ECG criteria
- Rhythm: sinus rhythm — regular P waves of sinus origin, usually positive in I, II, and aVF, negative in aVR; in III, the P wave amplitude and polarity may vary; a negative P wave in aVF is a sign of an ectopic (low atrial) rhythm rather than a normal variant; conduction of each P wave to the QRS is evaluated separately and is not considered a mandatory sign of sinus rhythm (in AV block, some P waves are not conducted)
- Rate: based on the RR interval (HR = 300 / number of large RR boxes at 25 mm/s or 60 / RR in seconds); for an irregular rhythm — the average over a 10-second recording or the number of QRS complexes in 6 sec × 10
- Regularity: RR intervals are visually estimated and quantitatively analyzed taking into account the clinical context (minor variability in sinus arrhythmia, significant irregularity in atrial fibrillation)
- QRS electrical axis: normal is approximately from −30° to +90° in adults; less than −30° is left axis deviation, greater than +90° is right axis deviation; evaluated by the wave amplitude ratio in I and aVF (quadrant method) or by the isoelectric lead
- P wave: morphology, amplitude in II and V1; P ≥ 2.5 mm in II is a criterion for right atrial abnormality (P pulmonale), which is non-specific and requires clinical correlation; P ≥ 120 ms, notching in II with an interpeak interval ≥ 40 ms, and a terminal negative portion in V1 ≥ 1 mm deep and ≥ 40 ms in duration are signs of left atrial abnormality
- PR (PQ) interval: 120–200 ms, usually constant; progressive prolongation with dropped QRS complexes is a pathological sign (Mobitz type I second-degree AV block)
- QRS complex: normal width up to 100–110 ms; pathological Q/QS wave — in two contiguous leads of the same territory, usually with a duration ≥ 40 ms and a depth ≥ 1 mm (or ≥ 25% of the subsequent R wave amplitude in the lead where specified separately); Q waves in III and aVR, as well as small septal Q waves in I, aVL, and V5–V6 may be normal variants, whereas QS/Q waves in V1–V3 require careful interpretation (they can be normal with cardiac rotation); the transition zone is usually in V3–V4
- ST segment and T wave: ST displacement is evaluated in contiguous leads relative to the TP/PR baseline, at the J point and subsequent points; criteria for a new ST elevation at the J point (Universal Definition/ESC): ≥ 1 mm in two contiguous leads (except V2–V3); in V2–V3 — ≥ 2 mm in men ≥ 40 years old, ≥ 2.5 mm in men < 40 years old, and ≥ 1.5 mm in women regardless of age; occlusion equivalents (posterior MI, ST elevation in aVR with multivessel disease, de Winter waves) and ischemic T waves are considered separately; a new bundle branch block by itself is not automatically considered a STEMI equivalent, and a single ECG without changes does not rule out ACS
- QT interval: measured from the onset of the QRS complex to the end of the T wave; QTc calculated using Bazett's formula (with caution outside of normal heart rate), normal up to ~440–450 ms in men, ~460 ms in women; for pronounced tachycardia/bradycardia and drug-induced prolongation, alternative formulas (Fridericia) are considered
What it looks like on ECG
The algorithm is not about "finding the diagnosis immediately", but rather a sequential check of each element of the tracing.
1. Rhythm. Evaluate whether regular P waves of sinus origin are present: usually positive in I, II, and aVF, negative in aVR; in III, polarity and amplitude may vary. Check separately whether each P wave is conducted to the QRS with a stable PR interval — this is not a mandatory condition for sinus rhythm, as some impulses are not conducted in AV block.
2. Rate (HR). At 25 mm/s and regular rhythm: HR = 300 / (number of large boxes in the RR interval), or 1500 / (number of small boxes). For an irregular rhythm: count QRS complexes for 6 seconds and multiply by 10.
3. Regularity. Compare RR intervals visually and, if necessary, measure them; minor variability is normal in sinus arrhythmia, while marked irregularity is typical for atrial fibrillation.
4. Electrical axis (QRS axis). Evaluated using limb leads: normal axis is approximately −30° to +90°. Use the quadrant method (ratio of R and S in I and aVF) or find the isoelectric lead.
5. P wave. Amplitude in II, morphology in V1; notching, duration, and the terminal portion in V1 indicate atrial abnormality.
6. PR interval. From the onset of P to the onset of QRS, normal is 120–200 ms, usually constant.
7. QRS complex. Width (normal ≤ 110 ms), morphology, Q wave in the context of two contiguous leads, transition zone.
8. ST segment and T wave. ST displacements in contiguous leads, T wave morphology, presence of secondary changes in block or hypertrophy.
9. QT interval. Measured from the onset of QRS to the end of T in the lead with its clearest termination, corrected for heart rate.
10. Conclusion. Formulate the result of each step indicating specific leads and numerical parameters.
Mechanism
Each step of the algorithm reflects a stage of the heart's electrical activity: the P wave corresponds to atrial depolarization, the PR interval reflects conduction through the intra-atrial system, the AV node, the bundle of His, and the Purkinje system; the QRS complex represents ventricular depolarization, the ST segment represents the plateau period (phase 2) and early repolarization, and the T wave represents the completion of ventricular repolarization. An abnormality at any stage manifests in the corresponding ECG element, so a systematic review helps localize the problem: a defect in impulse formation (P, rhythm), conduction (PR, QRS), or repolarization (ST-T).
How to measure
Practical steps at the bedside or at the desk:
- check calibration and recording speed (standard 25 mm/s, 10 mm/mV) before making any measurements;
- assess recording quality (baseline drift, interference, muscle tremor) before interpretation;
- measure intervals in the lead where wave boundaries are clearly visible rather than an arbitrary one;
- analyze all 12 leads, correlating findings across adjacent leads of the same wall;
- compare with the patient's previous ECGs if available — this significantly increases accuracy in doubtful cases.
Algorithm variants
There is no single strictly approved algorithm, but most textbooks (Hampton, Goldberger, Orlov) suggest a similar sequence: rhythm → rate → regularity → axis → P → PR → QRS → ST-T → QT. Some schemes shift the assessment of the axis or PQ relative to each other — the order between them is not critical, as long as the logic is maintained: atrial elements first, then ventricular elements, then repolarization. In an emergency situation (suspected ACS, hemodynamically significant arrhythmia), ST-T changes are sought immediately in parallel with a rapid rhythm assessment, without waiting for the full algorithm.
Common interpretation errors
- Reading "backwards": looking immediately at ST elevation while skipping rhythm and P wave assessment — you can miss atrial fibrillation, block, or another atrial pathology masking the true picture.
- P and QRS mismatch during dissociation: in AV dissociation or complete AV block, P waves and QRS complexes exist independently and are easily confused with extrasystoles.
- Measuring in only one lead: PQ, QT, and T wave amplitude should be checked where boundaries are most visible (II for PQ, V5 or II for QT), otherwise systematic error is introduced.
- Ignoring calibration: without checking 25 mm/s and 10 mm/mV, all numerical criteria (duration, amplitude) lose meaning.
- Diagnosis from a single lead: the ST segment or T wave is evaluated only in the context of adjacent leads of the same wall, otherwise an artifact will be mistaken for ischemia.
Clinical significance and management
A step-by-step algorithm reduces the risk of diagnostic error and makes the conclusion reproducible. The physician should record the result of each step (rhythm, heart rate, axis, intervals, QRS and ST-T morphology) in the report rather than limiting it to the final diagnosis — this facilitates verification by another specialist and comparison over time.
Any finding that alters clinical management (ST elevation/depression, wide QRS, pathological Q wave, pronounced QT prolongation) must be described with specific leads and numerical parameters.
If acute coronary syndrome with diagnostic ST elevation is suspected, immediate clinical assessment and activation of the reperfusion pathway (primary PCI or thrombolysis) are required; the same urgency applies to occlusion equivalents (posterior MI, ST elevation in aVR with multivigilant/multivessel disease, de Winter waves). In non-ST-segment elevation ACS (NSTE-ACS), management is chosen based on clinical presentation, ECG, troponin, and risk scores: an immediate invasive strategy is indicated for very high risk, and an early invasive strategy (<24 h) for high risk according to ESC criteria; for other patients, a selective invasive strategy or conservative management with mandatory risk stratification is possible. In cases of pronounced QT prolongation, medications, electrolytes, and the risk of torsades de pointes should be assessed. In complete AV block or unstable tachyarrhythmia with hemodynamic instability, emergency care is required. The ECG should not be interpreted in isolation from symptoms, hemodynamics, and medical history.
Rhythm-Rate-Axis-P-PQ-QRS-ST-T-QT — from atria to ventricles, from depolarization to repolarization
1. Which of the following steps comes first in the standard ECG interpretation algorithm?
Rhythm is assessed first because the interpretation of all subsequent ECG elements, including the ability to measure intervals and determine whether P and QRS are related, depends on it.
2. How to calculate heart rate from an ECG at a recording speed of 25 mm/s and a regular rhythm?
At 25 mm/s, one large square (5 mm) corresponds to 0.2 s, therefore 300 / RR (in large squares) = heart rate per minute.
3. In an emergency clinical situation (chest pain, suspected ACS), is it correct to first fully assess the rhythm and P wave, and only then look at the ST segment?
When acute coronary syndrome is suspected, immediate recognition of diagnostic ST-T changes is required to activate the reperfusion pathway; therefore, they are evaluated urgently, in parallel with a rapid rhythm assessment.
Frequently asked questions
What is the correct order for reading an ECG?
Standard order: rhythm, rate, regularity, electrical axis, P wave, PR interval, QRS complex, ST segment and T wave, QT interval. This sequence structures the analysis and reduces the risk of missing a finding. Record the result of each step in the conclusion.
Should ST elevation be checked immediately in case of chest pain?
In an emergency clinical situation (suspected ACS, syncope, hemodynamic instability), ST-T changes and signs of acute ischemia are assessed immediately, in parallel with a rapid rhythm assessment, without waiting to complete the full algorithm. A systematic analysis of rhythm, conduction, and other parameters is then performed.
Why is order important rather than just "looking at everything"?
Chaotic review of the tracing increases the risk of missing subtle findings (mild block, prolonged QT, small Q wave). A fixed algorithm makes the analysis reproducible among physicians and easier to verify.
How to quickly calculate heart rate from an ECG without a calculator?
At a speed of 25 mm/s, count the number of large squares (5 mm) between adjacent R waves and divide 300 by this number. For an irregular rhythm: count the number of QRS complexes in 6 seconds (30 large squares) and multiply by 10.
What to do if one of the algorithm steps is outside the normal range?
The deviation is recorded in the conclusion with indication of leads and numerical parameters, then the analysis continues through the remaining steps. A single finding should not interrupt the entire algorithm, as abnormalities frequently coexist (e.g., bundle branch block + hypertrophy).
ECG Course
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Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Goldberger A. Clinical Electrocardiography: A Simplified Approach
- Hampton J. The ECG Made Easy
- Orlov V.N. Guide to Electrocardiography
- 2023 ESC Guidelines for the management of acute coronary syndromes; AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram (2009)
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.