Atherosclerosis - narrowing of the arteries due to deposits of fatty material - is the most common reason, and believed to be behind this, Mr Yeltsin's second attack in four months.
Unmonitored and untreated, the condition can lead to a heart attack or stroke. Mr Yeltsin has other risk factors for heart disease: he is overweight, is believed to have high blood pressure, is alleged to drink heavily and has a stressful job.
However, a leading Russian doctor has ruled out any immediate danger to the President's life.
Vladimir Samarin, a cardiologist at the State Scientific Centre for Preventative Medicine in Moscow, said last night: "If I was talking to my patient I would tell him two attacks in four months was not bad. I would also give an optimistic prognosis - that is, if other attacks have not been concealed, of course."
Dr Samarin added: "As an expert, I would not be surprised if Yeltsin had an ischaemic attack after such a heavy and stressful trip to the United States."
Treatment of ischaemia includes vasodilator drugs and, in the most severe cases, coronary angioplasty to clear the heart arteries, or a by-pass operation.