PAC recognition: early narrow beat with different P-wave morphology
A premature atrial contraction (PAC) originates from an ectopic atrial focus outside the SA node. ECG features: (1) Early (premature) beat — arrives before the next expected sinus beat. (2) Different P-wave morphology — the ectopic P wave differs in shape, axis, or amplitude from sinus P waves because it activates the atria from a different starting point. (3) Narrow QRS — ventricular conduction is normal via the His-Purkinje system, producing a narrow QRS identical to sinus beats (unless aberrant conduction occurs). (4) Non-compensatory pause — the SA node is reset by the PAC, so the interval from the PAC to the next sinus beat is not full-compensatory.
Clinical significance: isolated PACs are benign in otherwise healthy patients. In neonates and infants, frequent PACs may trigger SVT — monitor closely. Frequent PACs (>1% of beats) warrant clinical evaluation.
PVC recognition: wide bizarre premature beat with compensatory pause
A premature ventricular contraction (PVC) originates from an ectopic ventricular focus. ECG features: (1) Early (premature) beat. (2) Wide, bizarre QRS (≥120 ms) — depolarization bypasses the normal His-Purkinje network, producing slow cell-to-cell conduction. (3) Morphology opposite to the sinus QRS — discordant ST-T changes (T wave opposite in direction to QRS). (4) Compensatory pause — the SA node is NOT reset; the post-PVC R-R interval is fully compensatory (the PVC-to-next-sinus R-R = 2× normal R-R).
PVC patterns: unifocal PVCs (all look identical — one ectopic focus); multifocal PVCs (different morphologies — multiple foci — more concerning); bigeminy (PVC every other beat); trigeminy (PVC every third beat); couplets (two consecutive PVCs — non-sustained VT); salvos (3+ consecutive PVCs = non-sustained VT — escalate immediately).