Quantitative analysis of high-resolution chest CT scan (QCT) is an established method for determining the severity and distribution of lung parenchymal destruction in patients with emphysema. Diffusing capacity of the lung for carbon monoxide (Dlco) is a traditional physiologic measure of emphysema severity and is probably influenced more by destruction of the alveolar capillary bed than by membrane diffusion per se. We reasoned that Dlco should correlate with tissue volume from QCT.
A total of 460 patients with upper-lobe-predominant emphysema were enrolled in the study. The mean (SD) of percent predicted values for FEV1, total lung capacity, and Dlco were 30.6% (8.0%), 129.5% (18.1%), and 6.7% (13.1%), respectively. QCT was performed using custom software; the relationship between Dlco and various metrics from QCT were evaluated using Pearson correlation coefficients.
On average, whole-body plethysmography volumes were higher by 841 mL compared with QCT-calculated total lung volume. However, there was a strong correlation between these measurements (r = 0.824, P < .0001). Dlco correlated with total lung volume (r = 0.314, P < .0001), total tissue volume (r = 0.498, P < .0001), and percentage of lung with low density (−950 Hounsfield units) (r = −0.337, P < .0001).
In patients with severe emphysema, Dlco correlates best with total tissue volume, supporting the hypothesis that pulmonary capillary blood volume is the main determinant of Dlco in the human lung. The relationships between Dlco and various anatomic metrics of lung parenchymal destruction from QCT inform our understanding of the relationship between structure and function of the human lung.