Objective Chronic heart failure is characterised by reduced exercise tolerance. We assessed the effects of different fractions of inspired oxygen (FiO₂) on exercise capacity using cycle ergometry to see if there is a dose–response relationship between FiO₂ and exercise performance.

Methods This was a single-centre, randomised, single-blinded, cross-over study. Thirty-one patients with chronic heart failure undertook three maximal incremental exercise tests. For each test, a different FiO₂ was used: room air (20.9%), 28% or 40%. The patient had to breathe in via a venturi mask allowing the investigator to control the FiO₂ and maintain the patient's blinding. The three tests were carried out in random order with a minimum of 4 days’ rest between any two tests.

Results Exercise time increased from (mean±standard deviations) 501±24.9 s on room air to 525±25.1 s (p=0.042) and 536±24.2 (p<0.001) seconds, with FiO₂ of 28% and 40%, respectively. Maximal metabolic equivalents were 3.47±0.16 on room air and 3.67±0.16 (p=0.002) and 3.70±0.15 (p<0.001) on 28% and 40% oxygen, respectively. Maximal workload was 78.4±4.5 W on room air and 82.6±4.3 (p=0.021) and 84.2±4.2 (p=0.005) on 28% and 40% oxygen, respectively. Increasing FiO₂ resulted in higher mean oxygen saturations during exercise. The mean heart rate during exercise was lower with FiO₂ of 28% with no further drop at 40%. Changing FiO₂ had no effect on blood pressure.

Conclusions Increasing FiO₂ to 28% or 40% acutely improves exercise capacity in patients with chronic heart failure.

Trial registration number Eudract number: 2014-003380-38; Results.