A heart attack produces distinct changes on an EKG, and the pattern depends on the type and severity of the blockage. The most recognizable sign is ST-segment elevation, where the line between the heartbeat’s spike and the next wave rises above the baseline. But heart attacks can also show ST depression, tall peaked T waves, inverted T waves, or new Q waves, each pointing to a different stage or type of damage.
The Normal EKG Baseline
To spot a heart attack, you need to know what normal looks like. Each heartbeat on an EKG traces a consistent pattern: a small P wave (the atria contracting), a tall QRS complex (the ventricles contracting), and a rounded T wave (the ventricles resetting). The flat line between the QRS complex and the T wave is the ST segment. In a healthy heart, this segment sits right at the baseline, neither elevated nor depressed. When blood flow to part of the heart muscle is cut off, the electrical activity in that area changes, and the ST segment is usually the first thing to shift.
STEMI: The Classic Heart Attack Pattern
An ST-elevation myocardial infarction, or STEMI, is the type most people picture when they think of a heart attack on an EKG. It means a coronary artery is completely blocked, and a section of heart muscle is losing blood supply right now. The hallmark is ST-segment elevation in at least two neighboring leads. The threshold for diagnosis is at least 2 mm of elevation in leads V1, V2, and V3, and at least 1 mm in all other leads, measured at the J point where the QRS complex ends and the ST segment begins.
This elevation creates a distinctive look. Instead of the ST segment sitting flat on the baseline, it curves upward and can merge with the T wave, sometimes forming a tombstone shape or a smooth dome. The pattern appears in leads that look at the damaged area of the heart, and it often appears alongside reciprocal changes: ST depression in the leads on the opposite side. For example, an inferior wall heart attack shows ST elevation in leads II, III, and aVF, and you’ll often see ST depression in leads I and aVL as a mirror image. These reciprocal changes can result from the electrical reflection of the injury or from actual reduced blood flow in other areas as blood gets diverted toward the damaged zone.
NSTEMI: Subtler but Still Dangerous
Not every heart attack raises the ST segment. A non-ST-elevation myocardial infarction, or NSTEMI, happens when a coronary artery is partially blocked or temporarily occluded. The EKG changes are less dramatic and harder to pin down. The most common findings are ST-segment depression (the segment dips below the baseline) and T-wave inversion (the T wave flips downside down instead of pointing upward).
These changes can be tricky because they’re not unique to heart attacks. Conditions like inflammation of the heart lining, thickened heart muscle, and stress-related heart dysfunction can produce similar-looking ST depression and T-wave inversions. That’s why an NSTEMI diagnosis relies on blood tests showing elevated troponin, a protein released when heart muscle cells are damaged, alongside the EKG findings and symptoms. New T-wave inversions that develop shortly after symptoms begin are considered a strong indicator of post-ischemic changes, meaning the heart muscle recently lost and then regained blood flow.
How the EKG Changes Over Time
A heart attack isn’t a single snapshot. The EKG evolves through predictable stages if the blockage isn’t treated. The earliest sign, appearing within minutes of a coronary artery shutting down, is hyperacute T waves. These are unusually tall, broad, and peaked T waves. Animal studies have shown they can develop within two minutes of blood flow being cut off. At this stage, the ST segment may still look relatively normal, so hyperacute T waves are easy to miss if you’re only watching for ST elevation.
Next comes the ST-segment elevation that defines a STEMI. As damage progresses, abnormal Q waves begin to appear, typically within the first nine hours. A pathological Q wave is deeper and wider than the small Q waves sometimes seen on a normal EKG, and it signals that a portion of heart muscle has died and is no longer generating electrical activity. The dead tissue essentially creates an electrical “window” that allows the EKG to pick up signals from the opposite wall of the heart, producing that deep Q wave.
After that, the T waves invert, flipping below the baseline in the affected leads. Eventually the ST segment returns to normal, but the Q waves and T-wave inversions can persist for weeks, months, or permanently. Old heart attacks often leave behind pathological Q waves as a lasting scar on the EKG, even when the ST segment has long since normalized.
Which Leads Point to Which Part of the Heart
A 12-lead EKG looks at the heart from 12 different angles, and the pattern of changes tells you which wall of the heart is affected and which artery is likely blocked. The leads are grouped by the region they face:
- Leads V1 and V2 look at the septum, the wall dividing the left and right ventricles. Changes here point to a blockage in the left coronary artery system.
- Leads V3 and V4 face the front (anterior) wall. An anterior heart attack, usually caused by a blockage in the left anterior descending artery, is one of the most dangerous because this artery supplies a large territory of muscle.
- Leads I, aVL, V5, and V6 look at the lateral wall. The circumflex artery typically supplies this region.
- Leads II, III, and aVF face the bottom (inferior) wall. Inferior heart attacks are most often caused by a blockage in the right coronary artery.
When ST elevation appears across multiple lead groups, it suggests a larger area of damage. An anterior STEMI with changes spanning V1 through V6 plus the lateral leads indicates widespread injury and a worse prognosis than changes confined to just two or three leads.
When Existing Conditions Mask the Pattern
Some people have EKGs that are already abnormal at baseline, making heart attack detection much harder. The most challenging scenario is a left bundle branch block (LBBB), a condition where the electrical signal to the left ventricle takes an abnormal path. LBBB causes wide QRS complexes and ST-segment changes that can mimic or completely obscure a heart attack pattern.
Specialized criteria exist for reading through the noise of an LBBB. The key findings that suggest a heart attack despite an existing LBBB include ST-segment changes that move in the same direction as the main QRS deflection (called concordant changes) of at least 1 mm, and ST changes moving in the opposite direction from the QRS in leads where the QRS voltage is very small. These criteria aren’t as sensitive as reading a clean EKG, which is one reason why patients with LBBB and chest pain are often treated urgently even when the EKG isn’t definitive.
Pacemaker rhythms create similar challenges because the paced beats produce wide, abnormal-looking QRS complexes that distort the ST segment. Prior heart attacks can also complicate interpretation, since old Q waves and persistent ST changes make it harder to identify new events layered on top.
What the EKG Can and Can’t Tell You
An EKG is fast, cheap, and available in virtually every emergency department and ambulance, which makes it the first-line tool for heart attack diagnosis. But it has real limitations. About 6% of STEMIs initially present with a normal or non-diagnostic EKG, particularly in the earliest minutes before the classic pattern develops. Serial EKGs taken 15 to 30 minutes apart can catch evolving changes that a single recording misses.
For NSTEMIs, the EKG alone is even less reliable. The diagnosis depends on combining EKG findings with troponin blood tests, which detect proteins leaking from injured heart cells. The current clinical definition of a heart attack requires a rise or fall in troponin above the 99th percentile of normal values, plus at least one supporting finding: symptoms, new EKG changes, new Q waves, or imaging showing a new area of damaged muscle. The EKG is one piece of a larger puzzle, not a standalone answer. But when ST elevation is clearly present in the right pattern, it’s enough to trigger emergency treatment without waiting for blood results.