A heart attack produces distinct changes on an ECG that vary depending on the type, location, and timing of the event. The most recognizable sign is ST-segment elevation, where a normally flat portion of the ECG tracing rises above its baseline. But heart attacks can also show up as subtle depressions, inverted waves, or new abnormalities that develop over minutes to hours. Understanding these patterns helps explain what doctors are looking for when they read those squiggly lines.
How a Normal ECG Tracing Works
Each heartbeat produces a characteristic wave pattern on the ECG. The P wave represents the upper chambers contracting. The tall, spiky QRS complex shows the main pumping chambers firing. After the QRS, there’s a brief flat stretch called the ST segment, followed by a rounded bump called the T wave, which represents the heart resetting electrically before the next beat. In a healthy heart, the ST segment sits right at the baseline, and the T wave curves gently upward. A heart attack disrupts blood flow to part of the heart muscle, and that disruption changes the electrical signals in predictable ways.
The Classic STEMI Pattern
The most dramatic and time-sensitive ECG finding is ST-segment elevation, the hallmark of a STEMI (ST-elevation myocardial infarction). Instead of sitting flat at the baseline, the ST segment pushes noticeably upward, creating what looks like a hill or dome between the QRS spike and the T wave. This elevation signals that a coronary artery is completely blocked and heart muscle is actively dying.
The threshold for a significant elevation depends on which leads are affected and who the patient is. In leads V2 and V3, the cutoffs from the 2018 Fourth Universal Definition of Myocardial Infarction are: 2.5 mm or more for men under 40, 2 mm or more for men 40 and older, and 1.5 mm or more for women of any age. In all other leads, 1 mm of elevation is considered significant. These millimeter measurements refer to the tiny grid squares on ECG paper, where each small square equals 1 mm.
How the Pattern Changes Over Time
A heart attack doesn’t produce a single static picture on the ECG. The tracing evolves through a sequence of changes, sometimes rapidly. The earliest sign, appearing within minutes of a coronary artery becoming blocked, is the hyperacute T wave. These T waves become unusually tall, broad-based, and symmetrical compared to their normal appearance. They’re most visible in the chest leads (V2 through V4) and are easiest to spot when a previous ECG is available for comparison. Hyperacute T waves are short-lived, quickly giving way to the more obvious ST-segment elevation.
After ST elevation develops, the next change is the appearance of pathological Q waves. These are deep, wide dips at the very start of the QRS complex that typically show up within the first 9 hours. Q waves indicate that a section of heart muscle has died and is no longer generating electrical activity. The ST segment then gradually returns toward the baseline, and the T waves flip upside down (T-wave inversion). In many cases, the Q waves persist permanently, serving as a lasting ECG fingerprint of the heart attack even years later.
NSTEMI: A Heart Attack Without ST Elevation
Not every heart attack pushes the ST segment upward. In an NSTEMI (non-ST-elevation myocardial infarction), the artery is severely narrowed but not completely blocked, or the blockage is temporary. The ECG changes are more subtle. Instead of rising, the ST segment drops below the baseline, a pattern called ST depression. The T wave may also flip upside down, curving downward like a bowl instead of its normal upward hill shape.
These findings can be present in just a few leads or may be widespread. Because the changes are less dramatic, an NSTEMI sometimes produces a near-normal-looking ECG, which is why doctors also rely on blood tests measuring troponin (a protein released by damaged heart cells) to confirm the diagnosis.
Which Leads Point to Which Part of the Heart
An ECG uses 12 leads that view the heart from different angles, and the pattern of which leads show abnormalities tells doctors exactly where the damage is occurring and which artery is likely blocked.
- Anterior wall (front of the heart): Changes in leads V1 through V4 point to a blockage in the left anterior descending artery (LAD), the largest and most critical coronary artery.
- Inferior wall (bottom of the heart): ST elevation in leads II, III, and aVF, typically from a blockage in the right coronary artery.
- Lateral wall (left side): Changes in leads I, aVL, V5, and V6, usually caused by a blockage in the left circumflex artery, which accounts for roughly 15% to 20% of all heart attacks.
- Posterior wall (back of the heart): This region doesn’t face any of the standard 12 leads directly, so it shows up as a mirror image. Instead of ST elevation, you see horizontal ST depression in leads V1 through V3, along with unusually tall R waves. The combination of flat ST depression, tall R waves, and upright T waves in those leads has a positive predictive value for posterior heart attack approaching 95%.
Large heart attacks often affect more than one region. An anterolateral STEMI, for example, shows ST elevation across both the anterior and lateral leads and typically signals a blockage high up in the LAD. An inferolateral pattern, with changes in both the inferior and lateral leads, usually points to a proximal left circumflex occlusion.
Reciprocal Changes
One of the most useful clues on an ECG during a heart attack is reciprocal changes: ST depression in leads that face the opposite side of the heart from the injury. If leads II, III, and aVF show ST elevation (inferior wall), you’ll often see ST depression in the anterior leads (V1 through V4), and vice versa. These mirror-image changes increase confidence that the ST elevation is a true heart attack rather than something else. They also help pinpoint the exact location of the blockage.
Patterns That Look Like a Heart Attack but Aren’t
ST elevation doesn’t always mean a heart attack. Pericarditis, an inflammation of the sac surrounding the heart, can produce ST elevation that looks alarmingly similar. The key differences are in the details. In pericarditis, the ST elevation tends to be concave (scooped upward like a smile) rather than convex, and it appears in many leads at once rather than following the territory of a single artery. The most distinguishing feature is PR-segment depression, where the short flat line just before the QRS dips below the baseline. This appears in about 82% of pericarditis cases. Another clue is Spodick’s sign, a subtle downward slope in the baseline between heartbeats, which is not seen in heart attacks.
Tricky Scenarios: Bundle Branch Blocks
Some people have a pre-existing electrical conduction abnormality called a left bundle branch block (LBBB) that distorts the entire ECG tracing, making it extremely difficult to spot a heart attack underneath. The Sgarbossa criteria were developed specifically for this situation. They assign points based on three findings: ST elevation of 1 mm or more that goes in the same direction as the QRS complex (5 points), ST depression of 1 mm or more in leads V1 through V3 (3 points), and ST elevation of 5 mm or more going in the opposite direction of the QRS (2 points). A score of 3 or more suggests an acute heart attack is occurring despite the confusing background pattern.
Warning Patterns Before a Full Heart Attack
Some ECG patterns signal a critically narrowed artery that hasn’t yet caused a full heart attack. Wellens syndrome is the most important of these. It shows up as deeply inverted, symmetrical T waves in leads V2 and V3 (the most common form, seen in about 75% of cases) or biphasic T waves that start positive and dip negative in those same leads (about 25% of cases). This pattern is highly specific for a severe blockage in the LAD. It typically appears when the patient is pain-free, between episodes of chest discomfort, and indicates that a major anterior heart attack could be imminent without intervention.