Security cameras caught Dr. Andrey Votyakov arguing with the patient before aggressively hitting him, first on the head and then in the area of his heart.
The patient died soon after in intensive care.
Britain's mailonline news website reports the incident happened at a prestigious hosptial in Perm, where Dr Votyakov is chief of the anaesthesiology and intensive care department.
He was clearly unaware that his vicious attack was being filmed, and blamed "a hard day after a 24-hour shift" when the footage went viral on the internet.
"I got carried away, and I punched him several times at the presence of my colleagues," Dr Votyakov said.
Care given to the patient after the attack is also being investigated, the mailonline reported.
The alarm was raised by the chief public affairs officer at the Russian Interior Ministry when he saw the video on YouTube.
"As soon as I came into the room with my team he started to call me various derogative names. And I just got blown away by it," Dr Votyakov said."We had spent so much time with his very complicated case to help him recover and he said not a single word of gratitude."
How I wish we can have some videos of our PDRM doing some shitty stuff to some poor detainees.