10 common ecg changes

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10 common ECG changes clinical interpretation

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ECG changes ST elevation STEMI myocardial infarction

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atrial fibrillation ECG irregularly irregular no P waves

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heart block first degree second degree third degree ECG PR interval

I now have sufficient material to provide a comprehensive, well-illustrated answer. Here are the 10 most common ECG changes:

10 Common ECG Changes

1. ST-Segment Elevation (STEMI)

Definition: ST elevation ≥1 mm in ≥2 contiguous limb leads or ≥2 mm in ≥2 contiguous precordial leads.

Causes: STEMI, Prinzmetal (vasospastic) angina, pericarditis (saddle-shaped, diffuse), left ventricular aneurysm, Brugada pattern.

Key features:

Convex ("tombstone") morphology in STEMI
Reciprocal ST depression in opposing leads
Hyperacute (peaked, broad) T-waves in earliest phase
Evolves → pathological Q waves over hours

Lead localization:

Leads	Territory	Artery
V1–V4	Anterior	LAD
II, III, aVF	Inferior	RCA
I, aVL, V5–V6	Lateral	LCx

Anterior STEMI with tombstoning morphology in V2–V5 and reciprocal inferior ST depression

2. ST-Segment Depression

Definition: ST depression >0.5–1 mm below the isoelectric line.

Causes: Subendocardial ischemia/NSTEMI, reciprocal change in STEMI, LVH strain pattern, digoxin effect (reverse "tick" / hockey stick shape), hypokalemia.

Morphologies:

Horizontal/downsloping → most ominous (ischemia)
Upsloping → less specific
Digoxin ("scooped") → concave downsloping with short QT

3. T-Wave Inversion

Definition: Negative T-wave in a lead where it is normally upright (T normally upright in I, II, V3–V6).

Causes:

Myocardial ischemia / NSTEMI (deep symmetrical inversion)
Wellens' syndrome — deep symmetric T-wave inversion in V2–V3 = critical proximal LAD stenosis
RV strain (acute PE) — V1–V4 ± III
LVH strain — lateral leads
Hypertrophic cardiomyopathy — deep diffuse inversion
Normal variant — V1–V2 (and V3 in women)

4. Pathological Q Waves

Definition: Q-wave duration >40 ms (>1 mm wide) or depth >25% of the following R wave, in ≥2 contiguous leads.

Significance: Indicates transmural myocardial necrosis (completed infarction). Develop within 6–24 hours of STEMI and may persist permanently.

Normal Q waves (septal depolarization) are small (<40 ms) in I, aVL, V5–V6 — do not confuse.

5. Atrial Fibrillation (AF)

Definition: Irregularly irregular rhythm with absent P waves replaced by chaotic fibrillatory (f) waves, most visible in V1.

Key ECG features:

No identifiable P waves
Irregular baseline (f-waves, 350–600/min)
Irregularly irregular R-R intervals
Narrow QRS (unless aberrant conduction/pre-excitation)
Ventricular rate depends on AV nodal conduction

Classic atrial fibrillation — irregularly irregular rhythm, absent P waves, fine f-waves in V1

6. AV Blocks (Heart Block)

Three degrees:

Degree	PR Interval	Dropped beats	Notes
1st degree	>200 ms (>5 small sq)	None	All P waves conducted; benign
2nd degree — Mobitz I (Wenckebach)	Progressive lengthening	Periodic	Group beating; AV node level
2nd degree — Mobitz II	Fixed	Sudden	Below AV node; risk of complete block
3rd degree (Complete)	AV dissociation	All	P & QRS completely independent; escape rhythm

Comparison of 1st, 2nd, and 3rd degree heart block showing P-QRS relationship

7. Bundle Branch Blocks (BBB)

Rule: BBB = QRS duration ≥120 ms (≥3 small squares).

Right Bundle Branch Block (RBBB):

RSR' ("M-shape") in V1
Wide slurred S wave in I, V6
Causes: PE, RV overload, congenital heart disease, normal variant

Left Bundle Branch Block (LBBB):

Broad notched R wave in I, aVL, V5–V6
Deep QS in V1
Causes: IHD, dilated cardiomyopathy, hypertension
⚠️ New LBBB with chest pain = STEMI equivalent (Sgarbossa criteria apply)

8. Left Ventricular Hypertrophy (LVH)

Common voltage criteria (Sokolow–Lyon):

S in V1 + R in V5 or V6 ≥35 mm
R in aVL ≥11 mm

Associated changes (strain pattern):

ST depression + asymmetric T-wave inversion in I, aVL, V5–V6
Left axis deviation
Broad, notched P wave (left atrial enlargement) — P-mitrale

Note: Voltage criteria alone have ~50% sensitivity; strain pattern increases specificity. — Harrison's Principles of Internal Medicine 22E

LVH ECG changes — increased voltage with repolarization abnormalities

9. Prolonged QT Interval

Definition: QTc >440 ms (men), >460 ms (women). QTc = QT ÷ √RR (Bazett formula).

Danger: QTc >500 ms significantly increases risk of Torsades de Pointes (polymorphic VT → VF → sudden death).

Causes — "ACUTE":

Antiarrhythmics (amiodarone, sotalol, quinidine)
Cardiac (ischemia, myocarditis)
Underelectrolytes (↓K⁺, ↓Mg²⁺, ↓Ca²⁺)
Toxins/drugs (antipsychotics, macrolides, TCAs)
Endocrine (hypothyroidism)
Congenital long QT syndromes (LQTS1, 2, 3)

10. Sinus Tachycardia / Sinus Bradycardia

Sinus Tachycardia (HR >100 bpm):

Regular rhythm, normal P-waves (upright in II), PR <200 ms
Causes: fever, pain, hypovolemia, anemia, PE, thyrotoxicosis, sympathomimetics
Never a primary diagnosis — always find and treat the cause

Sinus Bradycardia (HR <60 bpm):

Regular rhythm, normal P-waves
Causes: athlete's heart (normal), inferior MI (RCA → SA node), hypothyroidism, hypothermia, beta-blockers, raised ICP (Cushing reflex)
Treat only if symptomatic or hemodynamically compromised

Quick Reference Table

#	ECG Change	Key Feature	Classic Cause
1	ST elevation	Convex ↑, reciprocal depression	STEMI, pericarditis
2	ST depression	Horizontal/downsloping	NSTEMI, LVH strain, digoxin
3	T-wave inversion	Deep symmetrical	Ischemia, Wellens', PE
4	Pathological Q waves	>40 ms, >25% R wave	Old MI / transmural necrosis
5	Atrial fibrillation	Irregularly irregular, no P waves	HTN, valvular disease, thyrotoxicosis
6	AV block	PR prolonged / dropped beats / dissociation	Inferior MI, drugs, degenerative
7	Bundle branch block	QRS ≥120 ms, RSR' or broad R	IHD, PE, cardiomyopathy
8	LVH	High voltage + strain pattern	Hypertension, AS, HCM
9	Long QT	QTc >440–460 ms	Drugs, electrolytes, congenital
10	Sinus brady/tachy	Rate <60 or >100, normal morphology	Systemic illness, athlete, MI

Sources: Harrison's Principles of Internal Medicine 22E; Tintinalli's Emergency Medicine; Fuster & Hurst's The Heart 15E; Goldman-Cecil Medicine

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