Determinants of the slope of phase III of the single breath nitrogen test.

To assess whether the slope of phase III of the single breath nitrogen (SB-N2) test correlates better with inhomogeneity of the elastic properties within a lung or with peripheral airway inflammation, pressure-volume (PV) curves and SB-N2 tests were performed in 28 excised human lungs, including 11 emphysema-free and 17 emphysematous lungs. The degrees of emphysema and membranous as well as respiratory bronchiolar inflammation were graded in a semi-quantitative manner. The PV curves were analysed using an exponential curve fit, yielding the exponent K which is proportional to total incremental compliance. In the 11 emphysema-free lungs, the slope of phase III was significantly correlated with K, when both age (p less than 0.05) and bronchiolar inflammation scores (p less than 0.01) were held constant. On the other hand, the slope of phase III correlated less favourably with the bronchiolar inflammation scores when age was held constant and was not significantly correlated with the bronchiolar inflammation scores when K was held constant. In the emphysematous lungs alone, or when the emphysema-free and emphysematous lungs were pooled, these correlations disappeared. In 23 smoking subjects, the slope of phase III was also significantly correlated with K (r = 0.72; p less than 0.001). We conclude that in emphysema-free lungs the slope of phase III is better correlated with the overall elastic properties of the lungs than with bronchiolar inflammation, and we speculate that increases in K, implying a greater overall curvature of the PV relationship, reflect increasing inhomogeneity of elastic properties within a lung, rather than a uniform change affecting individual lung units equally.(ABSTRACT TRUNCATED AT 250 WORDS)