Purpose: Ventilatory efficiency, as measured by the slope of the relationship between minute ventilation and carbon dioxide production ((.)VE/(.)VCO2 slope) during cardiopulmonary exercise (CPX) testing, is an important prognostic measure in patients with heart failure (HF). An abnormal slope is linked to the skeletal muscle metaboreflex. In addition, skeletal muscle endurance is reduced in patients with HF. However, the relationship between (.)VE/(.)VCO2 slope and skeletal muscle endurance is not known. This investigation tests the hypothesis that reduced knee extensor muscle endurance is inversely related to an elevated (.)VE/(.)VCO2 slope during CPX testing in patients with HF and that these variables are not related in normal subjects.
Methods: Patients with HF (n = 32) and 6 age-matched normal subjects performed CPX testing and isokinetic dynamometry to determine the (.)VE/(.)VCO2 slope and knee extensor muscle endurance, respectively.
Results: The (.)VE/(.)VCO2 slope and leg muscle endurance percentage were significantly related in patients with HF (r = -0.68, P < .001), but not in normal subjects (r = -0.11, P = .83).
Discussion: Based on these data, abnormalities of skeletal muscle endurance of patients with HF partially account for (r = 0.46; standard error estimate = 6.6) abnormalities of ventilatory efficiency, as measured by the (.)VE/(.)VCO2 slope. This finding helps explain, in part, the factors that influence an established prognostic indicator, elevated (.)VE/(.)VCO2 slope. Future research is needed to determine whether the relationship between skeletal muscle dysfunction and ventilatory efficiency is directly mediated through the skeletal muscle ergoreflex.