We calculated the effect of 1) airway reactance, 2) extrathoracic airway shunt impedance, 3) heating and humidification of inspired air, 4) abdominal gas volume and 5) respiratory quotient on the measurement of phase difference between chest flow (Vc) and mouth flow (Vm) using computer simulation. 1) When airway impedance (Za) was approximated to be a simple airway resistance as we did in the former paper to evaluate the effect of airway impedance on Vc-Vm phase difference, the phase difference (θr) was calculated to be 0.8% less than that (θs) calculated from Za including airway inertance and shunt compliance in normal lung. θs became larger than θr when the peripheral resistance increased, but it was suspected that the difference ratio of θs and θr does not exceed 5%. 2) The influence of extra-thoracic shunt impedance was within 0.1% regardless of the variation of the respiratory frequency, airway impedance and VTG. 3) The difference ratio of the heating and humidification of the inspired air was calculated to be within -3% when the time constant of delay (d) was assumed to be 0.02 second. The difference ratio became higher as d increased, but it may not exceed -5% as far as d was within 0.1s. The influence of the abdominal gas was calculated to be much variable in correspondence with the abdominal gas volume or the respiratory pattern, but it was inferred to be within 5%. 5) The influence of the respiratory quotient was calculated to be negligible. As a result, it may be concluded that all the factors discussed above do not become an obstacle to the clinical application of this method to the evaluation of the pathological change in obstructive airway disorders.
