Measurement of total respiratory impedance in infants by the forced oscillation technique.

The forced oscillation technique according to Làndsér et al. (J. Appl. Physiol. 41:101-106, 1976) was modified for use in infants. Adaptations, including a flexible tube to connect the infant to the measuring system and a bias flow to avoid rebreathing, did not influence impedance values. The linearity of the respiratory system was assessed and confirmed by 1) applying pseudo-random noise oscillations at three different amplitudes to 7 infants and 2) comparing in 12 infants impedance values obtained with pseudo-random noise and with sinusoidal oscillations at 12 and 32 Hz. Intersubject variability, averaged for all frequencies, was 6%. In 17 infants the relative error (+/- SD) between two series of five measurements within a time interval of 15 min was 0.5 +/- 5.7%. No statistically significant difference was found between impedance values before and after repositioning of the infant's head, whereas rotation resulted in a decrease in resistance and no effect on reactance. Our results indicate that the infant-adapted forced pseudo-random noise oscillation technique has the potential to give valuable information about ventilatory lung function in infants.