Causes of breathing inefficiency during exercise in heart failure

Background: Patients with heart failure (HF) develop abnormal pulmonary gas exchange; specifically, they have abnormal ventilation relative to metabolic demand (ventilatory efficiency/minute ventilation in relation to carbon dioxide production [V_E/VCO₂]) during exercise. The purpose of this investigation was to examine the factors that underlie the abnormal breathing efficiency in this population. Methods and Results: Fourteen controls and 33 moderate-severe HF patients, ages 52 ± 12 and 54 ± 8 years, respectively, performed submaximal exercise (∼65% of maximum) on a cycle ergometer. Gas exchange and blood gas measurements were made at rest and during exercise. Submaximal exercise data were used to quantify the influence of hyperventilation (PaCO₂) and dead space ventilation (V_D) on V_E/VCO₂. The V_E/VCO₂ relationship was lower in controls (30 ± 4) than HF (45 ± 9, P < .01). This was the result of hyperventilation (lower PaCO₂) and higher V _D/V_T that contributed 40% and 47%, respectively, to the increased V_E/VCO₂ (P < .01). The elevated V _D/V_T in the HF patients was the result of a tachypneic breathing pattern (lower V_T, 1086 ± 366 versus 2003 ± 504 mL, P < .01) in the presence of a normal V_D (11.5 ± 4.0 versus 11.9 ± 5.7 L/min, P = .095). Conclusions: The abnormal ventilation in relation to metabolic demand in HF patients during exercise was due primarily to alterations in breathing pattern (reduced V_T) and excessive hyperventilation.