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Medical ECG Analysis

ECG ST-Segment Analysis Simulator

The ST segment of an ECG is the diagnostic window onto myocardial ischemia and infarction. Adjust the ST deviation, lead, sex, age, heart rate and J-point offset to see the 4th UDMI 2018 STEMI / NSTEMI classification, the corrected QTc and an acuity score update in real time, so you can build intuition for severity.

Parameters
ST deviation
mm
Deviation from baseline at J point +60-80 ms. Positive = elevation, negative = depression
Lead under analysis
STEMI threshold depends on the lead (anteroseptal is stricter)
Heart rate HR
BPM
QRS duration
ms
>120 ms suggests a bundle branch block (STEMI rule becomes unreliable)
J-point offset
ms
Where the ST is measured, in milliseconds after the J point
Age group
Young men get a higher V2-V3 threshold (early repolarisation is common)
Sex
Women use a 1.5 mm threshold in V2-V3
Results
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ST deviation (mm)
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STEMI threshold (mm)
—
Classification
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RR interval (ms)
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QTc estimate (ms)
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Acuity score
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ECG trace — P-QRS-ST-T with J point

Green = TP baseline, yellow = J point, red = ST measurement (J+60-80 ms). Magnitude and sign of the ST deviation update live.

12-lead ECG schematic
STEMI thresholds by lead (current sex / age)
Theory & Key Formulas

$$\Delta ST = V_{ST\,segment} - V_{TP\,baseline},\quad \text{STEMI iff} \quad \Delta ST \geq T_{lead,sex,age}$$

ΔST is the ST deviation (electrode potential at J point + 60-80 ms minus TP baseline), in mm. Tlead,sex,age is the sex-, age- and lead-specific STEMI threshold from the 4th UDMI 2018.

$$QTc = \frac{QT}{\sqrt{RR}} \approx \frac{470}{\sqrt{HR/60}}\ \text{ms}, \qquad RR = \frac{60000}{HR}\ \text{ms}$$

Bazett-corrected QT and RR interval, with HR in BPM and RR in ms. QTc > 450 ms (men) or > 460 ms (women) is considered prolonged.

$$\text{Acuity} = 10\,|\Delta ST| + 5\,[HR\gt 100] + 10\,[QRS\gt 120]$$

Educational acuity score that combines absolute ST deviation with tachycardia and wide-QRS penalties.

ECG ST-segment analysis — diagnosing ischemia and infarction

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You always hear "ST is up — this is a STEMI!" in medical dramas. What are they actually looking at?
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Good question. One ECG beat goes P wave → QRS complex → T wave. The point at the end of QRS is called the "J point", and the flat stretch from there to the start of the T wave is the ST segment. It is the brief moment when the heart muscle has finished depolarising and is getting ready to repolarise — normally it sits at the same level as the TP baseline. When it sits above the baseline, it is a sign that the full thickness of the myocardium is starving for oxygen, and that is the textbook STEMI pattern.
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So is the rule just "more than 1 mm = STEMI"? When I push the ST deviation to 1 mm on the left, the verdict still does not change.
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That is exactly the point. The threshold depends on the lead, the sex and the age. In the 4th UDMI 2018, a man 40 years or older needs 2.0 mm in V2-V3 or 1.0 mm in any other lead. Below 40, the V2-V3 cutoff goes up to 2.5 mm — healthy young men frequently show "early repolarisation" as a normal variant. Women, in contrast, use 1.5 mm in V2-V3. Switch the lead to V2 and the sex to female on the left and the verdict will become a lot more sensitive.
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I have also heard about ST being below baseline. Is that not a STEMI?
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Great question. ST depression tells you the inner layer (subendocardium) is ischemic. The coronary blood flow runs from the outside in, so when supply drops the innermost layer becomes hypoxic first — that is the NSTEMI or unstable angina pattern. While STEMI is a "open the artery within 90 minutes" emergency, NSTEMI is confirmed by a troponin rise and treated with angiography within 24-72 hours. This tool labels any ST below -0.5 mm as ST depression for that reason.
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Why does widening the QRS trigger a warning? That is independent of heart rate, right?
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When QRS exceeds 120 ms, a bundle branch block (LBBB or RBBB) is likely. Bundle branch block causes secondary ST-T changes that can mimic ischemic elevation, so the pure UDMI threshold rule starts to fail. If you suspect AMI in a patient with LBBB, you switch to the Sgarbossa criteria instead. This tool therefore penalises QRS > 120 ms in the acuity score as a "the simple ST rule may not apply here" flag.
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Last question — is ST analysis really used in ICUs and ambulances? How far has AI come?
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Yes — 12-lead ECG is built into AEDs, defibrillators, ICU monitors, 24-hour Holter recorders, and even wearables like the Apple Watch. On the AI side, FDA-cleared products like AliveCor KardiaAI and deep-learning CDx tools that detect AF, LBBB and PVCs with sensitivity over 95 percent are appearing every year. But the final call on "ST elevation = STEMI" is still made by a cardiologist who looks at morphology (concave vs convex), clinical course and troponins. As of 2026, AI is the triage accelerator, not the decision maker.

Frequently Asked Questions

The 4th Universal Definition of Myocardial Infarction (UDMI 2018) sets sex-, age- and lead-specific thresholds. For men 40 years or older: at least 2.0 mm of ST elevation in V2-V3, and at least 1.0 mm in all other leads. For men under 40: at least 2.5 mm in V2-V3. For women: at least 1.5 mm in V2-V3 and 1.0 mm in other leads. This simulator picks the correct threshold automatically and compares it with the ST deviation measured 60 to 80 ms after the J point.
ST elevation reflects transmural (full-wall) ischemia and triggers a STEMI pathway with primary PCI within 24 hours. ST depression of 0.5 mm or more reflects subendocardial ischemia, which is the hallmark of NSTEMI or unstable angina. NSTEMI is confirmed by a troponin rise and treated with antithrombotic therapy plus angiography within 24 to 72 hours. This tool labels any ST deviation at or below -0.5 mm as ST depression (ischemia / NSTEMI suspected).
The J point is the junction at the end of the QRS complex; ST deviation is measured 60 to 80 ms after it. Measuring too early picks up the QRS tail; too late picks up the T wave. QTc is the heart-rate-corrected QT interval using the Bazett formula QTc = QT / sqrt(RR). Values above 450 ms in men or 460 ms in women are prolonged and raise the risk of torsades de pointes. The acuity score combines ST deviation magnitude, tachycardia and wide QRS as an educational triage indicator.
Yes. Important differentials are (1) benign early repolarisation (common in healthy young men, concave ST elevation), (2) acute pericarditis (widespread concave ST elevation plus PR depression), (3) secondary ST changes from LVH or LBBB, (4) Brugada syndrome (V1-V3 saddleback pattern), and (5) hyperkalemia or hypothermia (Osborn waves). This simulator only checks numerical thresholds; final diagnosis needs waveform morphology, clinical evolution and troponin trend assessed by a cardiologist. Use it for education only.

Real-world applications

Emergency and cath-lab STEMI triage: When a patient presents with chest pain, a 12-lead ECG is recorded within 10 minutes, and any ST elevation triggers a "door-to-balloon" target of 90 minutes to primary PCI. The "lead × sex × age" threshold logic in this tool is exactly what is hard-coded into junior-resident checklists and triage software. Patients who do not meet the threshold are admitted to a CCU for serial troponin testing to rule in or out NSTEMI / unstable angina.

ICU monitors and arrhythmia management: Bedside ICU monitors continuously display leads II and V5 and run an "ST trend" feature that alarms on 0.1 mV (1 mm) deviations. After cardiac surgery or PCI the early detection of re-occlusion is critical, and lead-specific thresholds (essentially the logic in this tool) become the nursing intervention trigger.

Telemedicine, wearables and AI-ECG: Apple Watch, AliveCor KardiaMobile and Withings ScanWatch record a single-lead 30-second ECG. These devices are FDA-cleared for AF detection but their ST analysis is still limited. Commercial AI such as Mayo Clinic AI-ECG and Cardiologs (Philips) use ResNet-style deep learning to detect anterior STEMI with over 95 percent sensitivity and are being deployed for pre-hospital triage.

Exercise stress testing (treadmill / ergometer): Diagnosis of stable angina relies on watching for ST depression during graded exercise. A horizontal or downsloping ST depression of 1 mm or more at J+80 ms is considered positive, while upsloping ST depression is often a false positive. The HR-, age- and threshold-sensitivity panel of this tool serves as a sandbox for teaching stress-ECG interpretation.

Common misconceptions and pitfalls

The most common mistake is jumping from "ST is up" straight to "STEMI". Between 30 and 80 percent of healthy young men show "benign early repolarisation" with 1-4 mm of ST elevation in II and V2-V5. The key differentiators from STEMI are the shape (STEMI evolves towards a convex "tombstone" pattern over time, while early repolarisation stays concave and stable) and the evolution. The numerical check in this tool is only a threshold comparison; the final diagnosis requires morphology, evolution, symptoms and troponin. In practice, a 15-minute serial ECG is the key to confirming STEMI.

Second, not standardising the J-point measurement position. Many automated analysers in Japan measure ST at J+20 ms, while Western guidelines recommend J+60 or J+80 ms. Measuring too early picks up the terminal portion of the QRS and inflates ST; too late, it picks up the rising T wave and again inflates ST. When sites or devices disagree on this offset, the same patient can be classified as STEMI on one machine and not on another. The "J-point offset" slider here lets you feel that sensitivity directly.

Finally, applying the UDMI thresholds blindly to patients with LBBB. With left bundle branch block (QRS ≥ 120 ms), ST-T changes are secondary and cannot be cleanly separated from primary ischemia. If you suspect AMI in an LBBB patient, you switch to the Sgarbossa criteria (concordant ST elevation ≥ 1 mm in QRS-positive leads, discordant ST elevation ≥ 5 mm, or concordant ST depression ≥ 1 mm). This tool warns when QRS > 120 ms but is not a Sgarbossa calculator. The same caveat applies to ventricular-paced rhythms.

How to Use

  1. Set ST deviation (mm) using stNum slider; typical ischemia ranges 1–3 mm, STEMI ≥2 mm in contiguous leads
  2. Adjust heart rate (hrNum) between 40–180 bpm to model sinus bradycardia, normal rhythm, or tachycardia effects on QTc prolongation
  3. Configure QRS duration (qrsNum, 80–120 ms normal) and J-point elevation (jptNum) to distinguish benign early repolarization from acute injury patterns
  4. Select lead (stRange dropdown) to apply lead-specific ST thresholds: II/III/aVF for inferior wall, V1–V4 for anterior, I/aVL for lateral STEMI
  5. Review output metrics: ST deviation classification, RR interval calculation, QTc estimate (Bazett formula), and acuity score

Worked Example

Inferior STEMI case: ST deviation set to 2.5 mm in lead III, HR 88 bpm, QRS 95 ms, J-point 1.8 mm. RR interval = 60,000/88 = 682 ms. QTc = measured QT/√RR ≈ 420 ms (normal). Acuity score flags STEMI (≥2 mm ST in inferior leads). Compare to benign early repolarization: same J-point 1.8 mm but ST deviation only 0.8 mm in V2, HR 72 bpm, QTc 390 ms—classified as normal variant, acuity score low.

Practical Notes

  1. STEMI thresholds differ by sex and lead: ≥2.5 mm (men) or ≥2 mm (women) in V2–V3; ≥1 mm in other leads qualify as diagnostic
  2. PR depression accompanying ST elevation in multiple leads suggests pericarditis rather than coronary occlusion; adjust stRange and jptNum together to model this
  3. QTc prolongation (>450 ms men, >460 ms women) with high HR indicates repolarization abnormality; use hrNum >100 to simulate drug or electrolyte effects
  4. Sgarbossa criteria apply to LBBB: ST elevation discordant to QRS >1 mm, or ST depression in V1–V3 >1 mm; adjust qrsNum >120 ms to test LBBB scenarios