A comparison of pulmonary capillary blood volume and membrane diffusing capacity assessed via two pulmonary diffusing...

The combined measurement of pulmonary diffusing capacity for carbon monoxide (D _LCO) and nitric oxide (D _LNO) permits the interrogation of pulmonary capillary blood volume (V _c) and membrane diffusing capacity (D _m) from a single-breath hold, providing a less onerous means of evaluating pulmonary microvascular function than the classic Roughton-Forster multiple fractions of inspired oxygen approach (mF _iO2 -D _LCO). Despite its growing utility, the concordance between D _LCO, V _c and D _m derived from the D _LCO/D _LNO and mF _iO2 -D _LCO approaches remains unclear. Thus, we sought to evaluate the agreement between D _LCO, V _c and D _m measured via the single-breath D _LCO/D _LNO and the mF _iO2 -D _LCO approaches at baseline as well as during exercise and postural stress. We hypothesised that there would be strong agreement between the two approaches for D _LCO and V _c.

16 healthy young adults (5 female; age: 24±4 years) completed two experimental visits: 1) cycling at 30% and 60% of peak oxygen consumption (V'_O2peak), and 2) postural stress (supine-to-60°head-up tilt (HUT)). D _LCO, V _c and D _m were measured via the combined D _LCO/D _LNO (Hypair) and mF _IO2 -D _LCO approaches (randomised) at each stage.

During baseline and exercise, D _LCO demonstrated excellent agreement between measurement approaches (baseline intraclass correlation coefficient (ICC) 0.92 (95% CI 0.79-0.97); 30% V'_O2peak ICC 0.89 (95% CI 0.71-0.96); 60% V'_O2peak ICC 0.94 (95% CI 0.93-0.98), all p<0.001). V _c exhibited moderate-to-very good agreement (baseline ICC 0.61 (95% CI 0.17-0.85); 30% V'_O2peak ICC 0.77 (95% CI 0.46-0.91); 60% V'_O2peak ICC 0.51 (95% CI 0.01-0.81), all p≤0.022), while D _m had poor-to-moderate agreement (baseline ICC 0.19 (95% CI -0.32-0.62), p=0.228; 30% V'_O2peak ICC 0.43 (95% CI -0.08-0.77), p=0.046; 60% V'_O2peak ICC 0.51 (95% CI -0.01-0.82), p=0.026). Similar results were observed during postural stress.

These data indicate that D _LCO and V _c, but not D _m, derived from the D _LCO/D _LNO and mF _IO2 -D _LCO approach are comparable.