Background— CO ₂ vasodilates and O ₂ vasoconstricts the cerebral vascular bed; the opposite is true in the lungs. When the brain and lungs are connected exclusively in series, which feedback loop predominates is unknown. The circulation of the superior cavopulmonary connection (SCPC) provides a unique physiology to answer this question. Methods and Results— To determine cerebral and pulmonary blood flow and to establish the hierarchy of cerebral and pulmonary feedback mechanisms, 12 intubated, ventilated, single-ventricle patients in SCPC physiology (age 2.2±0.5 years) underwent magnetic resonance imaging velocity mapping of their jugular veins and aorta in room air, hypercarbia, and 100% O ₂ . Flows in these vessels and arterial blood gases were measured. With 22±6 torr CO ₂ (P co ₂ increased from 40 to 63 mm Hg, P <0.01), flow to the brain and lungs increased (1.5 to 2.7 L/min per m ² , P =0.0003), P o ₂ improved (48 to 60 mm Hg, P =0.0004), and cardiac index increased (4.3 to 5.4 L/min per m ² , P =0.0003). The increased cardiac index accounted for the increased cerebral and pulmonary blood flow ( R =0.73, P =0.02) and cerebral O ₂ transport increased by 80% ( P =0.0005) while preserving body O ₂ delivery. Hyperoxia did not change cerebral and pulmonary blood flow; P o ₂ increased 94% ( P =0.01). Conclusion— The cerebral CO ₂ feedback loop predominates over the pulmonary one when they directly compete with each other. CO ₂ has a major impact on flow distribution whereas O ₂ has little impact. Increased CO ₂ improves cerebral oxygenation in SCPC patients. This may provide a clue in determining neurological sequelae in SC physiology and may influence timing of Fontan completion.

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