Mr McBride, 47, was found dead in a hotel room in the Pakistani city of Lahore in March.
Police said they could find no sign of foul play and medical examiners said he died of natural causes. Dr Saeed Malik, head of the forensics department at Mayo Hospital, Lahore, said Mr McBride’s heart showed signs of damage.
Investigators had concluded that cause of death was a heart attack, he said.
“Some of his coronary blood vessels were already blocked and some were narrow,” he said.
Dr Malik said areas of fibrosis suggested Mr McBride had suffered earlier heart attacks perhaps without realising it. He added that he had asked for more details of drugs tests.
“The chemical examiner has also found alcohol and tranquilliser,” he said.
Derek Ogg, QC, a close friend of Mr McBride, said he was shocked to hear of the heart problems, and believes the lawyer was also unaware of them.