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Aims: In heart failure (HF), anaerobic threshold (AT) may be indeterminable but its value held a relevant prognostic role. AT is evaluated joining three methods: V-slope, ventilatory equivalent, and end-tidal methods. The possible non-concordance between the V-slope (met AT) and the other two methods (vent AT) has been highlighted in healthy individuals and named double threshold (DT)., Methods and Results: We reanalysed 1075 cardiopulmonary exercise tests of HF patients recruited in the Metabolic Exercise test data combined with Cardiac and Kidney Indexes (MECKI) score database. We identified DT in 43% of cases. Met AT precedes vent AT being met-ventΔVO 2 221 (interquartile range: 129-319) mL/min. Peak VO 2 , 1307 ± 485 vs. 1343 ± 446 mL/min (63 ± 17 vs. 63 ± 17 percentage of predicted), was similar between DT+ and DT- patients. Differently, DT+ showed a lower ventilatory vs. carbon dioxide production (VE/VCO 2 ) slope (29.6 ± 6.1 vs. 31.0 ± 6.3), a lower peak exercise end-tidal oxygen tension (PetO 2 ) 115.3 (111.5-118.9) vs. 116.4 (112.4-120.2) mmHg, and a higher carbon dioxide tension (PetCO 2 ) 34.2 (30.9-37.1) vs. 32.4 (28.7-35.5) mmHg. Vent AT showed a significant higher VO 2 , 957 ± 318 vs. 719 ± 252 mL/min, VCO 2 , 939 ± 319 vs. 627 ± 226 mL/min, ventilation, 31.0 ± 8.3 vs. 22.5 ± 6.3 L/min, respiratory exchange ratio, 0.98 ± 0.08 vs. 0.87 ± 0.07, PetO 2 , 108 (104-112) vs. 105 (101-109) mmHg, PetCO 2 , 37 (34-40) vs. 36 (33-39) mmHg, and VE/VO 2 ratio, 33.5 ± 6.7 vs. 32.6 ± 6.9, but lower VE/VCO 2 ratio, 33 (30-37) vs. 36 (32-41), compared with met AT. At 2 year survival by Kaplan-Meier analysis, even adjusted for confounders, DT resulted not associated with survival., Conclusions: Double threshold is frequently observed in HF patients. DT+ is associated to a decreased ventilatory response during exercise., (© 2022 The Authors. ESC Heart Failure published by John Wiley & Sons Ltd on behalf of European Society of Cardiology.)