Brain Versus Lung: Hierarchy of Feedback Loops in Single-Ventricle Patients With Superior Cavopulmonary Connection

Abstract

Background— CO₂ vasodilates and O₂ 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% O₂. Flows in these vessels and arterial blood gases were measured. With 22±6 torr CO₂ (Pco₂ increased from 40 to 63 mm Hg, P2, P=0.0003), Po₂ improved (48 to 60 mm Hg, P=0.0004), and cardiac index increased (4.3 to 5.4 L/min per m², P=0.0003). The increased cardiac index accounted for the increased cerebral and pulmonary blood flow (R=0.73, P=0.02) and cerebral O₂ transport increased by 80% (P=0.0005) while preserving body O₂ delivery. Hyperoxia did not change cerebral and pulmonary blood flow; Po₂ increased 94% (P=0.01). Conclusion— The cerebral CO₂ feedback loop predominates over the pulmonary one when they directly compete with each other. CO₂ has a major impact on flow distribution whereas O₂ has little impact. Increased CO₂ 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.