ECG Interpretation and Rhythm Recognition
Core concepts
Use a systematic method every time: rate, rhythm (regular/irregular), P waves and their relationship to QRS, PR interval, QRS width, axis, and ST/T changes. Rate can be estimated by the 300-150-100-75-60-50 sequence for regular rhythms or by counting QRS complexes in a 6-second strip × 10. Normal intervals: PR 0.12–0.20 s (3–5 small boxes), QRS <0.12 s, QT corrected for rate.
Recognize the core rhythms: sinus bradycardia/tachycardia, atrial fibrillation (irregularly irregular, no discrete P waves), atrial flutter (sawtooth flutter waves, often 2:1 conduction near 150 bpm), PSVT (regular narrow-complex, sudden onset), junctional rhythm (narrow QRS, absent/retrograde P), and ventricular rhythms (wide QRS: VT, VF, idioventricular). Distinguish first-degree AV block (prolonged PR), second-degree type I (progressive PR lengthening, Wenckebach), type II (constant PR with dropped beats), and third-degree (complete AV dissociation with P and QRS independent).
Question traps
- Calling an irregularly irregular narrow-complex rhythm anything other than AF.
- Missing a 2:1 flutter because the flutter waves hide in the T wave - look for a rate near 150 bpm.
- Confusing Wenckebach (usually benign, nodal) with Mobitz II (higher risk of progression, often infranodal).
- Forgetting that a wide-complex tachycardia is VT until proven otherwise.
Rule of thumb
Any regular narrow-complex tachycardia at ~150 bpm should prompt a search for atrial flutter with 2:1 block. Any wide-complex tachycardia should be treated as VT.