Validation of a new respiratory inductive plethysmograph

The respiratory inductive plethysmograph (RIP) can be used to monitor changes in end-expiratory lung volume (deltaEELV), and thus, used in intensive care when evaluating positive end-expiratory pressure (PEEP)-induced changes in lung volumes in order to optimise the ventilator settings. We validated the newest model of RIP (Respitrace Plus), both under laboratory and clinical conditions, and made a comparison with a previously validated RIP (Respigraph) in the measurement of tidal volume (V(T)), long-term EELV and PEEP-induced acute deltaEELV.

The in vitro validation was performed using a lung model and the in vivo evaluation in five spontaneously breathing normal subjects, four patients with acute lung injury (ALI) and eight anaesthetised patients.

The difference in V(T) values during PEEP manipulations between pneumotachometer (PNT) and RIP in spontaneously breathing volunteers and mechanically ventilated ALI patients was 0.1-0.9% (mean+/-SEM) and -5.2+/-0.6, respectively (effect of device NS). In anaesthetised patients, the difference between the new RIP and spirometer in the measurement of PEEP-induced acute deltaEELV was -2.6+/-1.5% (PEEP-device interaction, P<0.05). In ALI patients, the difference between PNT and RIP in the measurement of PEEP-induced acute deltaEELV was 1.0+/-9.7% (NS). The new RIP had a baseline drift over 50-fold when cold and over 10-fold when warm compared to the old RIP in vitro.

The new RIP is accurate enough for clinical and research purposes in the measurement of V(T). Semiquantitative measurements of acute deltaEELV can be done with accuracy sufficient for clinical use, but long-term deltaEELV monitoring is not possible. The new RIP should be kept on for several hours before measurements to minimise the drift.