Noninvasive Estimation of Pulmonary Vascular Resistance by Stroke Index Measurement With an Inert Gas Rebreathing Technique

¹ From the Section of Pulmonary and Critical Care Medicine, Norwalk Hospital, Norwalk, Conn
² From the Department of Thoracic Medicine, The National Heart and Lung Institute and Royal Brompton Heart and Lung Hospital, London, England

Patients with pulmonary hypertension (PH) have a cardiac limitation characterized by a decreased resting stroke index (SI) and an inability to augment stroke volume with exercise. We tested the hypothesis that a noninvasive estimation of SI, either at rest or with exercise, could be used to identify the presence and severity of PH. We used the inert gas rebreathing technique for measuring cardiac output and SI in nine subjects with PH and seven control subjects without pulmonary vascular disease. Noninvasive measurement of SI was compared with invasive measurement of pulmonary artery pressure (PAP), pulmonary vascular resistance index (PVRI), and SI. Inert gas estimation of cardiac output correlated well with invasive measurements (r=0.94). All PH subjects had a depressed resting SI while all control subjects had a normal resting SI. An inverse correlation between the SI measured by inert gas technique and mean PAP was seen with both rest (r=0.86) and exercise (r=0.79). Because the resting SI differentiated subjects with PH from controls and correlated with disease severity, exercise measurements were not required. Multiple serial measurements performed in two PH subjects while receiving prostacyclin infusion produced a high level of correlation between the inert gas SI and mean pulmonary artery pressure. We conclude that inert gas measurement of SI may serve a useful role in the diagnosis and management of patients with PH.