What happens if you're under the knife for a new surgery and your surgeon has a heart attack?

Heart surgery: The surgeon having the heart attack is in the hospital so he/she is in a good place to get treated with dispatch. The assisting surgeon in the or with the patient can take over and the patient(not surgeon)will continue to be cared for. If the assistant needs another lead surgeon, they'll send for one.

Backup: Conceivably, prior to incision a procedure could be stopped. Once started, another surgeon of similar type would be called to take over. This could be from same practice or a covering practice. Most hospitals require that we have "call/emergency coverage".