"I had to go and get the shock on my heart," the Napoli player told reporters ahead of Scotland's upcoming Nations League fixtures.
The 29-year-old also underwent a minor heart procedure in September, which forced him to miss the last international window.
McTominay said he has fully recovered and is delighted to be back with the national team.
"That's the most important thing in my life and it always will be, in terms of wanting to come back, prove myself again and make sure that I can go to another level," he said.
Scotland are third in Nations League Group B1 with four points from four matches, three behind Slovenia and eight adrift of leaders Switzerland. They host North Macedonia on November 13 before travelling to Switzerland three days later.