Examples of ventilatory efficiency slope (minute ventilation (V′E)/carbon dioxide production (V′CO2) slope) in a healthy subject (green line) and a patient with chronic heart failure (CHF) presenting with a peak oxygen uptake (V′O2) between 14 and 18 mL·kg−... Examples of ventilatory efficiency slope (minute ventilation (V′E)/carbon dioxide production (V′CO2) slope) in a healthy subject (green line) and a patient with chronic heart failure (CHF) presenting with a peak oxygen uptake (V′O2) between 14 and 18 mL·kg−1 (blue line) and a CHF patient presenting with a peak V′O2 <14 mL·kg−1 (red line). Bruno-Pierre Dubé et al. Eur Respir Rev 2016;25:317-332 ©2016 by European Respiratory Society