Senator Mike Duffy is taking a medical leave from the Red Chamber because of unstable angina, his office announced Thursday.

Mr. Duffy outlined his condition in a letter to the Speaker of the Senate Thursday morning. He said he sent the letter to explain his absence during Wednesday's Throne Speech.

The former Conservative is one of three sitting senators who are currently under investigation by the RCMP after external auditors found problems with their expense claims. A fourth, former Liberal Mac Harb, quit the Senate in August.

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Mr. Duffy wrote in the letter that he was hospitalized in August because of unstable angina. He was told this week that the disease has progressed, he said, and his doctor suggested a leave of absence from work to "avoid further stress on my system."

During open heart surgery in 2006, Mr. Duffy said doctors found three blockages but were unable to repair two of them. Because of complications, doctors fixed the main problem but did not repair the other vessels, he wrote.

Mr. Duffy received about $90,000 from Prime Minister Stephen Harper's former chief of staff, Nigel Wright, to repay expense claims that had been called into question. Mr. Wright resigned shortly after news of the payment became public.