ECG criteria
- Sinus rhythm: P wave is positive in II, negative in aVR, precedes every QRS, P morphology is constant
- Heart rate 60–100 bpm based on the RR interval (at 25 mm/s: 1500 / number of mm between R-R); bradycardia <60 bpm, tachycardia >100 bpm, in trained athletes and during sleep a heart rate of 50–59 bpm is physiological
- P wave: duration ≤110–120 ms, amplitude ≤2.5 mm in lead II
- PQ (PR) interval: 120–200 ms (3–5 small squares of 0.04 s), measured from the start of P to the start of Q
- QRS complex: usually <110 ms; 110–119 ms requires morphological evaluation (incomplete bundle branch block or nonspecific intraventricular conduction delay is possible); ≥120 ms — widened QRS
- The ST segment is usually close to the isoelectric line; a physiological minor elevation is possible predominantly in V2–V3 and in early repolarization. In clinical suspicion of STEMI, a new ST elevation at the J point in two contiguous leads is considered significant: ≥1.0 mm in all leads except V2–V3; in V2–V3 — ≥2.0 mm in men ≥40 years, ≥2.5 mm in men <40 years, and ≥1.5 mm in women (regardless of age), in the absence of left ventricular hypertrophy and left bundle branch block; criteria are always interpreted together with clinical presentation and ECG dynamics
- Corrected QT interval (QTc by Bazett): ≤450 ms in men, ≤460 ms in women (upper limit according to various guidelines 440–460/450–470 ms)
- Axis (electrical axis of the heart): normal position from −30° to +90° by the alpha angle in the frontal plane
What it looks like on ECG
A normal adult resting ECG is a regular sinus rhythm of 60–100 bpm with constant P-QRS-T morphology in each cycle.
| Element | Normal | Where to measure |
|---|---|---|
| P wave | ≤110–120 ms, ≤2.5 mm | Lead II |
| PQ | 120–200 ms | from the start of P to the start of QRS |
| QRS | <110 ms (110–119 ms — evaluate morphology) | any lead with max. width |
| QT/QTc | QTc ≤450/460 ms (m/f) | II or V5 |
| Q wave | physiological q <40 ms, <25% R in I, aVL, V5–V6 | lateral leads |
| R wave | progresses from V1 to V5–V6 | chest leads |
| T wave | positive in I, II, V3–V6; negative in aVR; variants possible in III, V1–V2 | all leads |
The q wave is physiological (septal q) in I, aVL, V5–V6 — narrow and low-amplitude, reflecting left-to-right interventricular septum depolarization. Q in III and QS in aVR are frequently physiological if there are no pathological Q waves in contiguous leads, R-wave progression abnormalities, or clinical signs of ischemia. The R/S transition zone in chest leads typically occurs at V3–V4. The ST segment is normally isoelectric or slightly elevated at the J point. Early repolarization is considered when there is characteristic J-point and ST-segment elevation in contiguous leads, most often inferior or lateral, especially with a stable pattern and absence of symptoms; based on the elevation amplitude in V2–V3 in young men alone, ischemia cannot be excluded — any new ST elevation with chest pain is evaluated according to the acute coronary syndrome protocol. The T wave is usually positive in I, II, V3–V6 and negative in aVR; in III, V1, and sometimes V2 (especially in young women), a negative or biphasic T wave can be a normal variant.
Mechanism
Each ECG element reflects a specific stage of excitation propagation. The P wave is atrial depolarization from the sinus node. The PQ interval is the time taken for the impulse to travel through the atria, AV node, and His-Purkinje system to the start of ventricular depolarization. The QRS complex is ventricular depolarization: first the septum (septal q), then the main mass of the ventricular myocardium (R), and finally the basal regions (S). The ST segment corresponds to the plateau phase of the ventricular cardiomyocyte action potential, when the potential difference between cells is minimal — which is why it is isoelectric. The T wave is ventricular repolarization. Depolarization generally spreads from endocardium to epicardium, whereas repolarization proceeds in the opposite direction — from epicardium to endocardium, since the epicardium repolarizes earlier than the endocardium. It is precisely this opposite direction of propagation, combined with the reverse polarity of the repolarization process itself, that causes the T wave to typically be concordant with QRS, meaning it points in the same direction.
Normal Variants
- Sinus arrhythmia (respiratory) — periodic heart rate increase during inspiration and decrease during expiration; a difference in adjacent RR intervals of approximately more than 120 ms is usually considered significant, the severity is variable and in itself does not indicate pathology in young individuals
- High QRS voltage in young, lean individuals and athletes
- Early ventricular repolarization (J-point and ST elevation in chest, inferior, or lateral leads)
- Negative T wave in V1–V3 in children and some young women (juvenile pattern)
- Incomplete right bundle branch block (rSr' in V1) as a normal variant in young individuals
- Sinus bradycardia of 50–59 bpm in trained athletes and during sleep
- Vertical or horizontal axis position in asthenics and hyperstenics, respectively
Pathological changes
Values going beyond normal limits indicate potential pathology and require targeted investigation:
- PQ prolongation >200 ms — first-degree AV block
- PR <120 ms along with a delta wave and QRS widening form a ventricular pre-excitation pattern; WPW syndrome is diagnosed when clinical tachyarrhythmias are present
- QRS widening ≥120 ms — differential diagnosis: bundle branch block, nonspecific intraventricular conduction delay, ventricular or paced rhythm, pre-excitation, hyperkalemia, antiarrhythmic drug effect; requires evaluation of morphology and rhythm
- QTc prolongation — congenital or acquired long QT syndrome, risk of torsades de pointes
- A persistent QTc roughly ≤330–340 ms is especially suspicious for short QT syndrome; values <360 ms require clinical evaluation, exclusion of hypercalcemia, drug-induced and other secondary causes, and, if necessary, an arrhythmologist consultation
- Pathological Q/QS: any Q in V2–V3, as well as a Q with duration ≥30 ms and depth ≥1 mm in two contiguous leads from I, II, aVL, aVF, V4–V6; QS in V1–V3 is also pathological in the appropriate context. Criteria are nonspecific: they also occur in bundle branch blocks, left ventricular hypertrophy, cardiomyopathies, and improper lead placement — requiring correlation with clinical presentation, troponin, previous ECGs, and imaging
- Axis deviation beyond −30°…+90° — ventricular hypertrophy, fascicular blocks, pulmonary pathology
Clinical significance and management
Knowledge of normal ranges is the foundation of interpreting any ECG: without them, it is impossible to distinguish a normal variant from pathology. A physician must measure intervals and amplitudes systematically (Heart rate → rhythm → axis → intervals → waves → ST segment → T wave), comparing them against age- and sex-specific norms, since thresholds for QTc, P wave amplitude, ST elevation in V2–V3 when suspecting STEMI, and certain left ventricular hypertrophy criteria differ between men and women. In borderline values (e.g., QRS 105–115 ms or QTc 440–460 ms), reliance is placed on the clinical context and dynamics compared to the patient's previous tracings, rather than a single measurement.
Common interpretation errors
- Applying Bazett's formula without heart rate correction during tachycardia/bradycardia — distorts QTc; one should cross-check with Fridericia's formula
- Ignoring different threshold values for men and women (QTc, ST elevation in V2–V3, LVH criteria)
- Measuring intervals in a single lead without checking across multiple leads (QRS width is not maximal everywhere)
- Mistaking a normal variant (early repolarization, juvenile T) for pathology in young patients without considering clinical presentation and dynamics
- Failing to account for paper speed (25 or 50 mm/s) when converting millimeters to milliseconds
1. What is the upper limit of the normal PQ (PR) interval in an adult?
A PQ interval of 120–200 ms is considered normal; prolongation above 200 ms indicates a first-degree AV block.
2. Which wave is considered physiological (septal q) and is not pathological?
The septal q wave is narrow (<40 ms) and low-amplitude (<25% of R), and is found in lateral leads due to left-to-right depolarization of the interventricular septum.
3. At what heart rate value does a sinus rhythm transition into tachycardia according to the generally accepted criterion?
Sinus tachycardia is defined as a heart rate over 100 bpm; values of 91–100 bpm remain within the normal range.
Frequently asked questions
What is the normal heart rate on an ECG in adults?
60–100 bpm for sinus rhythm. Below 60 is bradycardia, above 100 is tachycardia; values of 91–100 bpm are not considered tachycardia on their own. A heart rate of 50–59 bpm is physiological in trained athletes and during sleep.
What is the normal QT interval on an ECG?
First, the absolute QT interval is measured, then the heart rate-corrected QTc is calculated using the RR interval. The upper limit of QTc by Bazett's formula is usually 450 ms in men and 460 ms in women, with values of 440–470 ms found in various guidelines. When the heart rate differs significantly from normal, Fredericia's formula is preferable, as Bazett's formula overestimates QTc at high heart rates and underestimates it at low heart rates.
How do ECG norms differ between men and women?
Women have a higher upper limit of normal QTc and lower voltage criteria for left ventricular hypertrophy. Separately: when STEMI is suspected, the thresholds for significant ST-segment elevation in V2–V3 also depend on sex and age, but these are criteria for acute coronary syndrome rather than hypertrophy. The PQ and QRS intervals do not depend on sex.
Should I be concerned if QRS is 105 ms?
Values up to 110 ms are usually non-pathological if the morphology of the complex is unchanged. The 110–119 ms range requires morphological evaluation — incomplete bundle branch block or nonspecific conduction delay is possible; the duration is always evaluated together with the morphology of the complex, rather than in isolation.
How to quickly check if an ECG is normal?
Rhythm and HR, electrical axis of the heart, durations of PQ, QRS, and QTc, amplitude and morphology of P, Q, R, S, and T waves, and ST-segment position are evaluated sequentially. If all parameters are within the specified ranges and there are no pathological patterns, such an ECG can be considered normal based on this recording, but a normal ECG does not rule out disease: in the presence of symptoms, dynamic changes, or suspected acute coronary syndrome, clinical evaluation and additional tests are required.
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Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Goldberger's Clinical Electrocardiography, 9th ed.
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice, 6th ed.
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
- Orlov V.N. Guide to Electrocardiography
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.