Calibration of respiratory inductance plethysmograph in preterm infants with different respiratory conditions.

Respiratory inductance plethysmography (RIP) is a method for respiratory measurements particularly attractive in infants because it is noninvasive and it does not interfere with the airway. RIP calibration remains controversial in neonates, and is particularly difficult in infants with thoraco-abdominal asynchrony or with ventilatory assist. The objective of this study was to evaluate a new RIP calibration method in preterm infants either without respiratory disease, with thoraco-abdominal asynchrony, or with ventilatory support. This method is based on (i) a specifically adapted RIP jacket, (ii) the least squares method to estimate the volume/motion ribcage and abdominal coefficients, and (iii) an individualized filtering method that takes into account individual breathing pattern. The reference flow was recorded with a pneumotachograph. The accuracy of flow reconstruction using the new method was compared to the accuracy of three other calibration methods, with arbitrary fixed RIP coefficients or with coefficients determined according to qualitative diagnostic calibration method principle. Fifteen preterm neonates have been studied; gestational age was (mean +/- SD) 31.7 +/- 0.8 weeks; birth weight was 1,470 +/- 250 g. The respiratory flow determined with the new method had a goodness of fit at least equivalent to the other three methods in the entire group. Moreover, in unfavorable conditions--breathing asynchrony or ventilatory assist--the quality of fit was significantly higher than with the three other methods (P < 0.05, repeated measures ANOVA). Accuracy of tidal volume measurements was at least equivalent to the other methods, and the breath-by-breath differences with reference volumes were lower, although not significantly, than with the other methods. The goodness of fit of the reconstructed RIP flow with this new method--even in unfavorable respiratory conditions--provides a prerequisite for the study of flow pattern during the neonatal period.