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Aims: Heart failure (HF) has been defined classically as a condition in which the heart is unable to deliver sufficient oxygen to match the needs of the metabolizing tissues. Surprisingly, this definition has never been validated. The goal of this study was to determine the prevalence of elevated lactate levels in a cohort of patients with advanced heart failure., Methods and Results: We retrospectively analysed the arterio-venous oxygen difference (A-V O 2 ), haemodynamics, and plasma lactate levels in stage D heart failure patients who were being evaluated for a left ventricular assist device (LVAD). We identified 359 patients with a right heart catheterization (RHC) performed prior to LVAD implantation. Plasma lactate was available for 96 patients. RHC showed that 93% of the patients had an A-V O 2 above the upper limit of normal (>5 mL/100 mL). Among patients with measured lactate levels, the prevalence of elevated lactate (>2.1 mmol/L) was 25% (95% confidence interval 16.7-34.9). The A-V O 2 was widened in all patients with elevated lactate, but plasma lactate did not correlate with A-V O 2 (r = 0.02) and only 27% of patients with increased A-V O 2 had elevated plasma lactate., Conclusions: Lactate levels were normal in ∼75% of the patients with advanced heart failure and a widened A-V O 2 , suggesting that the cardiac output was sufficient to meet the metabolic needs of the peripheral metabolizing tissues. Given that ∼4% of heart failure patients are in NYHA class IV, these findings suggest that the classic definition of heart failure pertains to ∼1% of patients with HF., (© 2016 The Authors. European Journal of Heart Failure © 2016 European Society of Cardiology.)