Unstable Angina Pectoris

Inherent in the concept of stable angina pectoris is the principle that myocardial ischemia is caused by increased cardiac demand for oxygen. Attacks of chest pain are predictable since each attack has a definable cause, such as exercise or excitement. This predictability is missing in attacks of unstable angina, where oxygen supply to the heart muscle fluctuates irrespective of myocardial oxygen demands.

Unstable angina pectoris, in the broadest sense of the term, includes many situations in which chest pain does not follow a chronic, repetitive pattern. It occupies an intermediate position between stable angina and myocardial infarction and is classified as an acute coronary syndrome. As such, we can distinguish the following patterns:

rapid increase in frequency and severity of attacks of angina pectoris ("crescendo angina")

onset of angina provoked by a low level of activity

a combination of exercise-induced attacks of angina and unprovoked attacks at rest

occasional recurrent attacks of angina at night

a high concentration of unprovoked anginal attacks at rest (usually several attacks a day)

prolonged attacks of angina at rest (15 minutes or longer)

Current medical opinion places the cause of unstable angina close to that of myocardial infarction, namely an acute change inside a major coronary artery, most commonly rupture of an atherosclerotic plaque. Though the course of unstable angina is unpredictable,

the six patterns are listed here in order from least to most precarious. All cases of unstable angina should be considered for intensive hospital treatment, which is mandatory in the more serious varieties. Prolonged attacks of chest pain at rest are usually handled as suspected myocardial infarctions. Since the difference between unstable angina and myocardial infarction depends on the reversibility of ischemia, sometimes the two can be distinguished only after noting a series of changes in electrocardiographic readings and cardiac enzymes. Rapidly recurring attacks of angina at rest that are of shorter duration often are precursors of myocardial infarction. In some patients suffering occasional recurrent attacks of nocturnal angina, unstable angina may be caused by periodic spasm of a coronary artery (analogous to attacks of migraine caused by spasm of cerebral arteries). Coronary-artery spasms have distinctive features. An electrocardiogram taken during such an attack shows an elevated S-T segment instead of the depressed one typical of ischemia at rest. This angina, called variant angina or Prinzmetal's angina, displays electrocardiographic patterns identical with the early changes of myocardial infarction, but the patterns return to normal promptly after the attack. Many such patients have only a minor degree of coronary-artery disease; sometimes in fact the coronary arteries are entirely normal. The prognosis for such patients is much more favorable than for those with other varieties of unstable angina, and their response to medical treatment is usually excellent; in these cases intervention therapy does not help and is contraindicated.

Management of unstable angina is aimed at prompt control of symptoms for patients both in the hospital and at home. Hospitalized patients usually require continuous intravenous administration of drugs. In addition to antianginal drugs, anticoagulants (to prevent clot formation) and thrombolytics (to dissolve clots) may be used. Interventional therapy is often considered, requiring coronary angiograms. Both angiography and interventional treatment may have to be performed as emergency procedures if symptoms cannot be controlled by medical therapy.

As an acute coronary syndrome, unstable angina has an unpredictable outcome: it may progress to myocardial infarction, it may settle into stable angina, or it may cease altogether or change into stable angina. If interventional therapy is performed, either angioplasty

or bypass operation may restore the patient to an asymptomatic state.