Background: Changes in body position are associated with changes in functional residual capacity, elastic recoil pressure, and total lung capacity (TLC). Exponential analysis over the entire range of the lung inflation-expiratory pressure-volume (P-V) curve has not been studied using the esophageal balloon technique. Objective: A twin exponential fitting model was applied to the expiratory and inspiratory limbs of lung P-V data in healthy young adults in seated and supine postures using a modified exponential analysis. Methods: P-V curves were recorded between residual volume and TLC using the esophageal balloon technique in seated and supine positions in 10 healthy, nonsmoking young adults. Data points were obtained in both postures with the balloon placed at 5, 10, and 15 cm above the cardia. A modified “double exponential” model was used to fit the data and transform it into a function, enabling its use as a linear regression technique. Results: Key findings included: a) curvilinearity of the P-V curve was greater during deflation than during inflation at volumes ≥ 51% TLC in both postures, particularly with esophageal pressure recorded at the 5 cm and 15 cm esophageal levels, b) the position of the inflation curve was shifted to the right at volumes ≥ 51% TLC in both postures regardless of the esophageal level, but not at volumes < 51% TLC, and c) at all esophageal levels and body positions, correspondence of recorded data points with respect to theoretically computed P-V points based on the twin exponential fit was excellent, with mean r2 ranging between 96.9% and 99.7%. Conclusions: Using a modified inflation-deflation double exponential model permits accurate extrapolation of the P-V curve over a data range normalized according to TLC. Physiologically meaningful variables derived from the curve-fitting model better characterize the curve over its full range.
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