Effects of body position on exercise capacity and pulmonary vascular pressure-flow relationships

There has been revival of interest in exercise testing of the pulmonary circulation for the diagnosis of pulmonary vascular disease, but there still is uncertainty about body position and the most relevant measurements. Doppler echocardiography pulmonary hemodynamic measurements were performed at progressively increased workloads in 26 healthy adult volunteers in supine, semirecumbent, and upright positions that were randomly assigned at 24-h intervals. Mean pulmonary artery pressure (mPAP) was estimated from the maximum tricuspid regurgitation jet velocity. Cardiac output was calculated from the left ventricular outflow velocity-time integral. Pulmonary vascular distensibility α-index, the percent change of vessel diameter per millimeter mercury of mPAP, was calculated from multipoint mPAP-cardiac output plots. Body position did not affect maximum oxygen uptake (Vo_2max), maximum respiratory exchange ratio, ventilatory equivalent for carbon dioxide, or slope of mPAP-cardiac output relationships, which was on average of 1.5 ± 0.4 mmHg·l^-1·min^-1 Maximum mPAP, cardiac output, and total pulmonary vascular resistance were, respectively, 34 ± 4 mmHg, 18 ± 3 l/min, and 1.9 ± 0.3 Wood units. However, the semirecumbent position was associated with a 10% decrease in maximum workload. Furthermore, cardiac output-workload or cardiac output-Vo₂ relationships were nonlinear and variable. These results suggest that body position does not affect maximum exercise testing of the pulmonary circulation when results are expressed as mPAP-cardiac output or maximum total pulmonary vascular resistance. Maximum workload is decreased in semirecumbent compared with upright exercise. Workload or Vo₂ cannot reliably be used as surrogates for cardiac output.