A new application of an old method for respiratory mechanics measurements: the passive inflation method in newborn infants during pressure-controlled ventilation.

We have previously described a passive inflation method during constant inspiratory flow for measuring total respiratory elastance and resistance during mechanical ventilation in newborns. The simple method for measuring respiratory mechanics had been assessed during decelerating inspiratory flow obtained with pressure controlled ventilation (PCV). We report an application of this method to preterm and full-term mechanically ventilated newborn infants and a comparison with the occlusion method. Twenty-one newborn infants (birth weight 1,060 to 3,650 g; gestational age 26 to 41 weeks), between 1 to 55 days of postnatal age, were enrolled in the study. They were ventilated with a "Servo ventilator 900C," first set in the pressure-controlled mode and then in the volume-controlled mode without changing the tidal volume (VT), inspiratory time or ventilator rate. Flow was measured through a pneumotachograph inserted between the endotracheal tube (ETT) and the breathing circuit; VT was obtained by integration of flow and airway pressure measured directly at the airway opening. Flow, volume, and pressure were plotted on analog X/Y tables to obtain pressure-volume (P/V) and flow-volume (V/V) loops, as well as pressure-time curves. Occlusion was performed by using the end-inspiratory and the end-expiratory pause buttons of the ventilator. The passive inflation method during PCV was based on the analysis of P/V and V/V loops and provided compliance (Crs(PC)infl.), resistance Rrs(PC)infl.) of the respiratory system, and intrinsic positive end-expiratory pressure (PEEP(PC)i,infl.). These values were compared with (1) compliance (Crs(PC)occl.) and intrinsic positive end-expiratory pressure (PEEP(PC)i,occl.) measured by the occlusion method during PCV; (2) static (Crs(VC),occl.) and dynamic (Crs(VC),dyn.) compliance, airway (R(aw)(VC),), tissue (Rrs(VC),visc.) and total resistance (Rrs(VC),occl.), and intrinsic positive end-expiratory pressure (PEEP(VC)i,occl.) measured by the occlusion method during volume-controlled ventilation. Crs(PC),infl. correlated well with Crs(PC),occl., Crs(VC),occl., and Crs(VC),dyn.. Furthermore, Crs(PC),infl. and Crs(VC),dyn. did not differ significantly. Rrs(PC),infl. correlated well and did not differ significantly from total inspiratory resistance, i.e., the sum of R(aw)(VC) and Rrs(VC),vis. PEEPPC,i,infl. correlated well and did not differ significantly from PEEP(PC)i.occl. and from PEEP(VC),i,occl. The passive inflation method can be used during PCV with a decelerating flow waveform. It provides dynamic compliance, inspiratory resistance of the respiratory system, and intrinsic PEEP from the analysis of V/V and P/V loops recorded at the airway opening. This technique is simple to use and well tolerated by preterm and full-term ventilated newborn infants. It can be a good alternative to occlusion methods.