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Abstract   Aims  Peak exercise oxygen uptake (VO 2 ) and cardiac output (CO) are strong prognostic indexes in heart failure (HF) but unrelated to real‐life physical activity, which is associated to submaximal effort.    Methods and results  We analysed maximal cardiopulmonary exercise test with rest, mid‐exercise, and peak exercise non‐invasive CO measurements (inert gas rebreathing) of 231 HF patients and 265 healthy volunteers. HF patients were grouped according to exercise capacity (peak VO 2  &lt; 50% and ≥50% pred, Groups 1 and 2). To account for observed differences, data regarding VO 2 , CO, stroke volume (SV), and artero‐venous O 2  content difference [ΔC(a‐v)O 2 ] were adjusted by age, gender, and body mass index. A multiple regression analysis was performed to predict peak VO 2  from mid‐exercise cardiopulmonary exercise test and CO parameters among HF patients. Rest VO 2  was lower in HF compared with healthy subjects; meanwhile, Group 1 patients had the lowest CO and highest ΔC(a‐v)O 2 . At mid‐exercise, Group 1 patients achieved a lower VO 2 , CO, and SV [0.69 (interquartile range 0.57–0.80) L/min; 5.59 (4.83–6.67) L/min; 62 (51–73) mL] than Group 2 [0.94 (0.83–1.1) L/min; 7.6 (6.56–9.01) L/min; 77 (66–92) mL] and healthy subjects [1.15 (0.93–1.30) L/min; 9.33 (8.07–10.81) L/min; 87 (77–102) mL]. Rest to mid‐exercise SV increase was lower in Group 1 than Group 2 ( P  = 0.001) and healthy subjects ( P  &lt; 0.001). At mid‐exercise, ΔC(a‐v)O 2  was higher in Group 2 [13.6 (11.8–15.4) mL/100 mL] vs. healthy patients [11.6 (10.4–13.2) mL/100 mL] ( P  = 0.002) but not different from Group 1 [13.6 (12.0–14.9) mL/100 mL]. At peak exercise, Group 1 patients achieved a lower VO 2 , CO, and SV than Group 2 and healthy subjects. ΔC(a‐v)O 2  was the highest in Group 2. At multivariate analysis, a model comprising mid‐exercise VO 2 , carbon dioxide production (VCO 2 ), CO, haemoglobin, and weight predicted peak VO 2 ,  P  &lt; 0.001. Mid‐exercise VO 2  and CO, haemoglobin, and weight added statistically significantly to the prediction,  P  &lt; 0.050.    Conclusions  Mid‐exercise VO 2  and CO portend peak exercise values and identify severe HF patients. Their evaluation could be clinically useful.

Cardiac output changes during exercise in heart failure patients: focus on mid‐exercise