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

Aims Peak exercise oxygen uptake (VO2) 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 VO2 < 50% and >= 50% pred, Groups 1 and 2). To account for observed differences, data regarding VO2, CO, stroke volume (SV), and artero-venous O-2 content difference [Delta C(a-v)O-2] were adjusted by age, gender, and body mass index. A multiple regression analysis was performed to predict peak VO2 from mid-exercise cardiopulmonary exercise test and CO parameters among HF patients. Rest VO2 was lower in HF compared with healthy subjects; meanwhile, Group 1 patients had the lowest CO and highest Delta C(a-v)O-2. At mid-exercise, Group 1 patients achieved a lower VO2, 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 < 0.001). At mid-exercise, Delta C(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 VO2, CO, and SV than Group 2 and healthy subjects. Delta C(a-v)O-2 was the highest in Group 2. At multivariate analysis, a model comprising mid-exercise VO2, carbon dioxide production (VCO2), CO, haemoglobin, and weight predicted peak VO2, P < 0.001. Mid-exercise VO2 and CO, haemoglobin, and weight added statistically significantly to prediction, P < 0.050. Conclusions Mid-exercise VO2 and CO portend peak exercise values and identify severe HF patients. Their evaluation could be clinically useful.