Superior vena cava syndrome is an unusual complication of pacemakers and defibrillators. Basically, these devices are implanted through large veins in the chest that lead to the heart. For reasons that are unknown, in a very small percentage of patients, the pacemaker leads cause the formation of scar tissue on the inside of the veins, which can become narrowed to the point of obstructing blood flow. Surprisingly, other veins can enlarge and compensate by re-routing blood through them and back to the heart.
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