QRS complex: what it represents and normal duration
The QRS complex represents ventricular depolarization — the electrical activation of both ventricles from the His-Purkinje network outward through the ventricular myocardium. Normal QRS duration: < 120 ms (< 3 small boxes). A narrow QRS indicates normal conduction through the His-Purkinje system. A wide QRS indicates either abnormal conduction pathways (bundle branch block, pre-excitation) or ventricular origin (ectopic focus in the myocardium, ventricular tachycardia).
QRS components: Q wave (initial negative deflection, representing septal depolarization left-to-right), R wave (dominant positive deflection), S wave (terminal negative deflection after R). Not all leads show all three components. In lead V1, a normal pattern is rS (small r, deep S). In lead V6, the pattern is qRS or Rs.
R-wave progression: R waves should increase in amplitude from V1 to V5 (called 'normal R-wave progression'). Poor R-wave progression — small R waves persisting to V4 — suggests prior anterior MI, LBBB, left ventricular hypertrophy, or COPD.
Wide QRS: bundle branch block, VT, and the most important clinical rule
Wide QRS (≥ 120 ms) differential: RBBB (right bundle branch block): rSR' in V1 ('rabbit ear'), wide S wave in V5–V6. LBBB (left bundle branch block): broad notched R in V5–V6, QS in V1, no septal Q in V5–V6. VT (ventricular tachycardia): wide QRS tachycardia with AV dissociation, capture beats, or fusion beats.
The most critical clinical rule: wide-complex tachycardia = VT until proven otherwise. Never assume SVT with aberrancy as the first diagnosis. The consequence of treating VT as SVT (with verapamil) can be fatal. Apply the Brugada 4-step algorithm: (1) Absence of RS complex in any precordial lead → VT. (2) RS interval > 100 ms → VT. (3) AV dissociation → VT. (4) RBBB morphology criteria (monophasic R in V1, rS in V6) → VT.