MEASUREMENT OF LOW-FREQUENCY RESPIRATORY IMPEDANCE IN INFANTS

Low-frequency respiratory impedance (Zrs) data permit the separate est imation of the mechanical properties of the airways and the tissues, b ut they are difficult to collect in humans because of the need for apn eic conditions. We exploited the apneic phase produced by invoking the Hering-Breuer reflex with end-inspiratory airway occlusion in five se dated infants aged 9 to 16 mo. A computer-controlled pump and solenoid valves were used to inflate the supine infants through a face mask to a transrespiratory pressure of 20 cm H2O and to affect the airway occ lusion. A loudspeaker-in-box system was connected to the mask through a side-arm, and small-amplitude pseudorandom oscillations containing 2 3 frequency components between 0.5 and 20.75 Hz were applied for 6 s. Four consecutive measurements were made in each infant, and the averag ed Zrs spectra were evaluated on the basis of a model containing the f requency-independent resistance (Raw) and inertance (law) of the airwa ys, and the viscous damping (G) and elastance (H) parameters of the co nstant-phase compartment of the chest wall and parenchymal tissues. Th e measured Zrs values were consistent with the model up to 15 Hz, and the average fitting error was 0.89 +/- 0.11 (SD) cm H2O.s/L. The follo wing parameter values were obtained: Raw = 10.0 +/- 2.1 cm H2O.s/L, la w = 0.061 +/- 0.014 cm H2O.s(2)/L, G = 28.6 +/- 4.9 cm H2O/L, H = 141 +/- 55 cm H2O/L. The tissue hysteresivity (G/H) values were 0.218 +/- 0.061. Our results indicate that, in short apneic periods evoked by th e Hering-Breuer reflex, reliable low-frequency Zrs data can be collect ed to partition the tissue and airway impedances in sedated infants.