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



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



Bulletin Europeen de Physiopathologie Respiratoire 20(6): 521-527



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 analyzed 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 < 0.05) and bronchiolar inflammation scores (P < 0.01) were held constant. On the other hand, the slope of phase III correlated less favorably 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 < 0.001). 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 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. In emphysematous lungs, additional factors not measured in this study disturb the above relationship. It should be not noted that correlations demonstrate associations but do not imply cause and effect.

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Accession: 005120366

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PMID: 6518315



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