Impact of differential right-to-left shunting on systemic perfusion in pulmonary arterial hypertension

Objectives: This study aimed at identifying the ideal right-to-left shunt-fraction to improve cardiac output (CO) and systemic perfusion in pulmonary arterial hypertension (PHT). Background: Atrial septostomy (AS) has been a high-risk therapeutic option for symptomatic drug-refractory patients with PHT. Results have been unpredictable due to limited knowledge of the optimal shunt-quantity. Methods: In nine dogs, an 8-mm shunt-prosthesis was inserted between the superior vena cava (SVC) and the left atrium. With pulmonary artery (PA) banding, mean (6SEM) systolic right ventricular pressure increased from 37 ± 1 mm Hg at baseline to 44 ± 1 mm Hg (moderate PHT, P 5 0.005) and 50 ± 2 mm Hg (severe PHT, P < 0.001). Shunt-flow was adjusted by total (forcing all flow through the shunt) or partial occlusion of the SVC and partial or total clamping of the shunt. Caval-, shunt-, and aortic-flow were measured by ultrasonic flow-probes. Blood gases were drawn from the aortic root and PA. Results: At severe PHT, a shunt-flow of 11 ± 1% of CO (253 ± 90 mL/min) increased CO significantly by 25% (1.8 ± 0.1 to 2.4 ± 0.2 L/min, P = 0.005) causing an increase of systemic oxygen delivery index (DO2I) by 23% (309 ± 23 to 399 ± 32 mL/min/m ², P = 0.035). Arterial O₂- saturation did not change significantly until a shunt-flow of 18 ± 2% was exceeded, causing a drop from 96 ± 1% to 84 ± 4% (P = 0.013). At moderate PHT, CO or DO₂I did not improve significantly at any shunt-flow. Conclusions: In severe PHT, a shuntflow of 11% of CO represented the ideal shunt-fraction. Augmentation of CO compensated for declined O₂-saturation due to right-to-left shunting and improved DO₂I. In moderate PHT, AS is less promising.