane and isoflurane, on the airway dynamics by this method. The inhalation concentration of 1 MAC of halothane and/or enflurane was found to produce bronchodilation associated with a significant reduction in airway resistance, but isoflurane had no such effects. As the magnitude of changes in the real part at 1 Hz, the estimated airway resistance was -2.1 cmH2O/1/s (-25% of the mean value) for halothane, and -0.8 cmH2O/1/s (-11%) for enflurane.