Cor triatriatum is a rare birth defect where the left upper heart chamber is split in two by a thin membrane. Blood must squeeze through a small hole, so symptoms start early and get louder as the hole shrinks.

Fast breathing is the first clue. Babies breathe more quickly than normal, especially during feeding or crying.

Poor weight gain follows. Infants tire at the bottle, fall asleep mid-feed, and don’t add ounces like their peers.

Sweating is common. The forehead and upper lip bead with sweat, even when the room is cool.

Heart murmur is loud. A harsh, washing-machine sound is heard even without a stethoscope.

Frequent chest infections pop up. The extra wet lung tissue makes a good landing spot for viruses.

Sudden deeper blue spells or fainting mean the hole is kinking—an alarm that blood flow is critically low.