In 1944 Blalock, with Thomas by his side, performed the first "blue baby" operation on Eileen Saxon, a 15-month-old baby. The operation was successful, although the baby died a few months later. After the innovative first completion of the surgery, Blalock became comfortable with the procedure and performed it on thousands of children, often with Thomas by his side. The new operation not only directly saved thousands of lives, it marked the start of the modern era of cardiac surgery, as it was the first successful surgery on the human heart of the modern medical era.