A normal ECG shows: - Regular sinus rhythm - Appropriate rate, axis, intervals, voltages, and morphology - No pathological Q waves, ST/T changes, conduction defects, hypertrophy, or arrhythmia Normal ranges are for resting adults (≥16 y); variations exist in children, athletes, pregnancy, ethnicity, body habitus.

Systematic Interpretation Checklist

Rate: 60–100 bpm
Rhythm: Sinus (P before every QRS, consistent PR, upright P in II)
Axis: –30° to +90° (normal); physiological left axis –30° to –90° common
Intervals: PR 120–200 ms, QRS ≤110 ms (≤120 ms acceptable), QTc ≤440 ms (men), ≤460 ms (women)
P waves: Upright I/II/aVF, ≤2.5 mm amplitude, ≤120 ms duration
QRS voltage & morphology: Normal progression (small R V1 → large R V5/V6), no pathological Q waves
ST segment: Isoelectric or minimal elevation/depression (<1 mm)
T waves: Concordant with QRS, upright I/II/V3–V6, inverted aVR
U waves: Absent or small (<1 mm) in V2–V3

Detailed Normal Values & Features

Parameter	Normal Range (Adults)	Notes / Variations
Rate	60–100 bpm	Sinus bradycardia <60 (common in athletes); tachycardia >100
Rhythm	Sinus rhythm	P wave before QRS, PR constant, P positive in II
P wave	Duration ≤120 ms Amplitude ≤2.5 mm (limb leads)	Upright I, II, aVF; biphasic V1 (positive then negative); no notching
PR interval	120–200 ms	Shorter in young/athletes; longer in elderly (up to 220 ms still normal)
QRS duration	≤110 ms (≤120 ms acceptable)	Narrow complex; no BBB, WPW (delta wave), or fascicular block
QRS axis	–30° to +90°	Left axis deviation –30° to –90° physiological (obesity, pregnancy, ascites)
QRS voltage	Limb: R ≤20 mm, S ≤15 mm Precordial: R V5/V6 ≤27 mm, S V1/V2 ≤30 mm	No low voltage (<5 mm limb leads) or high voltage (LVH criteria)
R-wave progression	Small R V1 → transition V3–V4 → tall R V5/V6	Poor progression can be normal in obesity/COPD
ST segment	Isoelectric (±0.5 mm)	Early repolarisation: concave upward elevation (1–3 mm) in V2–V5 common in young males
T wave	Upright I, II, V3–V6; inverted aVR	Concordant with QRS; amplitude <5 mm limb, <10 mm precordial; no deep inversion
QTc (Bazett)	≤440 ms men, ≤460 ms women	Upper limit ~450–470 ms; shorter in hypercalcaemia, longer in drugs/hypokalaemia
U wave	Absent or small (<1 mm) after T in V2–V3	Prominent U waves abnormal (hypokalaemia, drugs)

Normal Variants (Not Abnormal)

Early repolarisation: Concave ST elevation (1–4 mm) in V2–V5, notched/slurred J point, tall T waves – common in young, black males, athletes
Sinus arrhythmia: Rate variation with respiration (increases on inspiration) – normal in young/athletes
Persistent juvenile T-wave pattern: T inversion V1–V3 (sometimes V4) – normal in young adults, especially women
Wandering atrial pacemaker: Slight P-wave morphology change with rate variation – benign
Borderline axis: –30° to –45° or +90° to +110° – often normal

Common Pitfalls & When to Call Abnormal

Pathological Q waves: ≥0.04 s wide or ≥25% R-wave depth in ≥2 contiguous leads
ST elevation: ≥1 mm limb leads or ≥2 mm precordial (men <40 y), ≥1.5 mm women – consider STEMI/early repolarisation
ST depression: ≥0.5 mm horizontal/downsloping – ischaemia
Left axis deviation < –30° or right > +90° – consider conduction defect
Prolonged QTc >470–500 ms – risk of TdP
Low voltage + electrical alternans – pericardial effusion

Clinical Pearls (Exam/OSCE/MCQ)

Report systematically: "Rate, rhythm, axis, intervals, ST/T changes, voltage, progression"
Always compare to previous ECGs if available
Normal ECG does NOT exclude acute ischaemia (up to 20–30% NSTEMI have normal ECG)
Athletes often have sinus bradycardia, early repolarisation, high voltages – physiological
Ethnic variations: Higher voltage and early repolarisation more common in black individuals
Age: PR/QRS slightly longer in elderly; T inversion V1–V2 more common in women

Sources (2025–2026)

AHA/ACC/HRS: ECG Standardization & Interpretation (2009, reaffirmed 2025)
ESC Guidelines: Acute Coronary Syndromes (2023–2025)
Life in the Fast Lane ECG Library (updated 2025)
Marriott's Practical Electrocardiography (13th ed.)
UpToDate: ECG Interpretation (2026)

NORMAL

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ECG - Normal 12 Lead