ECG Interpretation Online: Algorithm and Calculators

ECG interpretation is a core skill for any medical specialist. This article provides a step-by-step algorithm for interpreting an electrocardiogram and brings together free online calculators and a real-tracing trainer in one place.

What is an ECG and Why It Is Needed

An electrocardiogram is a recording of the heart's bioelectrical activity from the body surface. A standard 12-lead ECG includes 6 limb leads (I, II, III, aVR, aVL, aVF) and 6 chest leads (V1-V6). The standard paper speed is 25 mm/s (1 small grid square = 0.04 s = 40 ms), and the calibration is 10 mm/mV.

An ECG helps diagnose:

• Rhythm and conduction disorders (atrial fibrillation, AV blocks, WPW syndrome);
• Acute coronary syndrome (STEMI, NSTEMI);
• Cardiac chamber hypertrophy;
• Electrolyte disturbances (hypo-/hyperkalemia);
• Effects of cardioactive medications (digitalis, antiarrhythmics).

Step-by-Step ECG Interpretation Algorithm in 8 Steps

Step 1. Rhythm: Sinus or Non-Sinus?

The rhythm is considered sinus if each QRS complex is preceded by a positive P wave in lead II and a negative P wave in aVR, with a constant P-wave morphology. If P waves are absent or chaotic, it indicates atrial fibrillation or another supraventricular arrhythmia.

Step 2. Heart Rate: Bradycardia, Normal, or Tachycardia

The normal resting heart rate for an adult is 60-100 bpm. It is calculated using the R-R interval: HR = 60000 / RR (ms). A quick method for standard 25 mm/s paper is to divide 300 by the number of large grid squares between two R waves.

→ Online Heart Rate Calculator by R-R Interval

Step 3. Electrical Axis of the Heart

Normal axis range: -30° to +90°. Determined by the net amplitude of the QRS complex in leads I and aVF:

• I (+), aVF (+) — normal axis (Quadrant I);
• I (+), aVF (-) — left axis deviation (LBBB, left anterior hemiblock, LVH);
• I (-), aVF (+) — right axis deviation (RVH, COPD, pulmonary embolism);
• I (-), aVF (-) — extreme axis deviation, indeterminate axis.

→ Cardiac Electrical Axis Calculator

Step 4. P Wave and Atria

Normal P-wave duration is ≤ 110 ms (2.75 small squares), and amplitude is up to 2.5 mm. P-wave prolongation in lead II or a notched P wave indicates left atrial enlargement (P-mitrale). Tall P waves (>2.5 mm) indicate right atrial enlargement (P-pulmonale).

Step 5. PR Interval

Normal PR interval: 120-200 ms (3-5 small squares). Prolongation indicates first-degree AV block. Progressive prolongation with dropped QRS complexes indicates Mobitz type I (Wenckebach). A fixed P:QRS ratio of 2:1 or 3:1 indicates Mobitz type II or high-grade AV block. PR shortening (<120 ms) with a delta wave indicates WPW syndrome.

Step 6. QRS Complex

Normal QRS duration is less than 120 ms (3 small squares). Widening indicates bundle branch block:

• Right bundle branch block (RBBB): QRS >120 ms, rSR' pattern in V1, wide S wave in leads I and V6;
• Left bundle branch block (LBBB): QRS >120 ms, broad monophasic R wave in leads I, V5, V6, and absence of Q waves in these leads.

Pathological Q waves (>40 ms or >25% of the R-wave amplitude) indicate myocardial scar tissue following a myocardial infarction.

Step 7. ST Segment and T Wave

Normally, the ST segment is on the isoelectric line. ST-segment elevation >1 mm in two contiguous leads (≥2 mm in V2-V3) indicates STEMI:

• II, III, aVF — inferior myocardial infarction;
• V1-V4 — anterior/anteroseptal myocardial infarction;
• I, aVL, V5-V6 — lateral myocardial infarction;
• Depression in V1-V3 + tall R wave + positive T wave — posterior myocardial infarction.

In the presence of LBBB, standard criteria do not apply — use Sgarbossa's criteria.

→ Sgarbossa Criteria for STEMI in LBBB · Learn more about myocardial infarction

Step 8. QT Interval and QTc

The QT interval is measured from the onset of the QRS complex to the end of the T wave. Because QT depends on heart rate, the corrected QT (QTc) is used:

• Bazett's formula: QTc = QT / √RR — the most widely used formula;
• Fridericia's formula: QTc = QT / ∛RR — more accurate at tachy- and bradycardias;
• Framingham and Hodges — linear corrections.

Normal QTc: ≤450 ms for men, ≤460 ms for women. QTc >500 ms indicates a high risk of torsade de pointes.

→ QTc Calculator Using 4 Formulas · → Tisdale Risk Score: Inpatient QT Prolongation Risk

All Online ECG Calculators in One Place

The ECG Online page features all of our tools:

• QTc — corrected QT using Bazett, Fridericia, Framingham, and Hodges formulas;
• Heart Rate by R-R — calculation of rate from a single interval;
• Electrical axis — by QRS amplitudes in I and aVF;
• Sgarbossa criteria — AMI in the presence of LBBB;
• Tisdale risk score — inpatient QT prolongation risk;
• CHA₂DS₂-VASc — stroke risk in atrial fibrillation;
• HEART Score — risk stratification in chest pain.

Common ECG patterns and what to do with them

Atrial fibrillation

Absence of P waves, irregular R-R intervals, fine or coarse fibrillatory activity instead of P waves. Actions: assess heart rate (rate or rhythm control), assess stroke risk using CHA₂DS₂-VASc, initiate anticoagulation if score ≥2 in men and ≥3 in women.

Acute STEMI

ST-segment elevation with reciprocal depression in opposing leads. Time is myocardium: pre-hospital activation of a PCI center, aspirin 250 mg PO, P2Y12 inhibitor loading dose, anticoagulant. If PCI is unavailable within ≤120 min — thrombolysis.

Left ventricular hypertrophy

Sokolov-Lyon criteria: S(V1) + R(V5 или V6) ≥ 35 мм. Cornell voltage: R(aVL) + S(V3) >28 мм у мужчин или >20 мм у женщин. Accompanied by LV strain — ST depression and T-wave inversion in V5-V6.

Prolonged QT

Congenital (LQTS) or acquired (electrolytes, medications). Provoking drugs: class IA and III antiarrhythmics, fluoroquinolones, macrolides, antipsychotics, methadone. Vaughan Williams classification of antiarrhythmic drugs.

Practice on real tracings

Further reading

• Normal ECG: waves, intervals, and step-by-step interpretation
• Myocardial infarction: ECG diagnosis, troponins, and ACS treatment algorithm
• Cardiac cycle: phases, pressures, heart sounds, and ECG correlations
• Antiarrhythmic drugs: Vaughan Williams classification

FAQ: frequently asked questions about ECG

How many leads are in a standard ECG?

A standard ECG has 12 leads: 6 limb leads (I, II, III, aVR, aVL, aVF) and 6 precordial leads (V1-V6). For suspected right ventricular infarction, V3R-V4R are added; for suspected posterior infarction, V7-V9.

What is the normal paper speed?

The standard is 25 mm/s. At this speed, 1 small box = 1 mm = 40 ms, 1 large box (5 small boxes) = 5 mm = 200 ms. Sometimes 50 mm/s is used — all intervals will visually appear twice as long.

Can an ECG be interpreted online without a doctor?

Algorithmic processing (calculating heart rate, intervals, axis) — yes, via calculators like ours. However, clinical interpretation of an ECG requires professional medical qualification, especially when ischemia or rhythm disorders are suspected. This article and the calculators are educational tools and do not replace an in-person medical consultation.

What to do if QTc exceeds 500 ms?

High risk of torsade de pointes. Actions: 1) discontinue provoking medications, 2) correct electrolytes (K ≥ 4.0; Mg ≥ 2.0 mg/dL), 3) consider cardiac monitoring, 4) for torsades de pointes — IV magnesium sulfate and temporary overdrive pacing.

How to distinguish STEMI from pericarditis?

STEMI: ST elevation in the infarct zone + reciprocal depression + pathological Q waves over time + troponin dynamics. Pericarditis: diffuse ST elevation in all leads except aVR/V1, PR depression, troponins may be normal or mildly elevated in myopericarditis.

Abridged clinical guidelines in cardiology

If you need up-to-date clinical guidelines from the Russian Ministry of Health rather than educational courses, the platform features the Abridged Clinical Guidelines section. All cardiology guidelines are gathered in one place: acute myocardial infarction, ACS, atrial fibrillation, chronic heart failure, arterial hypertension, rhythm and conduction disorders. Abridged to the clinically significant essentials, supplemented with interactive lessons, clinical checklists, and reinforcement quizzes.

Medical students also have access to the full catalog of 31 medical school subjects featuring an AI tutor and an accreditation exam simulator.