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The QRS complex represents ventricular depolarization - the rapid electrical activation of the left and right ventricular myocardium. It is the most prominent waveform on the ECG and reflects the spread of electrical impulse through the ventricular conduction system (bundle branches and Purkinje fibers) and the ventricular muscle mass.
Understanding QRS morphology is essential for interpreting:
- Conduction system integrity: Bundle branch blocks, fascicular blocks, accessory pathways
- Chamber abnormalities: Ventricular hypertrophy (LVH, RVH)
- Prior myocardial injury: Pathologic Q waves indicating old infarction
- Rhythm origin: Supraventricular (narrow QRS) vs ventricular (wide QRS)
- Metabolic/toxic effects: Hyperkalemia, sodium channel blockers, antiarrhythmics
Key concept: The QRS complex is your window into ventricular electrical activation. Width, morphology, and amplitude patterns unlock diagnoses ranging from conduction disease to chamber remodeling to life-threatening arrhythmias.
QRS Nomenclature
Individual QRS components are labeled by specific conventions:
- Q wave: First negative (downward) deflection
- R wave: First positive (upward) deflection
- S wave: Negative deflection following an R wave
- Capital vs lowercase: Capital letters (Q, R, S) denote large deflections; lowercase (q, r, s) denote small deflections
- R' (R-prime): Second positive deflection (as in rSR' pattern)
- QS complex: Entirely negative deflection with no R wave
Example nomenclature:
- qRs: small q, large R, small s
- rSR': small r, large S, large R' (classic RBBB in V1)
- QS: entirely negative, no R wave (seen in anterior MI or V1 normally)
Normal QRS Measurements
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| Parameter | Normal Range | Clinical Significance |
|---|---|---|
| Duration | <0.12 seconds (3 small boxes) | ≥0.12 sec = wide QRS (bundle branch block, ventricular rhythm, drug effect) |
| R wave amplitude (V5-V6) | 5-26 mm | Tall R waves may indicate LVH; low voltage (<5 mm) suggests pericardial effusion, COPD, obesity |
| R wave progression | R wave increases V1→V6; transition zone V3-V4 | Poor R wave progression suggests anterior MI; early transition can indicate RVH or posterior MI |
| Q wave (physiologic) | <0.04 sec wide, <25% R wave height | Pathologic Q waves (wide/deep) indicate prior transmural MI |
Normal QRS Morphology by Lead
- V1 (right precordial): Predominantly negative (rS or QS); small r reflects septal depolarization left→right
- V2: Similar to V1, often with deeper S wave
- V3-V4 (transition zone): R and S waves approximately equal (equiphasic); marks electrical "transition" from right to left
- V5-V6 (left precordial): Predominantly positive (qR or R); reflects dominant left ventricular mass
- Limb leads: Variable based on cardiac axis; typically upright in I and II in normal axis
R wave progression: The R wave normally grows taller from V1 to V6 as the electrode "sees" more of the left ventricle. Loss of this progression (poor R wave progression) suggests anterior wall pathology.
Bundle branch blocks occur when conduction is delayed or blocked in the left or right bundle branch, causing sequential rather than simultaneous ventricular activation. This produces a wide QRS (≥0.12 seconds) with characteristic morphology patterns.
Right Bundle Branch Block (RBBB)
Pathophysiology: Right ventricle depolarizes late (after left ventricle), via slow cell-to-cell conduction.
ECG Criteria:
- QRS duration ≥0.12 seconds
- V1-V2: rSR' pattern ("M-shaped" or "rabbit ears") - terminal R' represents delayed RV activation
- I, aVL, V5-V6: Wide, slurred S wave (represents late RV depolarization moving away from lateral leads)
- ST-T changes: Discordant (opposite direction to terminal QRS) - normal finding in RBBB
Common causes:
- Normal variant (especially in athletes, young patients)
- Right ventricular strain (PE, pulmonary hypertension, cor pulmonale)
- Structural heart disease (cardiomyopathy, congenital defects)
- Ischemic heart disease
- Degenerative conduction system disease (Lenegre-Lev disease)
Memory aid - "WiLLiaM MaRRoW":
- WiLLiaM: LBBB has W in V1, M in V6
- MaRRoW: RBBB has M in V1, W in V6
Left Bundle Branch Block (LBBB)
Pathophysiology: Left ventricle depolarizes late via slow cell-to-cell conduction from right ventricle, altering normal septal depolarization.
ECG Criteria:
- QRS duration ≥0.12 seconds
- V5-V6, I, aVL: Broad, monophasic R wave (often notched or "M-shaped") - no q wave
- V1-V2: Deep QS or rS complex
- Absent septal Q waves: No small q in lateral leads (I, aVL, V5-V6) because septal depolarization is reversed
- ST-T changes: Discordant (ST depression/T inversion in leads with tall R; ST elevation in leads with deep S)
Common causes:
- Ischemic heart disease (most common)
- Hypertensive heart disease / LVH
- Cardiomyopathy (dilated, hypertrophic)
- Aortic valve disease (stenosis, regurgitation)
- Degenerative conduction system disease
Clinical significance: LBBB is almost always pathologic and suggests underlying structural heart disease. New LBBB in the setting of chest pain = STEMI equivalent (Sgarbossa criteria for MI diagnosis).
Incomplete Bundle Branch Block
- Definition: QRS duration 0.10-0.119 seconds with typical RBBB or LBBB morphology
- Significance: Represents partial conduction delay; may progress to complete block
- IRBBB: Very common, often benign in young patients
- ILBBB: Less common; consider similar causes as complete LBBB
- Normal QRS duration: <0.12 seconds (less than 3 small boxes)
- QRS represents: Ventricular depolarization - the electrical activation of both ventricles
- Wide QRS (≥0.12 sec): Think bundle branch block, ventricular pacing, hyperkalemia, drugs (TCA, antiarrhythmics), ventricular tachycardia
- RBBB pattern: rSR' in V1 (M-shaped or "rabbit ears"), wide S in I and V6
- LBBB pattern: Broad monophasic R in I, aVL, V5-V6; no septal Q waves; ST-T discordance
- Q waves: Small physiologic q waves are normal; pathologic Q waves (≥0.04 sec wide or >25% R wave height) suggest prior MI
- Key pearl: QRS morphology reveals conduction system integrity, chamber hypertrophy, and prior infarction patterns
"WiLLiaM MaRRoW" - Bundle branch mnemonic: LBBB shows W pattern in V1 and M pattern in V6. RBBB shows M pattern in V1 and W pattern in V6. This simple memory tool helps you instantly recognize bundle branch blocks.
"Wide is worry": A wide QRS (≥0.12 sec) in the setting of tachycardia demands immediate attention. Assume ventricular tachycardia until proven otherwise - it's safer to treat VT as VT than to dismiss it as SVT with aberrancy.
"New LBBB = STEMI equivalent": New or presumably new LBBB in a patient with chest pain is a STEMI equivalent and requires immediate cath lab activation. Don't wait for biomarkers - the ECG diagnosis is sufficient for reperfusion therapy.
Sgarbossa criteria saves lives: In LBBB, use Sgarbossa criteria to diagnose MI: (1) ST elevation ≥1mm concordant with QRS, (2) ST depression ≥1mm in V1-V3, or (3) ST elevation ≥5mm discordant with QRS. Concordance is the most specific finding.
"RBBB can be normal, LBBB rarely is": RBBB is common in healthy individuals, especially athletes and young people. LBBB almost always indicates underlying structural heart disease - don't dismiss it as benign.
"R wave progression tells a story": Poor R wave progression (small R waves V1-V4) suggests anterior MI, but consider mimics: COPD, lead misplacement, LVH, normal variant in young women. Early transition (tall R in V1-V2) suggests RVH or posterior MI.
Q wave width matters more than depth: When evaluating Q waves for MI, focus on width first. A Q wave ≥0.04 seconds (1 small box) wide is more significant than depth. Q waves ≥25% of R wave height are also pathologic.
Lead III is a liar: Isolated Q waves in lead III are unreliable and often represent normal variation or positional changes. Always confirm inferior Q waves in leads II and aVF before diagnosing inferior MI.
"Low voltage has high differential": Low QRS voltage (<5mm in limb leads or <10mm in precordial leads) suggests pericardial effusion, COPD, obesity, hypothyroidism, amyloidosis, or anasarca. Context and clinical presentation guide diagnosis.
Transitional zone matters: Normal transition is V3-V4 (where R=S). Early transition (V1-V2) suggests RVH or posterior MI. Late transition (V5-V6) suggests LVH or anterior MI. Clockwise/counterclockwise rotation describes these shifts.
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