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Figure 4 The pulmonary vascular resistance index (PVRI) remained unchanged at arterial carbon dioxide tension (PaCO₂) 35, 45, 55, and 40 mm Hg. There was a significant decrease in superior vena cava pressure between PaCO₂ 55 and 40 mm Hg but no change in pulmonary vascular resistance as pulmonary blood flow decreased.