buy a return ticket and send a photocopy of that along with a doctor's note.. the note should clearly mention the diagnosis and the fact that your mom would love to see you at this time.
Or you could just go and then apply for a PRTD while there. Take your PR renewal information with you when you go to submit to them. Just my opinion, but if your mom had a heart attack and they aren't scheduling the bypass for over a month, IRCC might not see it as urgent, given the time frame for the surgery. If they can wait that long it may not seem really urgent. Just thinking like a bureauocrat.