An electrocardiogram (ECG or EKG) is a medical test that records the heart’s electrical activity. It involves placing small electrodes on the skin to detect the tiny electrical impulses generated by the heart as it beats. A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow to a part of the heart muscle is severely reduced or completely blocked, typically due to a clot in a coronary artery. This lack of blood supply causes damage or death to the heart tissue.
The ECG’s Role in Detecting Heart Attacks
The electrocardiogram serves as a crucial and rapid diagnostic tool for identifying a myocardial infarction. It works by translating the heart’s electrical signals into a visual wave pattern, which healthcare providers can interpret to assess cardiac function. This non-invasive procedure involves attaching electrodes to the chest, arms, and legs, allowing the machine to capture the heart’s electrical impulses.
An ECG can quickly reveal irregularities in these electrical signals, which are often indicative of underlying heart conditions, including a heart attack. Its speed and accessibility make it invaluable, particularly in emergency settings where early diagnosis is paramount. Within minutes, an ECG can provide insights that help determine the need for immediate medical interventions to restore blood flow to the heart.
Recognizing a Healthy Heart on ECG
A typical ECG displays a sequence of waves, each representing a specific electrical event within the heart’s cycle. The P wave, for instance, reflects the electrical activation of the heart’s upper chambers, the atria, as they contract.
Following the P wave is the QRS complex, which represents the electrical activation of the lower, larger chambers, the ventricles, as they contract to pump blood. This complex is typically a sharp, spiked waveform. Finally, the T wave signifies the electrical recovery or repolarization of the ventricles, as they relax and prepare for the next beat. In a healthy heart, the ST segment, the flat line between the QRS complex and the T wave, typically appears as a straight, level line.
Specific ECG Signs of a Heart Attack
A heart attack significantly alters the heart’s electrical activity, leading to distinct changes on the ECG. One of the most definitive signs is ST-segment elevation, which indicates acute injury to the heart muscle. This elevation occurs when the ST segment, normally a flat line, rises above the baseline, often appearing convex or arched upwards. A particularly severe form, known as “tombstone” ST elevation, features the ST segment merging with the T wave and rising higher than the preceding R wave, resembling a tombstone. This pattern is associated with extensive and rapid heart muscle damage.
Another important sign is the appearance of pathological Q waves. While small Q waves can be normal, pathological Q waves are typically deeper and wider, often indicating an area of dead heart tissue, or infarction. These Q waves are not an early sign of a heart attack; they generally take several hours to days to develop and, once present, often persist indefinitely, marking a past heart attack.
T-wave inversion, where the T wave appears downward (negative) instead of its usual upright position, can also be a sign of myocardial ischemia, meaning reduced blood flow to the heart muscle. Ischemic T-wave inversions are often symmetric, contrasting with the normal asymmetric T wave. While T-wave inversion can signify ischemia, it is not always specific to a heart attack and can also occur in other conditions. The presence and pattern of these ECG changes provide crucial clues about the extent and location of heart muscle involvement.
Distinguishing Types of Heart Attack on ECG
The primary distinction is often made between ST-elevation myocardial infarction (STEMI) and non-ST-elevation myocardial infarction (NSTEMI). Both are types of heart attacks, but their ECG presentations differ significantly.
STEMI is characterized by significant ST-segment elevation in at least two contiguous leads, indicating a complete and sustained blockage of a coronary artery. This type of heart attack typically involves extensive heart muscle damage and requires immediate interventions to restore blood flow. The leads showing ST elevation can also help identify the specific area of the heart affected, such as anterior or inferior regions.
In contrast, NSTEMI usually presents without significant ST-segment elevation. Instead, ECG findings in NSTEMI may include ST-segment depression, T-wave inversion, or no specific changes at all, despite clinical evidence of heart muscle injury. NSTEMI typically results from a partial or temporary blockage of a coronary artery. While the ECG changes in NSTEMI are often less pronounced than in STEMI, both conditions involve heart muscle damage and require prompt medical attention.