Limitation in the use of peak exercise oxygen consumption in the selection of candidates for heart transplantation

Purpose: Peak exercise oxygen consumption (VO₂) is an indicator of prognosis in advanced heart failure, and is currently a major criterion in the selection of candidates for heart transplantation. Available studies suggest that patients with peak VO₂ < 14 ml/min/Kg have high mortality if not transplanted and derive significant functional benefit from transplantation. Since patients may terminate symptom-limited exercise tests for a variety of reasons, peak VO₂ does not necessarily reflect maximum VO₂, leading to the possibility of inappropriate selection for transplantation. Therefore we investigated the proportion of transplant candidates referred for exercise testing considered to have achieved maximal studies. Methods: 55 patients with heart failure aged 51±9 (SD) years underwent maximum symptom-limited exercise tests on a cycle ergometer utilizing a Jones Stage I incremental protocol. Tests were considered maximum if subjects achieved peak heart rate (HR) > 85% predicted ("cardiocirculatory limitation") or peak minute ventilation (VE) > 85% predicted ("ventilatory limitation"), and achieved an anaerobic threshold (AT) by non-invasive measures. Results: 7 tests were terminated because of chest pain, ST-segment abnormalities or ventricular arrhythmias. Of the remaining 48 studies, the reasons for stopping exercise were leg fatigue in 52%, dyspnea in 16%, and both in 23%. 16 of the 48 patients (33%) had peak VO₂ < 14 ml/min/Kg. In 8 of these 16 patients, both peak HR and VE were < 85% predicted. Of these 8 without apparent HR or ventilatory limitation, none had oxygen desaturation below 90% or fall in BP, 2 were in atrial fibrillation and only 3 had evidence that an AT was achieved. Conclusions: In half of our patients with peak VO₂ < 14 ml/min/Kg, there was no objective evidence that the study was maximal. Use of peak VO₂ may misclassify an appreciable proportion of candidates. Clinical Implications: Clinical exercise studies indicating low peak VO₂ must be interpreted in the context of whether or not a defined objective exercise limitation is evident in order to avoid selecting inappropriate patients for transplantation.