Complex sentence: In 1933, Fernos-Isern resigned as health commissioner and went to New York, here he completed his residency in cardiology at Columbia University and so became the "first" Puerto Rican cardiologist.
Split into simpler sentences: In 1933, Fernos-Isern resigned from his position as health commissioner and went to New York. In New York he completed his residency in cardiology at Columbia University, thus becoming the "first" Puerto Rican cardiologist.