Background: Ventilatory efficiency, the relationship between ventilation (VE) and carbon dioxide production (VCO2), predicts mortality in heart failure patients, but its determination has not been standardized. Additionally, it is unclear if the prognostic power of ventilatory efficiency is independent of exercise intensity. Therefore, we investigated the relative prognostic power of different measures of ventilatory efficiency calculated from maximal and sub-maximal exercise in patients with heart failure.
Methods and results: Heart failure patients (n = 355, 72% males, age 51 +/- 10 years) had follow-up for at least 5 years from an exercise test. There were 145 events (133 deaths and 12 emergent cardiac transplants). Ventilatory efficiency calculations were not equivalent. Of the different measures of ventilatory efficiency, the VE/VCO2 slope to peak exercise was the most significant predictor of mortality in a multivariable Cox model, including ejection fraction, systolic blood pressure, peak oxygen consumption (VO2), gender, etiology, and heart rate. A 5 U increment of the VE/VCO2 slope to peak exercise corresponded to a 9% increase in mortality risk. When tests were grouped by peak exercise intensity, by quartiles of peak respiratory exchange ratio, the VE/VCO2 slope to peak exercise was always a better predictor than peak VO2.
Conclusion: Peak and sub-maximal measures of ventilatory efficiency were not equivalent, and the VE/VCO2 slope to peak exercise was the best predictor of mortality in patients with heart failure. Thus, the prognostic power of ventilatory efficiency is enhanced when exercise extends beyond the ventilatory threshold and includes all of the available exercise data.