IS PROXIMAL AIRWAY PRESSURE A GOOD REFLECTION OF PERIPHERAL AIRSPACE PRESSURE IN INFANTS AND CHILDREN MODELS UNDER HFJV

This experimental study was carried out to determine if an alveolar po sitive end-expiratory pressure (PEEP) could occur during high frequenc y jet ventilation (HFJV) in infants, and if tracheal pressure is a goo d estimation of alveolar pressure. We used physical models simulating a 1.5 kg premature (P), a 3 kg newborn (N) and a 6 kg child (C) with n ormal compliance and normal resistance. Moreover, in the N model, we u sed two different resistances and lung compliance heterogeneity was st udied in the P model. Pressure was measured simultaneously in the tube simulating trachea (Paw) and in the bottle simulating the lung (Palv) . HFJV was performed either via an endotracheal tube (ETT) or via a lo ng catheter as in laryngoscopy. The ratio of injection time upon cycle duration (Ti/Ttot) was 20% or 30%, jet frequency was altered from 150 to 300 min(-1) and the driving pressure was set as in clinical practi ce (0.5 and 0.6 bar). PEEP occurred mainly in N (1.1 to 3.2 cm H2O) an d C models (0 to 3.5 cm H2O). It was inversely related to expiratory t ime (Te). The end-expiratory pressure drop between Palv and Paw (Delta EEP) was higher in N and increased from 0.5 to 2 cm H2O with the shor tening of Te and with airway resistances, i.e. the presence of ETT. In the heterogeneous model, PEEP and Delta EEP were greater in the highe r compliance alveolus. This study shows that the end-expiratory Palv i s underestimated by end-expiratory Paw, This is particularly important in the presence of an heterogeneity of distribution in lung complianc e. In this case the airway PEEP overestimates the PEEP in the lower co mpliance alveolus and underestimates the PEEP in the higher compliance alveolus.