Influence of resting lung diffusion on exercise capacity in patients with COPD

Background: Lung diffusing capacity for carbon monoxide (DLCO) gives an overall assessment of functional lung surface area for gas exchange and can be assessed using various methods. DLCO is an important factor in exercise intolerance in patients with chronic obstructive pulmonary disease (COPD). We investigated if the intra-breath (IBDLCO) method may give a more sensitive measure of available gas exchange surface area than the more typical single breath (SBDLCO) method and if COPD subjects with the largest resting DLCO relative to pulmonary blood flow (Qc) would have a more preserved exercise capacity. Methods: Informed consent, hemoglobin, spirometry, SBDLCO, IBDLCO, and Qc during IBDLCO were performed in moderate to severe COPD patients, followed by progressive cycle ergometry to exhaustion with measures of oxygen saturation (SaO₂) and expired gases. Results: Thirty two subjects (47% female, age 66±9yrs., BMI 30.4±6.3kg/m², smoking hx 35±29 pkyrs, 2.3±0.8 on the 0-4 GOLD classification scale) participated. The majority used multiple inhaled medications and 20% were on oral steroids. Averages were: FEV₁/FVC 58±10%Pred, peak VO₂ 11.4±3.1ml/kg/min, and IBDLCO 72% of the SBDLCO (r=0.88, SB vs IB methods). Using univariate regression, both the SB and IBDLCO (% predicted but not absolute) were predictive of VO₂peak in ml/kg/min; SBDLCO/Qc (r=0.63, p<0.001) was the best predictor of VO₂peak; maximal expiratory flows over the mid to lower lung volumes were the most significantly predictive spirometric measure (r=0.49, p<0.01). However, in multivariate models only BMI added additional predictive value to the SBDLCO/Qc for predicting aerobic capacity (r=0.73). Adjusting for current smoking status and gender did not significantly change the primary results. Conclusion: In patients with moderate to severe COPD, preservation of lung gas exchange surface area as assessed using the resting SBDLCO/Qc appears to be a better predictor of exercise capacity than more classic measures of lung mechanics.