Treadmill-Based Cardiopulmonary Assessment

A modified Astrand-Saltin incremental treadmill protocol was used to determine peak exercise capacity.^{13 (link)} Measures of ventilatory gas exchange were made by use of the Douglas bag technique. Gas fractions were analyzed by mass spectrometry (Marquette MGA 1100), and ventilatory volumes were measured with a Tissot spirometer. V̇O₂max was defined as the highest oxygen uptake measured from at least a 40-second Douglas bag. Cardiac output was measured with a modification of the acetylene rebreathing method, with acetylene as the soluble gas and helium as the insoluble gas.^{14 (link)} Measurement of cardiac output by acetylene rebreathing has been validated at rest and maximal exercise.^{14 (link)–17 (link)} This method assumes that cardiac output is equal to effective pulmonary blood flow to ventilated lung, which can be assessed by the rate of decay of acetylene concentration during rebreathing.^{15 (link)} Adequate mixing of the rebreathing gas in the lung was confirmed by a constant level of helium in all cases. Arterial-venous oxygen difference (a-vDo₂) was calculated by the Fick equation. The ventilatory threshold was determined by commercial software (First Breath, Marquette). The heart rate at the work rate that elicited the ventilatory threshold was defined as the heart rate at maximal steady state (MSS), which was generally equivalent to ≈85% to 90% of the maximal heart rate.