Impaired lung diffusing capacity for nitric oxide and alveolar-capillary membrane conductance results in oxygen desaturation during exercise in patients with cystic fibrosis

Background: Exercise has been shown to be beneficial for patients with cystic fibrosis (CF), but for some CF patients there is a risk of desaturation, although the predicting factors are not conclusive or reliable. We sought to determine the relationship between the diffusion capacity of the lungs for nitric oxide and carbon monoxide (DLNO and DLCO) and the components of DLCO: alveolar-capillary membrane conductance (D_M), and pulmonary capillary blood volume (V_C) on peripheral oxygen saturation (SaO₂) at rest and during exercise in CF. Methods: 17 mild/moderate CF patients and 17 healthy subjects were recruited (age=26±7 vs. 23±8 years, ht=169±8 vs. 166±8 cm, wt=65±9 vs. 59±8 kg, BMI=23±3 vs. 22±3 kg/m², VO_2PEAK=101±36 vs. 55±25%pred., FEV₁=92±22 vs. 68±25%pred., for healthy and CF, respectively, mean±SD, VO_2PEAK and FEV₁ p<0.001). Subjects performed incremental cycle ergometry to exhaustion with continuous monitoring of SaO₂ and measures of DLNO, DLCO, D_M and V_C at each stage. Results: CF patients had a lower SaO₂ at rest and peak exercise (rest=98±1 vs. 96±1%, peak=97±2 vs. 93±5%, for healthy and CF, respectively, p<0.01). At rest, DLNO, DLCO, D_M were significantly lower in the CF group (p<0.01). The difference between groups was augmented with exercise (DLNO=117±4 vs. 73±3ml/min/mmHg; DLCO=34±8 vs. 23±8ml/min/mmHg; D_M=50±1 vs. 34±1, p<0.001, for healthy and CF respectively). Peak SaO₂ was related to resting DLNO in CF patients (r=0.65, p=0.003). Conclusions: These results suggest a limitation in exercise-mediated increases in membrane conductance in CF which may contribute to a drop in SaO₂ and that resting DLNO can account for a large portion of the variability in SaO₂.