Study Objective To assess the accuracy of the respiratory inductive plethysmography in the measurement of PEEP-induced changes in end-expiratory lung volume during mechanical ventilation and its accuracy and stability in the measurement of ventilation during controlled mechanical ventilation and spontaneous breathing. Design An open comparison between two methods using a criterion standard. Either a pneumotachometer (mechanically ventilated patients) or a spirometer (spontaneously breathing subjects) was used as the reference method. Setting Tertiary care center; a multidisciplinary intensive care unit and a metabolic research unit. Patients Six mechanically ventilated, paralyzed postoperative open heart surgery patients, six spontaneously breathing COPD patients, and eight healthy volunteers. Interventions Stepwise increases and reductions of PEEP from zero to 12 cm H 2 O during controlled mechanical ventilation; repeated validation of the calibration of the respiratory inductive plethysmography (RIP) in both mechanically ventilated and spontaneously breathing subjects. Measurements and Results The baseline drift of the RIP in vitro was 10 ml/150 min and in a ventilated model it was 20 ml/150 min. In mechanically ventilated patients, the mean error of the calibration after 150 min was within ± 5 percent. Change in end-expiratory lung volume (EELV) during the stepwise increase of PEEP up to 12 cm H 2 O was 849 ± 136 ml with the RIP and 809 ± 125 ml with the pneumotachometer (PT), and during the stepwise reduction of PEEP it was 845 ± 124 ml and 922 ± 122, respectively (not significant [NS]. The mean difference between methods in the measurement of change in EELV was −6.6 ± 3.5 percent during increasing and 6.6 ± 6.7 percent during decreasing PEEP (NS). Both in mechanically ventilated and spontaneously breathing subjects, the difference between methods was significant for VT and VT/TI. The difference in VT was −2.2 ± 0.2 percent during mechanical ventilation, −1.1 ± 0.5 percent in spontaneously breathing COPD patients, and 2.9 ± 0.4 percent in healthy volunteers (NS between groups). Conclusions The RIP is sufficiently accurate for the measurement of PEEP-induced changes in EELV during controlled mechanical ventilation. The accuracy of tidal volume measurement is similar during mechanical ventilation and spontaneous breathing. The calibration of the RIP is stable enough for bedside monitoring of changes in lung volumes. (Chest 1992; 102:234–38)
