10 ecg explanation and explanation conditions in details
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10 Essential ECG Patterns: Explanation & Associated Conditions
How to Read an ECG: Foundations
Before the 10 patterns, a quick reference on the ECG waveform components — sourced from Goldman-Cecil Medicine:
| Wave/Interval | Represents | Normal Value |
|---|---|---|
| P wave | Atrial depolarization | Duration 0.06–0.12 sec; amplitude 2–3 mm |
| PR interval | AV nodal conduction (atria → ventricles) | 120–200 ms (3–5 small boxes) |
| QRS complex | Ventricular depolarization | ≤ 120 ms (narrow) |
| ST segment | Early ventricular repolarization | Isoelectric (flat) at baseline |
| T wave | Ventricular repolarization | Concordant (same direction) with QRS |
| QT interval | Total ventricular electrical cycle | QTc ≤ 440 ms (Bazett's formula) |
| J point | Junction of QRS end and ST start | — |
The standard 12-lead ECG uses 10 electrodes to produce 12 vantage points: 6 limb leads (I, II, III, aVR, aVL, aVF) and 6 precordial leads (V1–V6). Paper speed is 25 mm/sec: each small box = 0.04 sec, each large box = 0.2 sec.
1. Normal Sinus Rhythm (NSR)
ECG Features
- Rate: 60–100 bpm
- P wave: Upright and uniform in leads I, II, aVF; negative in aVR; before every QRS
- PR interval: 120–200 ms (constant)
- QRS: Narrow (< 120 ms)
- Axis: −30° to +90°
- Rhythm: Regular (R-R intervals equal)
Condition
The normal baseline rhythm. The SA node acts as the dominant pacemaker. Slight variations in rate with breathing = sinus arrhythmia (normal variant, especially in young people and athletes).
FIGURE 42-1 from Goldman-Cecil shows a normal 12-lead ECG with rate ~78 bpm, axis +60°, PR 140 ms, QRS 90 ms, QT 360 ms — all normal.
2. ST-Elevation Myocardial Infarction (STEMI)
ECG Features
- ST elevation ≥ 1 mm in ≥ 2 contiguous limb leads, OR ≥ 2 mm in ≥ 2 contiguous precordial leads
- Hyperacute T waves — early sign (tall, peaked, asymmetric)
- Pathological Q waves — develop 1–4 hours after infarct (> 0.04 sec wide, > 25% of R-wave height)
- Reciprocal ST depression in opposite leads
- New LBBB is a STEMI equivalent
Localisation by Lead
| Territory | Leads with STE | Artery Occluded |
|---|---|---|
| Anterior | V1–V4 | LAD |
| Inferior | II, III, aVF | RCA (or LCx) |
| Lateral | I, aVL, V5–V6 | LCx |
| Posterior | ST depression V1–V3 + tall R in V1 | RCA/LCx |
| Right ventricle | RV4 elevation | RCA proximal |
Clinical Significance
STEMI is diagnosed by ST elevation in the clinical context of chest pain. Troponin elevation follows on bloods. Immediate reperfusion (PCI or thrombolysis) is mandatory. Time-to-balloon < 90 minutes is the target. — Tintinalli's Emergency Medicine; ROSEN's Emergency Medicine
3. Atrial Fibrillation (AF)
ECG Features
- Absent P waves — replaced by chaotic, irregular fibrillatory baseline (f-waves at 350–600/min)
- Irregularly irregular R-R intervals — the hallmark
- Narrow QRS (unless aberrant conduction or pre-excitation)
- Rate: ventricular response typically 100–180 bpm if uncontrolled
Conditions Associated
- Hypertension (most common cause)
- Valvular heart disease (especially mitral stenosis)
- Heart failure
- Thyrotoxicosis
- Alcohol ("holiday heart")
- Ischaemic heart disease
- Post-cardiac surgery
Clinical Significance
AF is the most common sustained arrhythmia. Major risk: cardioembolic stroke (5× normal risk). CHA₂DS₂-VASc score guides anticoagulation. Rate control (beta-blockers, CCBs) or rhythm control (cardioversion, flecainide, amiodarone) depending on clinical context. — Guyton & Hall; Goldman-Cecil Medicine
4. Atrial Flutter
ECG Features
- "Sawtooth" flutter waves at ~300/min — best seen in leads II, III, aVF, and V1
- Regular atrial rate ~300/min, ventricular rate typically 150/min (2:1 block)
- Variable block (3:1 or 4:1) gives ventricular rates of 100 or 75 bpm
- Narrow QRS (unless aberrant)
- Absent discrete P waves
Conditions Associated
Similar to AF: hypertension, heart failure, pulmonary disease (COPD, PE), post-cardiac surgery (especially after atrial repairs). Distinguished from AF by the regular sawtooth pattern rather than chaotic fibrillation.
Clinical Significance
Often less stable than AF and frequently converts to AF or sinus rhythm. Rate control, anticoagulation, and cardioversion principles similar to AF. Catheter ablation of the cavotricuspid isthmus is highly effective (>95% cure rate).
5. Ventricular Tachycardia (VT)
ECG Features
- Wide QRS tachycardia (QRS ≥ 120 ms) at rate > 100 bpm (often 150–250 bpm)
- AV dissociation — P waves march independently from QRS (pathognomonic when seen)
- Fusion beats — partial capture of ventricle from sinus beat merging with VT complex
- Capture beats — occasional narrow QRS from sinus capture (confirms AV dissociation)
- QRS morphology: LBBB or RBBB pattern depending on origin
- Axis often extreme ("northwest")
Conditions Associated
- Ischaemic cardiomyopathy (most common — scar-based re-entry)
- Dilated cardiomyopathy
- Hypertrophic cardiomyopathy
- Arrhythmogenic right ventricular cardiomyopathy (ARVC)
- Electrolyte disturbances (hypokalaemia, hypomagnesaemia)
- Post-MI scar
Clinical Significance
Any wide-complex tachycardia should be treated as VT until proven otherwise. Haemodynamically unstable → immediate DC cardioversion. Haemodynamically stable → amiodarone IV, procainamide, or cardioversion. Sustained VT in structural heart disease → ICD.
— Goldman-Cecil Medicine; Tintinalli's
Key rule (Brugada criteria): AV dissociation, concordance across all precordials, and QRS > 160 ms strongly favour VT over SVT with aberrancy.
6. AV Heart Block (1°, 2°, 3°)
6A. First-Degree AV Block
- PR interval > 200 ms (> 5 small boxes), constant
- All P waves conduct; no dropped beats
- Often benign; may be seen with increased vagal tone, inferior MI, digoxin, beta-blockers, athletes
6B. Second-Degree AV Block
Mobitz Type I (Wenckebach):
- Progressive PR prolongation → dropped QRS
- Grouped beating; shortest PR after the dropped beat
- Block usually in AV node; generally benign
- May be seen in inferior MI, athletes, increased vagal tone
Mobitz Type II:
- Fixed PR interval → sudden dropped QRS (no warning)
- QRS often wide (infranodal/bundle branch block)
- High risk of progression to complete heart block
- Requires pacemaker even if asymptomatic
2:1 Block:
- Every other P wave conducts; cannot distinguish Mobitz I from II on ECG alone
- If PR < 160 ms and QRS wide → likely Type II (infranodal)
6C. Third-Degree (Complete) Heart Block
- Complete AV dissociation: P waves and QRS complexes beat independently
- Atrial rate > ventricular rate
- Escape rhythm maintains ventricular output: narrow QRS (junctional, ~40–60 bpm) or wide QRS (ventricular, ~20–40 bpm)
- Wide QRS escape = infranodal block; unstable
- Requires urgent pacing
Conditions Associated
Idiopathic fibrosis (Lev's/Lenègre's disease), inferior MI (RCA supplies AV node), Lyme disease, sarcoidosis, myocarditis, digoxin toxicity, congenital (maternal anti-Ro/La antibodies).
— Harrison's Principles of Internal Medicine 22E; Goldman-Cecil Medicine
7. Bundle Branch Block (RBBB & LBBB)
Key ECG Rule
QRS ≥ 120 ms = complete bundle branch block.
Right Bundle Branch Block (RBBB)
- rSR' (RSR') in V1/V2 — "M" or "rabbit ears" pattern
- Wide S wave in leads I, V5, V6
- ST-T discordance in V1–V2 (T wave inverted)
- QRS axis usually normal
Conditions: Pulmonary embolism (new RBBB = poor prognosis), atrial septal defect, anterior MI, right heart strain, may be normal variant
Left Bundle Branch Block (LBBB)
- Broad notched R ("W") in V5–V6, I, aVL
- QS or rS in V1 (broad negative)
- No septal Q waves in V5/V6
- Discordant ST-T changes throughout
- "WiLLiaM MaRRoW" mnemonic: W in V1, M in V6 = LBBB; M in V1, W in V6 = RBBB
Conditions: Hypertension, coronary artery disease, dilated cardiomyopathy, aortic stenosis. New LBBB in acute chest pain = STEMI equivalent (Sgarbossa criteria apply).
Fascicular Blocks (Hemiblocks)
From the Goldman-Cecil table:
- LAFB (Left anterior fascicular block): QRS < 120 ms, left axis deviation −45° to −90°, qR in aVL
- LPFB (Left posterior fascicular block): Right axis deviation +90° to +180°, rS in I/aVL — Goldman-Cecil Medicine; Harrison's Principles
8. Left Ventricular Hypertrophy (LVH)
ECG Features (Voltage Criteria)
- Sokolow-Lyon: SV1 + RV5 (or RV6) > 35 mm
- Cornell voltage: RaVL > 28 mm (men), > 20 mm (women)
- "Strain" pattern: ST depression + T wave inversion in lateral leads (I, aVL, V5–V6)
- Left atrial abnormality (broad, notched P wave in II, biphasic in V1) — increases LVH likelihood
Limitations
Low sensitivity (~50%) — especially in obese, elderly, smokers, and those with RBBB. Echocardiography is definitive.
Conditions Associated
- Hypertension (most common)
- Aortic stenosis
- Hypertrophic cardiomyopathy
- Coarctation of the aorta
Clinical Significance
ECG evidence of LVH is an independent marker of increased cardiovascular morbidity and mortality, including sudden cardiac death.
— Harrison's Principles of Internal Medicine 22E
9. Long QT Syndrome (LQTS)
ECG Features
- QTc > 440 ms (males), > 460 ms (females)
- Calculated using Bazett's formula: QTc = QTm / √(R-R interval)
- T wave morphology abnormalities: notched T (LQT2), late-onset T (LQT3)
- May degenerate into Torsades de Pointes — polymorphic VT with twisting QRS axis around the baseline
Types
| Type | Gene | Trigger |
|---|---|---|
| LQT1 | KCNQ1 | Exercise/swimming |
| LQT2 | KCNH2 | Auditory stimuli, sleep |
| LQT3 | SCN5A | Sleep/rest bradycardia |
Acquired Causes
Hypokalaemia, hypomagnesaemia, hypocalcaemia, ischaemia, CNS injury, drugs (antiarrhythmics — class Ia/III, antipsychotics, fluoroquinolones, azithromycin, antihistamines). Check QTdrugs.org for updated drug list.
Genetic Forms
- Autosomal recessive with deafness: Jervell–Lange-Nielsen syndrome
- Autosomal dominant without deafness: Romano-Ward syndrome
Management
- Avoid QT-prolonging drugs and high-intensity sport
- Beta-blockers for prophylaxis
- ICD for syncope or VF/torsades despite beta-blockade — Tintinalli's Emergency Medicine
10. Wolff–Parkinson–White (WPW) Syndrome & Pre-excitation
ECG Features
- Short PR interval < 120 ms (< 3 small boxes) — accessory pathway bypasses AV node delay
- Delta wave — slurred upstroke at the start of QRS
- Wide QRS > 120 ms — fusion of delta wave with normal QRS
- Discordant ST-T changes — ST/T in opposite direction to delta/QRS
- Mimics RBBB, LBBB, or LVH depending on lead
Classification
- Type A: Positive delta in V1 (left-sided pathway)
- Type B: Negative delta in V1 (right-sided pathway)
Arrhythmias in WPW
- Orthodromic AVRT (narrow QRS, 70%): Anterograde through AV node, retrograde via accessory pathway
- Antidromic AVRT (wide QRS, 5–10%): Anterograde via accessory pathway — mimics VT exactly
- AF with pre-excitation: Rapid, irregular, wide-complex AF with varying QRS morphologies — rates can exceed 300 bpm → risk of VF and sudden death
Critical Warning
Never give AV-nodal blocking agents (adenosine, verapamil, diltiazem, digoxin) in WPW + AF — they may accelerate accessory pathway conduction → VF.
Management
- Haemodynamically unstable: DC cardioversion
- Haemodynamically stable: procainamide IV or ibutilide
- Curative: catheter ablation of accessory pathway (>95% success rate) — Tintinalli's Emergency Medicine; ROSEN's Emergency Medicine
Quick Reference Summary Table
| # | ECG Pattern | Key Feature | Key Condition |
|---|---|---|---|
| 1 | Normal Sinus Rhythm | Regular, P before every QRS, rate 60–100 | Baseline normal |
| 2 | STEMI | ST elevation ≥ 1 mm, 2 contiguous leads, Q waves | Acute coronary artery occlusion |
| 3 | Atrial Fibrillation | Absent P waves, irregularly irregular | Hypertension, valvular disease |
| 4 | Atrial Flutter | Sawtooth at 300/min, 2:1 block = 150 bpm | Pulmonary disease, post-cardiac surgery |
| 5 | Ventricular Tachycardia | Wide QRS > 120 ms, AV dissociation, rate > 100 | Ischaemic cardiomyopathy, scar re-entry |
| 6 | AV Block (1°/2°/3°) | PR prolongation / dropped beats / AV dissociation | Fibrosis, inferior MI, Lyme disease |
| 7 | Bundle Branch Block | Wide QRS; RSR' in V1 (RBBB); broad R in V6 (LBBB) | PE (RBBB), hypertension, CAD (LBBB) |
| 8 | LVH | SV1+RV5 > 35 mm, strain pattern | Hypertension, aortic stenosis |
| 9 | Long QT / Torsades | QTc > 440–460 ms | Drugs, electrolytes, channelopathy |
| 10 | WPW (Pre-excitation) | Short PR, delta wave, wide QRS | Accessory pathway (bundle of Kent) |
Sources: Goldman-Cecil Medicine 2-Vol Set | Harrison's Principles of Internal Medicine 22E (2025) | Tintinalli's Emergency Medicine | ROSEN's Emergency Medicine | Harrison's Principles (bundle branch block section)
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