RESPIRATORY MECHANICS DURING MECHANICAL VENTILATION - A MODEL STUDY ON THE EFFECTS OF LEAK AROUND A TRACHEAL TUBE

Air leak around a tracheal tube (TT) during mechanical Ventilation is likely to occur during the inspiratory phase because airway pressure i s high for a prolonged period. The presence of a leak may introduce er rors in measurements of respiratory mechanics made at the airway openi ng. If so, respiratory mechanics can be measured more accurately when data are collected during the expiratory phase of ventilation. We exam ined this phenomenon in a lung model. When a leak was introduced into the model, simulating a leak around the TT, the leak occurred predomin antly during the inspiratory phase of respiration. As the magnitude of the leak increased, the overestimation of resistance progressively in creased, when calculated from pressure and flow measured at the airway opening. A large leak (38%) resulted in an overestimation of respirat ory system resistance by 51% and an underestimation of elastance (Ers) by 23% when calculated from the entire ventilatory cycle. However, th ere was no under- or overestimation in Rrs when calculated from the ex piratory phase only, and ERS was overestimated by only 6.1%, Varying p eak inspiratory pressure, end-expiratory pressure, and expiratory time did influence the effect of leak, however, respiratory mechanics coul d still be calculated accurately from the expiratory phase under these conditions. We conclude that measurements of lung mechanics from the expiratory phase is a promising approach to dealing with the problem o f measuring respiratory mechanics in mechanically ventilated infants w ith leaks around the tracheal tube. (C) 1997 Wiley-Liss, Inc.