Angina, characterized by chest pain or discomfort, indicates that the heart muscle is not receiving sufficient oxygen-rich blood, a condition known as myocardial ischemia. This often results from narrowed coronary arteries. An electrocardiogram (ECG) is a non-invasive test that records the heart’s electrical activity, providing a visual representation of its rhythm and electrical impulses. By analyzing these signals, an ECG can offer important clues about angina.
The Basics of Angina and ECG Interpretation
Angina arises when the heart muscle experiences a mismatch between its oxygen supply and demand, typically due to reduced blood flow. This ischemia is most commonly caused by atherosclerosis, a condition where fatty deposits narrow the coronary arteries. When the heart’s demand for oxygen increases, such as during physical exertion or stress, but the narrowed arteries cannot deliver enough blood, angina symptoms may occur.
The ECG records the heart’s electrical events through waves and segments. The P wave, QRS complex, and T wave represent atrial activation, ventricular activation, and ventricular recovery, respectively. The baseline of an ECG, known as the isoelectric line, represents periods of no electrical activity and serves as a reference point. The ST segment, between the end of the QRS complex and the beginning of the T wave, is particularly important for identifying ischemia. A normal ST segment typically appears flat and aligns with the isoelectric line.
Distinct ECG Signatures of Angina
Myocardial ischemia produces specific visual changes on an ECG. ST-segment depression is a common ECG sign of angina, appearing as the ST segment shifting below the isoelectric line. It can take various forms, including horizontal, downsloping, or less commonly, upsloping depression. Horizontal or downsloping ST depression of 0.5 mm or more in at least two adjacent leads is generally considered indicative of myocardial ischemia, often reflecting subendocardial ischemia where only the inner layer of the heart muscle is affected.
T-wave inversion is another ECG change associated with ischemia. In ischemia, T waves may appear inverted, pointing downwards. This occurs because ischemia alters the normal repolarization process of the heart muscle.
While less frequent in typical angina, ST-segment elevation can also be a sign of severe ischemia. This occurs when the ST segment rises above the isoelectric line. Such elevation often indicates transmural ischemia, where the entire thickness of the heart muscle wall is affected, usually due to a complete blockage of a coronary artery. Reciprocal changes may also be observed, where ST depression appears in ECG leads opposite to those showing ST elevation, suggesting widespread or extensive ischemia.
Different Types of Angina and Their ECG Appearance
The manifestation of angina on an ECG varies depending on the type. In stable angina, ECG changes typically occur during periods of increased myocardial oxygen demand, such as physical exertion or emotional stress. These changes, often ST depression or T-wave inversion, are usually transient and resolve with rest or medication. The ECG may appear normal when the individual is not experiencing symptoms.
Unstable angina represents a more serious condition, and its ECG changes can be more dynamic and persistent. Individuals with unstable angina may show new or worsening ST depression, T-wave inversion, or even transient ST elevation, which can occur at rest or with minimal exertion. These ECG findings suggest a greater degree of myocardial ischemia and indicate a heightened risk of a heart attack.
Prinzmetal angina, also known as variant angina, is distinct in its ECG presentation. This type of angina is caused by a spasm in the coronary arteries, which temporarily restricts blood flow. During an episode, the ECG typically shows transient ST-segment elevation, often occurring at rest or during sleep. Once the coronary artery spasm resolves, the ST segment usually returns to the baseline.
Other Conditions Mimicking Angina on ECG and Clinical Context
ECG changes indicative of angina are not exclusive to this condition. Various other non-ischemic conditions can produce similar ECG patterns, making a comprehensive clinical evaluation essential. These include pericarditis (inflammation of the sac surrounding the heart), early repolarization, and structural heart changes like left ventricular hypertrophy (where the heart muscle thickens).
Bundle branch blocks, electrolyte imbalances (such as high or low potassium levels), and certain medications (including digitalis) are also known to influence ECG readings and may mimic ischemic changes. An ECG is only one component of the diagnostic process. Medical professionals integrate ECG findings with a patient’s symptoms, medical history, physical examination, and other diagnostic tests, such as blood tests for cardiac enzymes or stress tests, to arrive at an accurate diagnosis.