Non-invasive measurement of stroke volume during exercise in heart failure patients

The objective of the present study was to determine the variability of the arterio–venous O₂ concentration difference [C(a–v)O₂] at anaerobic threshold and at peak oxygen uptake (VO₂) during a progressively increasing cycle ergometer exercise test, with the purpose of assessing the possible error in estimating stroke volume from measurements of VO₂ alone. We sampled mixed venous and systemic arterial blood every 1 min during a progressively increasing cycle ergometer exercise test and measured, in each blood sample, haemoglobin concentration and blood gas data. Ventilation, VO₂ and CO₂ uptake were also measured continuously. We studied 40 patients with normal haemoglobin concentrations and with stable heart failure due to ischaemic or idiopathic cardiomyopathy. Mean values (±S.D.) for C(a–v)O₂ were 7.8±2.6, 13.0±2.4 and 15.0±2.7 ml/100 ml at rest, anaerobic threshold and peak VO₂ respectively. The patients with heart failure were divided into classes according to their peak VO₂. Classes A, B and C contained patients with peak VO₂ values of > 20, 15–20 and 10–15 ml·min^-1·kg^-1 respectively. At anaerobic threshold, C(a–v)O₂ was 12.3±1.3, 13.1±2.7 and 13.5±2.6 ml/100 ml for classes A, B and C respectively (class A significantly different from classes B and C; P < 0.05). At peak exercise C(a–v)O₂ was 13.6±1.4, 15.6±2.5 and 15.4±3.2 ml/100 ml for classes A, B and C respectively (class A significantly different from classes B and C; P < 0.05). Stroke volume was estimated for each subject using the mean values of the measured C(a–v)O₂ in each functional class and individual values of VO₂ and heart rate using the Fick formulation. The average difference between the stroke volume estimated from mean C(a–v)O₂ and that obtained using the patient's actual C(a–v)O₂ value was 9.2±9.7, 1.0±8.8 and -0.2±6.1 ml at anaerobic threshold, and -1.9±11.3, 0.9±10.0 and -2.3±8.5 ml at peak exercise, in classes A, B and C respectively. Among the various classes, the most precise estimation of stroke volume was observed for class C patients. We conclude that stroke volume during exercise can be estimated with the accuracy needed for most purposes from measurement of VO₂ at the anaerobic threshold and at peak exercise, and from population-estimated mean values for C(a–v)O₂ in heart failure patients.