Patients with mild COPD without significant hypoxemia have preserved resting cardiac output, RV end-diastolic pressure, and right atrial pressure.6 Exercise produces an increase in cardiac output proportional to increased oxygen consumption,35 which is contrary to the exercise limitation being a function of RV pump failure. The RV stroke work index and the RV end-diastolic pressure remain normal in most patients with mild COPD but may rise in response to exercise with the attendant rise in mPAP.35,36 The increase in exercising RV end-diastolic pressure may be a manifestation of other factors, such as RV hypertrophy and diastolic dysfunction,37 increased blood volume, and changes in intrathoracic pressure. Most patients with COPD-associated PH have preserved RV contractility if studied during periods of clinical stability,38 and studies have demonstrated RV systolic failure only among patients in the acutely decompensated state.11,28,38-42 Vizza et al13 reported low values of RV ejection fraction, measured by radionuclide ventriculography, in 59% of patients with advanced COPD being evaluated for lung transplantation. However, these values typically were in the low-normal range, as opposed to values in patients with pulmonary arterial hypertension, and the cardiac index typically was preserved. Early studies of autopsy in COPD demonstrated anatomic evidence of RV hypertrophy in up to two-thirds of patients with chronic bronchitis43 and one-third of patients with emphysema.44 A study that used MRI in patients with COPD45 found that concentric RV hypertrophy was the earliest sign of RV pressure overload and did not alter RV systolic function. Hypoxia may worsen RV relaxation in humans by producing myocyte hypoxia and impaired calcium transport, and another study suggested a correlation between RV hypertrophy and hypoxemia in patients with COPD.46 It may be argued that in patients with COPD, because of the mild elevations in afterload and the slow disease progression, the right ventricle has a chance to adapt with hypertrophy, and RV systolic failure may not occur in the absence of comorbidities when patients are in a long-term stable state.