Electrocardiogram (ECG)

An electrocardiogram (ECG or EKG) measures and records the electrical activity of the heart. The waves measured by the ECG correspond to the contraction and relaxation pattern of the different parts of the heart. Specific waves seen on an ECG are named with letters:P. The P wave is associated with the contractions of the atria (the two chambers in the heart that receive blood from outside).
QRS. The QRS is a series of waves associated with ventricular contractions. (The ventricles are the two major pumping chambers in the heart.)
T and U. These waves follow the ventricular contractionsDoctors use a term called the P-Q or P-R interval, which is the time taken for an electrical impulse to travel from the atria to the ventricle.
The most important wave patterns in diagnosing and determining treatment for a heart attack are called ST elevations and Q waves.
Elevated ST Segments: Heart Attack. Elevated ST segments are strong indicators of a heart attack in patients with symptoms and other indicators. They suggest that an artery to the heart is blocked and that the full thickness of the heart muscle is damaged. When this finding coincides with a heart attack, the condition is sometimes referred to as either a Q-wave myocardial infarction or a STEMI (ST-segment elevation myocardial infarction). STEMI heart attacks are very severe and usually have complete artery blockage. ST-elevations are strong indicators for aggressive treatments (thrombolytic drugs or angioplasty) to reopen blood vessels. (ST segment elevations do not always mean the patient has a heart attack. Also, some patients do not have elevated ST segments. Other factors are important in making a diagnosis.)
Non-Elevated ST Segments: Angina and Acute Coronary Syndrome. A depressed or horizontal ST wave suggests some blockage and the presence of heart disease, even if there is no angina present. It occurs in about half of patients with other signs of a heart event. This finding, however, is not very accurate, particularly in women, and can occur without heart problems. In such cases, laboratory tests are needed to determine the extent, if any, of heart damage. In general, one of the following conditions may be present:
Stable Angina (blood test results or other tests show no serious problems and chest pain resolves). Most patients with angina can go home. (Between 25 - 50% of people who have angina or silent ischemia have normal ECG readings.)
Acute Coronary Syndrome (ACS). This includes severe and sudden heart conditions that require aggressive treatment but have not developed into a full-blown heart attack. ACS, refers to either unstable angina or NSTEMI (non ST-segment elevation myocardial infarction) -- also referred to as non Q-wave myocardial infarction. Unstable angina is potentially serious, and chest pain is persistent, but blood tests do not show markers for heart attack. With NSTEMI, the blood tests suggest a developing heart attack, but most likely, injury in the arteries is less serious than with a full-blown heart attack