Exponential Analysis of the Pressure-Volume Curve: Correlation with Mean Linear Intercept and Emphysema in Human Lungs

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Abstract
We investigated the effect of obvious emphysematous lesions and the mean airspace size in the lung surrounding these lesions on the exponential constant K of the lung pressure-volume curve. The severity of the centrilobular emphysematous (CLE) lesions was determined in resected lung specimens, and the mean linear intercept (Lm) was measured on random histologic sections taken from regions without obvious emphysema. The exponential constant K was determined by fitting lung pressure-volume data obtained from the patient just prior to resection to the equation V = A − Be−KP. This allowed us to compare patients with little or no emphysema that had either normal (0.16 ± 0.03 SD; n = 12) or increased (0.27 ± 0.04 SD; n = 12) K to other patients that had severe emphysema but either normal (0.17 ± 0.01 SD; n = 10) or increased (0.25 ± 0.03 SD; n = 10) K. In subjects without emphysema, K was significantly related to Lm, suggesting that K is a measure of mean alveolar size. In the subjects with emphysema, a lower value for K was associated with more severe airway dysfunction and gas trapping. We conclude that K reflects airspace size except when airway closure subtracts the contribution of lung units from the deflation pressure-volume curve.