72 yr old man on life support 15bpm after major cardiac event. what is the chance for survival?

Very complicated: Prognosis is a very complicated thing. The whole clinical picture, including medical history, clinical course, etc is required. The answer to this question will ultimately be based on a discussion with the patient's doctor, indeed his whole team of care providers, with his wishes (advanced directives) kept squarely in mind.

Uncertain: If physicians feel patient might survive, a temporary pacemaker should be placed to keep the heart rate at or above 50/minute.

Sounds critical: He sounds critically ill. May not make it. My condolences.