Relationship between extent of pulmonary emphysema by high-resolution computed tomography and lung elastic recoil in patients with chronic obstructive pulmonary disease

We investigated the relationship between the extent of pulmonary emphysema, assessed by quantitative high-resolution computed tomography (HRCT), and l ung mechanics in 24 patients with chronic obstructive pulmonary disease (CO PD). The extent of emphysema was quantified as the relative lung area with CT numbers < -950 Hounsfield Units (HU). Patients with COPD had severe airf low obstruction (FEV1 35 +/- 15% pred) and severe reduction of CO diffusion constant (Dco/V-A 32 +/- 19% pred). Maximal static elastic recoil pressure (Pst(max)) averaged 54 +/- 24% predicted, and the exponential constant K o f pressure-volume curves was 258 +/- 116% predicted. Relative lung area wit h CT numbers < -950 HU averaged 21 +/- 11% (range 1 to 38%). It showed a hi ghly significant negative correlation with Dco/VA (r = -0.84, p < 0.0001), a weak correlation with FEV1% predicted, and no correlation with either Pst (max) or constant K. A significant relationship was found between the natur al logarithm of K and the full width at half maximum of the frequency distr ibution of CT numbers, taken as an index of the heterogeneity of lung densi ty (r = 0.68, p < 0.0005). We conclude that currently used methods of asses sing the extent of emphysema by HRCT closely reflect the reduction of CO di ffusion constant, but cannot predict the elastic properties of the lung tis sue.