How to Read a Normal ECG: Waves, Intervals, and Values 2026
Clinical guide for doctors, students, and healthcare professionals · Colombia 2026
Normal electrocardiogram: waves, intervals, and how to interpret it step by step
From the P wave to corrected QTc — the most comprehensive guide to confidently interpreting a normal ECG: sinus rhythm, heart rate, normal intervals, and the most common reading errors
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GSMED Imaging Clinical-Technical Team
Content based on the MSD Manual Professional Edition (Electrocardiography, 2025), My-EKG.com (ECG intervals and segments), FAPap — Journal of Primary Care Pediatrics (pediatric ECG), Salusplay (Topic 3: ECG interpretation), UNAM Physiology (ECG interpretation workshop), and the normal ECG interpretation document from the Rosario Department of Health (November 2025).
The electrocardiogram (ECG) is the most frequently requested test in general medical practice and, paradoxically, one of the tests that causes the most uncertainty when interpreted. The first step in detecting an abnormality is knowing exactly what a completely normal ECG looks like. This guide walks you through every element of the tracing, with exact reference values and the most common interpretation errors.
1. The ECG paper: how to read the scale before reading the tracing
Before interpreting any waves, you need to understand the paper on which the ECG is recorded. ECG paper is ruled with two types of grid:
📄 Standard ECG paper — speed 25 mm/s · Calibration 10 mm/mV
Small square (1 mm)Horizontally (time): 0.04 seconds · Vertically (voltage): 0.1 mV
Large square (5 mm = 5 small squares)Horizontally (time): 0.20 seconds · Vertically (voltage): 0.5 mV
Standard calibrationA 10 mm-high calibration signal = 1 mV. If the calibration is different, all amplitudes must be adjusted proportionally before interpretation.
Standard speed: 25 mm/sIn some situations, 50 mm/s is used (the tracing is "spread out," making fine details easier to see). All times in this guide assume 25 mm/s.
The quick trick for calculating heart rate Find an R wave that lines up with a thick vertical line on the paper. From there, count the large squares to the next R wave and apply the rule: 1 large square = 300 bpm · 2 squares = 150 · 3 squares = 100 · 4 squares = 75 · 5 squares = 60 · 6 squares = 50. This is the 300 rule — the fastest method in clinical practice. For irregular tracings: count the QRS complexes on a 6-second strip (30 large squares) and multiply by 10.
2. Normal ECG waves, intervals, and segments
P wave
Duration: <0.12 s · Amplitude: <0.25 mV
Represents atrial depolarization. It should be positive (upward) in leads I, II, aVF, and the left precordial leads. Negative in aVR. It may be biphasic in V1 and II.
⚠️ Abnormal: absent (AF), inverted in II (ectopic rhythm), peaked >0.25 mV (right atrial enlargement), notched or wide >0.12 s (left atrial enlargement)
PR interval
Normal: 0.12–0.20 s (3 to 5 small squares)
From the start of the P wave to the start of the QRS. Represents conduction time from the sinus node to the ventricles through the AV node. Varies with heart rate: the higher the heart rate, the shorter the PR.
⚠️ Abnormal: >0.20 s = first-degree AV block · <0.12 s = pre-excitation (WPW) or junctional rhythm
QRS complex
Duration: <0.12 s (<3 small squares)
Represents ventricular depolarization. The Q wave is the first negative deflection, the R is the first positive deflection, and the S is the negative deflection after the R. Amplitude varies by lead.
⚠️ Abnormal: QRS >0.12 s = bundle branch block · Pathological Q wave: >25% of R or >0.04 s = myocardial necrosis
ST segment
Normal: isoelectric (± 0.5 mm from baseline)
From the end of the QRS (J point) to the start of the T wave. Represents the plateau phase of the ventricular action potential — the period between depolarization and repolarization. It should not be significantly elevated or depressed.
⚠️ Abnormal: elevation >1 mm in 2+ contiguous leads = STEMI · Depression = ischemia or digitalis effect
T wave
Positive in I, II, V3–V6 · Negative in aVR
Represents ventricular repolarization. It should be rounded and asymmetric (slow upstroke, rapid downstroke). Its polarity usually follows that of the QRS in the same lead.
⚠️ Abnormal: inversion in V1–V4 = anterior ischemia · peaked and symmetric = hyperkalemia or hyperacute ischemia
QT / QTc interval
Normal QTc: 340–440 ms (men) / ≤460 ms (women)
From the start of the QRS to the end of the T wave. Varies with heart rate — corrected using Bazett's formula: QTc = QT / √(RR in seconds). Visual shortcut: a normal QT is shorter than half the RR interval.
⚠️ Abnormal: QTc >450 ms in men / >470 ms in women = risk of Torsades de Pointes · Short QT <340 ms is also pathological
3. Normal sinus rhythm: the 5 criteria that must be met
Sinus rhythm is the normal heart rhythm — it originates in the sinus node and propagates in an orderly manner through the conduction system. For an ECG to be considered normal sinus rhythm, all of the following criteria must be met:
1
Heart rate between 60 and 100 bpm
This is the intrinsic firing rate of the sinus node in a resting adult. Below 60 bpm = sinus bradycardia (may be normal in athletes). Above 100 bpm = sinus tachycardia.
✓ Normal: 60–100 bpm
2
P wave positive in II, III, and aVF — negative in aVR
Indicates that atrial depolarization is directed downward and to the left — the activation pattern produced by a normally located sinus node in the right atrium. If the P wave is negative in II, the pacemaker is not the sinus node.
✓ Normal: P+ in II · P- in aVR
3
Regular RR interval
The QRS complexes should be equally spaced — the heart beats at a steady rhythm. A variation of up to 0.12 s between consecutive RR intervals may be normal (respiratory sinus arrhythmia, especially in young people). Greater variation warrants investigation for atrial fibrillation or other arrhythmias.
✓ Normal: regular RR (variation ≤0.12 s between beats)
4
Each QRS preceded by a P wave with a constant PR
Each ventricular contraction should be preceded by atrial depolarization, with the same PR in every cycle. This confirms that the impulse travels from the sinus node to the AV node and from there to the ventricles through the His-Purkinje system with every beat.
✓ Normal: 1 P wave for every QRS · constant PR 0.12–0.20 s
5
Narrow QRS — duration less than 0.12 s
A narrow QRS indicates normal intraventricular conduction — the impulse travels down the bundle of His and is distributed simultaneously to both ventricles through the Purkinje system. A wide QRS (>0.12 s) suggests bundle branch block or aberrant conduction.
✓ Normal: QRS <0.12 s (fewer than 3 small squares)
4. The electrical axis: how to calculate it and what it means
The QRS electrical axis is the average direction of ventricular depolarization, expressed in degrees in the frontal plane. It is calculated by observing which leads have a predominantly positive QRS.
| Electrical axis | Range in degrees | Meaning | Clinical associations |
|---|---|---|---|
| Normal | -30° to +90° | Normal | Healthy heart, normal position |
| Left axis deviation | -30° to -90° | Pathological | Left anterior fascicular block, inferior infarction, COPD, obesity |
| Right axis deviation | +90° to +180° | Pathological | Left posterior fascicular block, cor pulmonale, PE, vertical position (may be normal in thin young people) |
| Indeterminate axis | -90° to ±180° | Rare | Generally associated with severe disease or artifact |
Quick method for determining the axis — the lead I and aVF rule Look at the QRS in lead I and aVF. If it is positive in both → normal axis. If positive in I and negative in aVF → left axis deviation (check whether it is also negative in II → pathological deviation). If negative in I and positive in aVF → right axis deviation. This is a quick approximation that is sufficient for most clinical situations without needing to calculate exact angles.
5. Table of normal adult ECG reference values
| Parameter | Normal value | Unit | Pathological if |
|---|---|---|---|
| Heart rate | 60–100 | bpm | <60 = bradycardia · >100 = tachycardia |
| PR interval | 0.12–0.20 | seconds | >0.20 s = first-degree AV block · <0.12 s = pre-excitation |
| QRS duration | <0.12 | seconds | ≥0.12 s = bundle branch block |
| P wave amplitude | <0.25 | mV | >0.25 mV = right atrial enlargement |
| P wave duration | <0.12 | seconds | >0.12 s = left atrial enlargement |
| QTc (Bazett), men | 340–440 | ms | >450 ms = prolonged QT · <340 ms = short QT |
| QTc (Bazett), women | 340–460 | ms | >470 ms = prolonged QT |
| ST segment | Isoelectric ±0.5 mm | mm | Elevation >1 mm in ≥2 contiguous leads = STEMI |
| QRS electrical axis | -30° to +90° | degrees | <-30° = left axis deviation · >+90° = right axis deviation |
| Pathological Q wave | <0.04 s and <25% of R | seconds | >0.04 s or >25% of R = necrosis/old infarction |
6. The 6 most common ECG interpretation errors
- Failing to check the paper calibration or speed: if the ECG is recorded at 50 mm/s (double speed), all intervals will appear twice as long — you will interpret a normal PR of 0.16 s as 0.32 s and falsely diagnose first-degree AV block. Always check the calibration mark at the beginning of the tracing.
- Confusing artifact with arrhythmia: muscle tremors (Parkinson's disease, chills) produce high-frequency oscillations that mimic atrial fibrillation or flutter. Check for regular P waves beneath the artifact and see whether the baseline also oscillates in an organized manner.
- Not checking for T-wave inversion in specific leads: T-wave inversion in V1 is normal in adults. Inversion in V1–V2 may be normal. Inversion in V3–V4 is more suggestive of anterior ischemia.
- Ignoring QTc because you haven't calculated it: Long QT is one of the most preventable causes of sudden death. Always calculate QTc — apply Bazett's formula (QT / √RR in seconds) or use the visual rule: QT should not exceed half of RR.
- Diagnosing LBBB based on a wide QRS without checking all criteria: Complete left bundle branch block requires QRS >0.12 s + an "M"-shaped morphology in V5/V6 + an rS or QS pattern in V1/V2. A wide QRS without these criteria may indicate right bundle branch block or other conditions.
- Failing to correlate with the clinical picture: an ECG must always be interpreted in context. Sinus tachycardia at 115 bpm in an anxious patient in the emergency department is normal. The same tachycardia in a postoperative patient may indicate bleeding, PE, or sepsis. The ECG provides information — the clinician makes the diagnosis.
7. Frequently asked questions about the normal electrocardiogram
How long does an electrocardiogram take, and how is it performed?
A conventional 12-lead ECG takes 5–10 minutes. The technician or nurse places 10 electrodes: 4 on the limbs (right arm, left arm, right leg, left leg) and 6 on the chest (V1 to V6 in specific anatomical positions). The patient should be resting, still, and breathing normally. It does not hurt at all — the electrodes only record; they do not deliver current. The machine (electrocardiograph) prints or digitally stores the simultaneous 12-lead tracing, which usually records for 10 seconds.
What is the difference between a 12-lead ECG and a 3-lead ECG?
A 3-lead ECG (used in bedside ICU monitors or portable monitors) records only three "views" of the heart — enough to detect arrhythmias and continuously monitor the rhythm. A 12-lead ECG provides twelve different perspectives of the heart — the 6 in the frontal plane (I, II, III, aVR, aVL, aVF) and the 6 precordial leads (V1–V6). Only a 12-lead ECG allows you to: diagnose a heart attack (and locate the affected area), assess the electrical axis, detect complete bundle branch blocks, characterize arrhythmias accurately, and properly assess QTc.
Does a normal ECG rule out a myocardial infarction?
Not completely. This is one of the most important pitfalls in ECG interpretation. Up to 20% of acute myocardial infarctions may have a normal or non-diagnostic initial ECG, especially in the first few hours, in posterior infarctions, or in non-ST-elevation myocardial infarctions (NSTEMI). A normal ECG in a patient with typical chest pain does not rule out acute coronary syndrome — it requires serial troponins (at 0 and 3 hours at minimum) and a complete clinical evaluation. The phrase that all residents should memorize: "treat the patient, not the ECG".
What electrocardiograph does a clinic need to perform quality ECGs?
A 12-channel electrocardiograph with thermal printing and digital storage is the minimum recommended standard for any clinic or hospital in Colombia. Key features to evaluate: minimum 12-bit ADC resolution for faithful signal capture; artifact filters (60 Hz line filter for Colombia, muscle filter); automatic ECG analysis with preliminary interpretation (does not replace the physician but helps); connectivity (USB, Wi-Fi, or Ethernet for storage in the electronic health record); and an internal battery for cordless use. At GSMED, we have electrocardiographs available in Colombia for all levels of complexity.
12-lead electrocardiographs available in Colombia
Equipment for clinics, hospitals, and medical offices — with automatic analysis, digital connectivity, and local technical support.
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