The ST Segment & T Wave

The ST segment runs from the J point, where the QRS ends, to the start of the T wave. Think of it as the plateau. In phase 2 of the ventricular action potential, every ventricular myocyte is depolarized at once. No net current flows, so the lead reads flat.

The T wave is ventricular repolarization (phase 3). That is the recovery back to resting membrane potential. Recovery is where the trouble shows up, so the ST segment and T wave react to injury, ischemia and metabolic change:

• Acute ischemia or injury: the most urgent cause of ST-T change
• Electrolytes: Potassium and calcium both reshape the T wave and the ST segment.
• Conduction and pacing: Bundle branch block and ventricular pacing change the order in which the ventricles repolarize.
• Drugs and structural disease: digoxin, pericarditis, and hypertrophy

A shifted segment is not automatically ischemic. Measure every shift from the same baseline, every time. Injury current shows as ST elevation in the leads facing the injured myocardium. Subendocardial ischemia shows as ST depression in the leads facing the non-injured wall.

1. Identify the J point in each lead as the end of the QRS, using a 12-lead with ECG paper at 25 mm/s.
2. Set the baseline at the TP segment, or the PR segment when the TP is not usable.
3. Measure the ST deviation at the J point. Some protocols also read at a fixed point after the J point (protocol-dependent).
4. Compare contiguous leads to define the territory and check reciprocal depression.
5. Check the shape: convex (tombstone), concave (smiley), horizontal or downsloping.
6. Compare with the prior ECG and repeat the tracing when symptoms continue.

ST elevation is a current of injury. Injured muscle on the epicardial side sits at a different voltage from normal tissue during the plateau. The fourth universal definition of MI sets millimeter criteria for STEMI. Those criteria pick out who needs emergent reperfusion. They are not perfectly sensitive or specific.

ECG Criteria

• New ST elevation at the J point in 2 contiguous leads: ≥1 mm in all leads except V2–V3
• V2–V3, men ≥40 years: ≥2 mm
• V2–V3, men <40 years: ≥2.5 mm
• V2–V3, women: ≥1.5 mm
• Reciprocal ST depression: in leads opposite the territory; it adds specificity but is not always present

Causes

• Ischemic: STEMI from coronary occlusion
• Non-ischemic: acute pericarditis, early repolarization, left ventricular hypertrophy, left bundle branch block, Brugada pattern, ventricular aneurysm, and takotsubo cardiomyopathy

Clinical Significance

• Regional, convex or straight ST elevation with reciprocal depression? Treat it as STEMI until proven otherwise.
• Diffuse, concave ST elevation with no reciprocal change points to a non-ischemic cause. Chest pain plus risk factors still means you exclude ischemia.

Elevation tells you which wall is at risk. Reciprocal depression shows the wall across from it. Localize the territory before the cath lab team asks.

• Inferior STEMI: Record V4R and V7–V9. You are checking for right ventricular and posterior involvement.
• Posterior STEMI: ≥0.5 mm of ST elevation in V7–V9 confirms the posterior wall. No sex or age modifier applies.
• Right ventricular infarction: ST elevation in V4R with an inferior STEMI is the sign. These patients depend on preload.

Not every occlusion meets the classic millimeter criteria. Some patterns point to one anyway. The 2022 ACC Expert Consensus Decision Pathway treats posterior STEMI, left bundle branch block or paced rhythm with Sgarbossa criteria, de Winter sign and hyperacute T waves as STEMI equivalents. A growing body of work on occlusion MI (OMI) says the STEMI criteria miss many occlusions. Inconclusive ECG and high suspicion? Repeat the tracings and add a posterior and right-sided lead set.

De Winter Sign

• Pattern: upsloping ST depression >1 mm at the J point in the precordial leads, with tall, prominent, symmetric T waves
• Significance: described in proximal LAD occlusion. It counts as a STEMI equivalent and can occur without ST elevation.
• Action: Activate cardiology or the cath lab right away, per protocol. Don't wait for ST elevation.

Hyperacute T Waves

• Pattern: broad, asymmetric or peaked T waves early in STEMI, often before ST elevation appears; the ECDP wording is broad, asymmetric, peaked
• Action: Repeat the ECG at short intervals. Treat it as a possible occlusion when the clinical picture agrees.

Posterior Pattern

• Standard ECG: horizontal ST depression in V1–V3, dominant R wave (R/S >1) in V2, and upright T waves in the anterior leads
• Confirmation: posterior leads V7–V9 with ST elevation ≥0.5 mm

Occlusion MI (OMI) and Non-Occlusion MI (NOMI)

• Concept: OMI is defined by the occlusion itself, not by millimeter thresholds. NOMI is injury without occlusion, and it is often diffuse.
• Bedside tools: 2026 reviews report higher sensitivity than STEMI criteria for some signs. Those signs don't yet replace guideline activation criteria.

ST depression usually means subendocardial ischemia. The inner myocardium has the highest oxygen demand and the least perfusion. So it is the first to suffer. Horizontal or downsloping depression in two or more contiguous leads (≥0.5 mm at the J point) suggests ischemia.

Morphology

• Horizontal: the most specific shape for ischemia when it appears in contiguous leads.
• Downsloping: also points to ischemia, most of all with a convex ST segment.
• Upsloping: the least specific. It is common with tachycardia and digoxin effect. Use context and serial ECGs.

Causes

• NSTE-ACS: subendocardial ischemia. It is the most important cause.
• Reciprocal change: the mirror image of ST elevation when an occlusion is present.
• Posterior MI: what you see as ST depression in V1–V3 is posterior ST elevation seen from the front.
• Digoxin effect: sagging ST depression with a reverse-tick shape
• Hypokalemia and LVH strain: Read the potassium and hypertrophy pages for the detail.
• Rate-related: demand ischemia or physiologic depression with tachycardia

ST Elevation in aVR With Diffuse ST Depression

• Pattern: ST elevation in aVR with multilead ST depression in I, II, V4–V6 and no contiguous STE elsewhere
• Significance: the 2022 ECDP files this under ischemia findings, not STEMI equivalents. It most often comes from diffuse subendocardial ischemia with left main or multivessel disease.
• Threshold: ST elevation in aVR, commonly ≥1 mm, with multilead ST depression
• Action: Take the high-risk NSTE pathway. Get urgent cardiology review and serial ECGs.

Wellens pattern is a T-wave signature of a critical proximal LAD stenosis. It shows up after recent angina, usually once the pain has settled and the ECG looks close to normal. Treat it as a high-risk NSTE finding. It is not an active occlusion. The original de Zwaan and Wellens series describes it.

ECG Criteria (2022 ECDP)

• Biphasic or deeply inverted, symmetric T waves in V2–V3 (may extend to V6)
• ST segment: isoelectric or minimally elevated (under 1 mm); troponin normal or minimally elevated
• Recent angina with pain resolved or minimal
• No pathologic Q waves in the same leads

Causes and Significance

• Cause: a critical proximal LAD stenosis. The T-wave change reflects myocardium that is reperfused but still ischemic.
• Risk: without urgent evaluation and revascularization, extensive anterior infarction is highly likely.
• Action: Get urgent cardiology review under the NSTE pathway. Don't discharge from the ED without that review.

In LBBB and ventricular paced rhythm, the ST-T changes run opposite the QRS. That is expected, and it is called discordant. Standard STEMI criteria do not apply. The modified Sgarbossa criteria were built to catch the concordant changes that suggest occlusion.

Smith-Modified Sgarbossa Criteria

• Concordant ST elevation: ≥1 mm in leads with a positive QRS
• Concordant ST depression: ≥1 mm in V1–V3
• Discordant ST elevation: ≥1 mm at the J point and ≥25% of the preceding S-wave depth

Causes and Significance

• New or presumed-new LBBB alone is not a STEMI equivalent. Apply the criteria to the tracing.
• Paced rhythm: the same criteria apply to ventricular pacing.
• Compare with a prior ECG when you have one. A previous LBBB changes how you read this tracing.

Acute pericarditis and early repolarization are the main look-alikes for ST elevation. Both can clear the millimeter thresholds. Use the table below to split them from STEMI at the bedside.

Early repolarization means a J-point elevation (Jp) of ≥0.1 mV with an end-QRS notch or slur. You don't need ST elevation to meet that definition. Features that raise the risk: a J-point elevation of 0.2 mV or more, inferior or inferolateral leads, and a horizontal or descending ST segment. Pericarditis is a clinical call, with the ECG as one of several features (2015 ESC guideline).

Other STEMI Mimics

• LVH: high voltage with discordant ST elevation in V1–V3
• Brugada pattern: coved ST elevation of 2 mm or more with a negative T wave in V1–V2
• Ventricular aneurysm: persistent ST elevation with Q waves that does not evolve
• Takotsubo: wide-territory ST change after stress; a wall-motion abnormality that crosses coronary territories on echo
• Hyperkalemia: peaked, narrow-based T waves and a widening QRS. It can mimic a de Winter pattern, so check potassium.
• SAH or stroke: diffuse deep T-wave inversion with a long QT and no coronary occlusion
• Tricyclic toxicity: wide QRS with a tall R wave in aVR (3 mm or more)

Artifact & Lead Misplacement

• Limb-lead reversal: it shifts the axis and can make lead I or aVR look changed.
• Precordial electrodes placed too high: they can fake ST elevation in V1–V2.
• Tremor, shivering or motion: they can fake ST deviation and VT or VF.
• Transport vibration: it can make the baseline unreadable. Stop and re-record.

Your ECG picks the pathway. STEMI or a STEMI equivalent means reperfusion. A high-risk NSTE pattern means urgent cardiology review. Every drug and threshold below runs through local protocols and online medical direction.

Doses vary by protocol; follow local protocols and medical direction.

Pediatrics

• Pattern: Repolarization changes shift with age and heart rate. Use age-based pediatric ECG references.
• Cause: In children, ST changes are more often non-ischemic (myocarditis, pericarditis, early repolarization). Ischemia is uncommon. When it is suspected, think about coronary anomalies and Kawasaki-related coronary disease.
• Action: Call pediatric cardiology or medical direction early for a new ST change with symptoms. For rate and BP norms, see the pediatric vital signs reference.
• Drugs: Pediatric dosing for nitrates, fibrinolytics and antiplatelet agents is not established. Use the weight-based dosing reference and specialist input.

Other Populations

• Older adults and women: often present atypically. ST depression may be the only finding.
• Pacemaker or ICD: paced rhythms call for the Sgarbossa criteria. Get a prior ECG to compare.
• Heart transplant: denervated hearts can show abnormal ST-T patterns and blunted symptoms. Compare with prior tracings.
• Pregnancy: ST-T changes can be normal in pregnancy. A true STEMI in pregnancy or the peripartum period is often spontaneous coronary artery dissection (SCAD). Fibrinolysis is generally avoided. It can extend the dissection, so PCI or conservative therapy is preferred. Bring in cardiology, obstetrics and the PCI center early. Follow medical direction for antithrombotics.

Quiet the stretcher and cut the motion before you record. In transport, a clean tracing is the only reliable input for a diagnosis.

1. Identify the J point and TP baseline in each lead before reading any shape.
2. Measure the ST deviation at the J point and note its shape: convex, concave, horizontal or downsloping.
3. Map the distribution to a territory, and check reciprocal leads.
4. Check the STEMI equivalents: de Winter sign, hyperacute T waves, posterior ST depression, Sgarbossa criteria in LBBB or paced rhythm.
5. Check the mimics: diffuse concave STE with PR depression (pericarditis), early repolarization, LVH, and artifact.
6. Decide the pathway: STEMI or equivalent to reperfusion; high-risk NSTE (STE in aVR with diffuse depression, Wellens) to urgent cardiology review.
7. Repeat and compare: serial ECGs and the prior tracing, and document each change with a time stamp.

Common Pitfalls

• Don't wait for classic ST elevation when a de Winter sign or posterior pattern is present.
• Don't label isolated V1–V3 ST depression as NSTEMI without posterior leads.
• Don't use STEMI thresholds in LBBB or a paced rhythm. Use the Sgarbossa criteria.
• Don't give nitrates to a hypotensive patient with inferior changes without checking V4R.
• Don't order an exercise stress test for a Wellens pattern.