Idiopathic pulmonary fibrosis: new insights on functional characteristics at diagnosis

The lung function of patients with idiopathic pulmonary fibrosis (IPF) has not been characterized in detail. objective: To characterize the heterogeneous physiological abnormalities that exist in patients with IPF during their initial clinical evaluation.

Lung function tests from 93 patients, performed within six months of the initial diagnosis of IPF, were obtained from a referral pulmonary function laboratory at a tertiary care hospital in Canada. A restrictive pattern was defined as total lung capacity (TLC) <95th percentile of predicted value. Patients with obstructive lung disease, lung cancer, emphysema and other restrictive lung diseases were excluded.

On diagnosis, 73% of patients with IPF had a restrictive pattern, with a mean TLC of 72% of predicted. Mean forced vital capacity (FVC) was 71% and 44% of patients had an FVC <95th percentile. Mean diffusing capacity for carbon monoxide (DLCO) was 60% and DLCO⁄alveolar volume (VA) 92% of predicted. Increased severity of restriction - based on TLC - was associated with lower DLCO (74% of predicted in mild restriction and 39% of predicted in severe restriction) and higher forced expiratory volume in 1 s (FEV1)⁄FVC ratio (82% of predicted in mild restriction and 90% of predicted in severe restriction) but not with age (76 years in mild restriction and 69 years in severe restriction). Regardless of severity of restriction, the average DLCO⁄VA (≥86% of predicted) remained within normal limits.

One in four patients with IPF had normal TLC and more than one-half had a normal FVC during initial evaluation. As the severity of the restriction increased, FEV1⁄FVC increased, DLCO decreased but DLCO⁄VA remained normal.