Quantifying leg muscle deoxygenation during incremental cycling in hypoxemic patients with fibrotic interstitial lung disease

BackgroundHypoxaemia and cardiocirculatory abnormalities may impair muscle oxygen (O-2) delivery relative to O-2 requirements thereby increasing the rate of O-2 extraction during incremental exercise in fibrotic interstitial lung disease (f-ILD). Using changes in deoxyhaemoglobin concentration ([HHb]) by near-infrared spectroscopy (NIRS) as a proxy of O-2 extraction, we investigated whether a simplified (double-linear) approach, previously tested in heart failure, would provide useful estimates of muscle deoxygenation in f-ILD. MethodsA total of 25 patients (23 men, 72 +/- 8 years; 20 with idiopathic pulmonary fibrosis, lung diffusing capacity for carbon monoxide = 44 +/- 11% predicted) and 12 age- and sex-matched healthy controls performed incremental cycling to symptom limitation. Changes in vastus lateralis [HHb] assessed by NIRS were analysed in relation to work rate (WR) and O-2 uptake throughout the exercise. ResultsPatients showed lower exercise capacity than controls (e.g., peak WR = 67 +/- 18% vs. 105 +/- 20% predicted, respectively; p < 0.001). The [HHb] response profile was typically S-shaped, presenting three distinct phases. Exacerbated muscle deoxygenation in patients versus controls was evidenced by: (i) a steeper mid-exercise [HHb]-WR slope (0.30 +/- 0.22 vs. 0.11 +/- 0.08 mu mol/W; p = 0.008) (Phase 2), and (ii) a larger late-exercise increase in [HHb] (p = 0.002) (Phase 3). Steeper [HHb]-WR slope was associated with lower peak WR (r = -0.70) and greater leg discomfort (r = 0.77; p < 0.001) in f-ILD. ConclusionThis practical approach to interpreting [HHb] during incremental exercise might prove useful to determine the severity of muscle deoxygenation and the potential effects of interventions thereof in hypoxemic patients with f-ILD.