Background: Peak oxygen consumption (peak Vo₂) is well accepted as a measure of cardiopulmonary fitness and has proven prognostic capabilities in patients with and without heart disease, including those with heart failure. However, it is effort dependent and potentially of less value in the morbidly obese. We sought to evaluate the utility of the minute ventilation/carbon dioxide production slope (V_E/Vco₂ slope) as an alternative measure to classify functional limitations, breathing economy, and prognosis in this population, as it is not limited by exercise effort.

Methods: We evaluated peak Vo₂ and related cardiorespiratory and hemodynamic responses in 76 patients (mean +/- SD age = 44.3 +/- 10.8 years, 69.7% female) with morbid obesity (mean +/- SD body mass index [BMI] = 49.4 +/- 7.0 kg/m²) referred for diagnostic exercise testing with simultaneous metabolic measurements.

Results: Forty-three (57%) and 33 (43%) patients achieved a peak respiratory exchange ratio (RER) <= 1.10 and < 1.10, respectively. For the entire cohort (n = 76), mean peak Vo₂ and V_E/Vco₂ slope were 17.0 +/- 3.7 mL kg^-1 min^-1 and 27.8 +/- 4.0, respectively. The ventilatory derived anaerobic threshold occurred at 76% and 66% of the peak Vo₂ values for the cohorts who achieved RERs < 1.10 and <= 1.10, respectively. Peak Vo₂ was correlated with BMI, r = -0.45, P < .0001, while V_E/Vco₂ slope was not, r = -0.04, P = .73. In addition, there was a linear trend for declining mean peak Vo₂ (P = .001) and not for V_E/Vco₂ slope (P =.59) with increasing BMI quintile. Although only 13 of the 76 morbidly obese patients (17%) were taking [beta]-blockers, peak heart rates were generally well-below age predicted values, suggesting chronotropic impairment as a contributor to their reduced cardiorespiratory fitness.

Conclusions: The V_E/Vco₂ slope is an effort-independent measure of dynamic cardiorespiratory function that is also independent of BMI. Because nearly half of our morbidly obese population (33/76 [43%]) did not achieve an RER <= 1.10, the V_E/Vco₂ slope may serve as a valuable parameter when evaluating functional status, respiratory gas exchange, and prognosis in this escalating patient population.