Viscoelastic properties of the visceral pleura and its contribution to lung impedance.

The mechanical impedances of 10 dog lung lobes (ZL) and circular pleura samples 1.6 cm in diameter (Zpl) were measured with small-amplitude forced oscillations between 0.2 and 4.2 Hz. Two to four samples were ablated from each lobe after their in situ tension had been fixed at 5 cmH2O transpulmonary pressure with plastic rings. Lobe resistance was inversely proportional to frequency (f) and lobe elastance increased linearly with the logarithm of f by 23%/decade. The real part of Zpl (Rpl) decreased hyperbolically with f. Pleural elastance (Epl) showed only a 5%/decade increase with log f. The regional variability of Epl was large and the data allowed the lumping of Epl into only two groups. The variability of Epl was higher in the group of Epl values from the coastal surfaces than in the group of Epl values from any other surfaces (3105 +/- 2741 (SD) vs 2263 +/- 1152 cm H2O/L). The mean intraindividual variation of Epl corresponding to costal and to other surfaces was 38 +/- 25 and 32 +/- 23%, respectively. The hysteresivity index (Fredberg and Stamenovic, J. Appl. Physiol., 67: 2408-2419, 1989) of the pleura was significantly smaller than that of the lobes (0.025 vs 0.148). Extrapolation of Zpl to the entire lobe surface predicted pleural/lobar resistance and elastance ratios of 2.7 +/- 0.4 and 16.8 +/- 10.6%, respectively, at 0.2 Hz, and 0.4 +/- 0.4 and 15.5 +/- 10.6%, respectively, at 4.2 Hz. This suggests that for small deformations and medium lung volumes the pleural contribution to ZL is almost ideally elastic and only slightly frequency-dependent.