| Summary: | Oxygen pulse (O<sub>2</sub>P) is a function of stroke volume and cellular oxygen extraction and O<sub>2</sub>P curve pattern (O<sub>2</sub>PCP) can provide continuous measurements of O<sub>2</sub>P. However, measurements of these two components are difficult during incremental maximum exercise. As cardiac function is evaluated using ejection fraction (EF) according to the guidelines and EF can be obtained using first-pass radionuclide ventriculography, the aim of this study was to investigate associations of O<sub>2</sub>P%predicted and O<sub>2</sub>PCP with EF in patients with heart failure with reduced or mildly reduced ejection fraction (HFrEF/HFmrEF) and chronic obstructive pulmonary disease (COPD), and also in normal controls. This was a prospective observational cross-sectional study. Correlations of resting left ventricular EF, dynamic right and left ventricular EFs and outcomes with O<sub>2</sub>P% and O<sub>2</sub>PCP across the three participant groups were analyzed. A total of 237 male subjects were screened and 90 were enrolled (27 with HFrEF/HFmrEF, 30 with COPD and 33 normal controls). O<sub>2</sub>P% and the proportions of the three types of O<sub>2</sub>PCP were similar across the three groups. O<sub>2</sub>P% reflected dynamic right and left ventricular EFs in the control and HFrEF/HFmrEF groups, but did not reflect resting left ventricular EF in all participants. O<sub>2</sub>PCP did not reflect resting or dynamic ventricular EFs in any of the subjects. A decrease in O<sub>2</sub>PCP was significantly related to nonfatal cardiac events in the HFrEF/HFmrEF group (log rank test, <i>p</i> = 0.01), whereas O<sub>2</sub>P% and O<sub>2</sub>PCP did not predict severe acute exacerbations of COPD. The findings of this study may clarify the utility of O<sub>2</sub>P and O<sub>2</sub>PCP, and may contribute to the currently used interpretation algorithm and the strategy for managing patients, especially those with HFrEF/HFmrEF. (Trial registration number NCT05189301.)
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