EFFECT OF EXPIRATORY FLOW LIMITATION ON RESPIRATORY MECHANICAL IMPEDANCE - A MODEL STUDY

Large phasic variations of respiratory mechanical impedance (Zrs) have been observed during induced expiratory flow limitation (EFL) (M. Vas siliou, R. Peslin, C. Saunier, and C. Duvivier. Eur. Respir. J. 9: 779 -786, 1996). To clarify the meaning of Zrs during EFL, we have measure d from 5 to 30 Hz the input impedance (Zin) of mechanical analogues of the respiratory system, including flow-limiting elements (FLE) made o f easily collapsible rubber tubing. The pressures upstream (Pus) and d ownstream (Pds) from the FLE were controlled and systematically varied . Maximal flow (Vmax) increased linearly with Pus, was close to the va lue predicted from wave-speed theory, and was obtained for Pus-Pds of 4-6 hPa. The real part of Zin started increasing abruptly with flow (i i) >85% Vmax and either further increased or suddenly decreased in the vicinity of Vmax. The imaginary part of Zin decreased markedly and su ddenly above 95% Vmax. Similar variations of Zin during EFL were seen with an analogue that mimicked the changes of airway transmural pressu re during breathing. After pressure and ii measurements upstream and d ownstream from the FLE were combined, the latter was analyzed in terms of a serial (Zs) and a shunt (Zp) compartment. Zs was consistent with a large resistance and inertance, and Zp with a mainly elastic elemen t having an elastance close to that of the tube walls. We conclude tha t Zrs data during EFL mainly reflect the properties of the FLE.