DYNAMIC ELASTANCE AND TISSUE RESISTANCE OF ISOLATED LIQUID-FILLED RATLUNGS

The effect of the surface forces of the alveolar air-liquid interface on the dynamic behavior of lung tissue was investigated in five isolat ed liquid-filled rat lungs. The lungs were subjected to 0.04-Hz sinuso idal oscillation (1.5-ml tidal volume) at lung volume (VL) levels rang ing from volume at zero pressure (V-o) + 4 ml to V-o + 10 ml. Oscillat ions were performed at each VL after inflation of the lungs from V-o. Alveolar pressure (PA) was measured with an alveolar capsule attached to the visceral pleura. Dynamic elastance (Edyn), tissue resistance (R ti), and hysteresivity [eta = Rti omega/Edyn, where omega is angular f requency (2 pi X frequency)] were computed from PA and VL changes. Edy n was 59.6 +/- 4.3 Pa/ml at V-o + 4 ml and varied little up to V-o + 7 ml. Thereafter, Edyn increased markedly with VL, reaching 102 +/- 16 Pa/ml at V-o + 10 ml. No significant difference was found between elas tance computed from PA and that computed from pressure recorded at the airway opening. Rti was 35.2 +/- 3.6 Pa . s . ml(-1) and exhibited a VL dependence similar to that of Edyn. As a result, eta was 0.16 and d id not vary significantly in the explored VL range. We conclude that P A can be reliably measured in the liquid-filled lung by means of alveo lar capsules. In the liquid-filled lung, Edyn was smaller than and eta was similar to values reported for air-filled lungs. Hence, surface t ension accounts for a considerable part of elastance and Rti of the ai r-filled lung within the volume range of normal breathing.