VOLUME DEPENDENCE OF RESPIRATORY IMPEDANCE IN INFANTS

We previously studied low-frequency respiratory impedance (Zrs) data a t an elevated lung volume to separate airway and tissue mechanical pro perties in normal Infants (Am. J. Respir, Crit. Care Med. 1996; 154:16 1-166). The aim of the present study was to determine the volume depen dence of the airway and tissue mechanics by extending Zrs measurements to tower lung volumes. Zrs spectra between 0.5 and 21 Hz were measure d in supine sleeping infants (n = 8; 7 to 26 mo of age) at mean transr espiratory pressures (Ptr(mean)) of 20, 10, and 0 cm H2O, during perio ds of apnea induced by inflating the infants' lungs to a pressure of 2 0 cm H2O through a face mask. At each inflation pressure, a model cont aining airway resistance (Raw) and inertance (law) and tissue damping (G) and elastance (H) was fitted to Zrs data. At FRC, the values of Ra w, law, G, and H were 20.6 +/- 4.9 (SD) cm H2O . s/L, 0.037 +/- 0.014 cm H2O . s(2)/L, 39.6 +/- 10.3 cm H2O/L, and 147 +/- 35 cm H2O/L, resp ectively. increase of Ptr(mean) caused a monotonous decrease in Raw (4 2 +/- 7% of the value at FRC), while law remained constant. The tissue parameters were minimal at a Ptr(mean) of 10 cm H2O (68 +/- 10% and 7 8 +/- 6% in G and H, respectively) and significantly higher at both 0 and 20 cm H2O. Although irs measurements can be made in most infants a t lung volumes as low as FRC, an inflation pressure of 20 cm H2O provi des a higher success rate and is therefore a more suitable condition f or general use.