GEOMETRIC HYSTERESIS IN PULMONARY SURFACE-TO-VOLUME RATIO DURING TIDAL BREATHING

We investigated the dynamic history dependence of lung surface area-to -volume ratio (S/V) during tidal breathing in live rabbits with use of our recently developed technique of diffuse optical scattering. We al so examined the effect of methacholine (continuous intravenous infusio n, 1-10 mug . kg-1 . min-1) on lung micromechanics with the same techn ique. Animals were anesthetized, tracheostomized, and mechanically ven tilated, and the left lung was exposed through a thoracotomy. An optic al fiber delivering light from a He-Ne laser was attached normal to th e pleural surface, producing a circular light pattern on the pleural s urface from diffusively scattered light within the parenchyma. The pat tern of light intensities was measured using a CCD video camera connec ted to a computer. S/V during tidal breathing changed in a manner qual itatively consistent with geometric similarity. There was a small but significant hysteresis in S/V vs. volume, with S/V inspiration greater than S/V expiration at the same volume. However, during methacholine challenge, the sense of hysteresis reversed; S/V inspiration was less than S/V expiration at isovolume points. Moreover, S/V during methacho line challenge systematically decreased at all lung volumes compared w ith control. These findings suggest that 1) during normal tidal breath ing, stress hysteresis in ductal tissue is larger than septal stress h ysteresis (septal tissue plus surface tension) and 2) the effect of me thacholine on tissue in the septa is greater than the corresponding ef fect in ductal tissue.