Contribution of airway resistance to airway pressure during mechanical ventilation: an experimental study

Although airway resistance (R) is an important parameter of the pulmonary condition, its determination during mechanical ventilation is not easy. Most physicians estimate R from peak airway pressure during mechanical ventilation. We assessed the relationship between R and peak airway pressure by a computer simulation. The time course of airway pressure (Ptr) during mechanical ventilation was calculated from the airway pressure at end-inspiration and respiratory flow, allowing for the buffering effect of the dead space. The parameters for computer simulation were obtained from 5 paralyzed and mechanically ventilated dogs. The predicted Ptr curve was a function of airway resistance. Since R was not directly determined by the animal experiments, we determined R by using the Ptr curve most closely approximating the original Ptr curve as the true R. The R-peak tracheal pressure relationship predicted by computer simulation showed that the peak airway pressure increased almost linearly with increases in R. However, in computer simulation, when R was increased 10-fold from the value at airway relaxation, the peak airway pressure increased only 6-fold from the corresponding value. We conclude that peak airway pressure is a relatively insensitive parameter for the estimation of airway constriction during mechanical ventilation.