Cyanosis produced by superior vena caval stenosis.

Superior vena cava syndrome is a rare complication of pacemaker implantation. Narrowing and occlusion of the superior vena cava is often asymptomatic due to the formation of decompressive collateral pathways to the right atrium. We present a case in which an anomalous venous pathway allowed decompression of the systemic venous return into the left atrium, resulting in arterial desaturation, cyanosis, and fatigue.