Cardiac Arrhythmias and Conduction Abnormalities on ECG
Cardiac Arrhythmias and Conduction Abnormalities
This domain tests rhythm recognition. First decide whether the rhythm is regular, whether every QRS has a P wave, and whether the QRS is narrow or wide.
Core patterns
- Sinus rhythm: upright P in lead II, regular rate 60-100/min, constant PR.
- Sinus bradycardia is under 60/min; sinus tachycardia is over 100/min.
- Atrial fibrillation: irregularly irregular QRS, no distinct P waves. Atrial flutter: sawtooth flutter waves.
- PVCs are premature, wide and often followed by a compensatory pause.
- First-degree AV block: PR over 0.20 s. Mobitz I: progressive PR lengthening before a dropped beat. Mobitz II: constant PR with sudden dropped QRS. Complete AV block: P and QRS dissociate.
Worked example
For a regular rhythm, count large squares between R waves. If R-R is 4 large squares, rate = 300/4 = 75/min. Reasoning: at 25 mm/s, one large square is 0.20 s, and 300 large squares pass per minute; dividing 300 by the number of large squares gives the rate.
Traps that cost marks
Do not call every irregular rhythm atrial fibrillation; sinus arrhythmia and ectopics also cause irregularity. A wide QRS does not always mean ventricular tachycardia. In Mobitz II, PR stays constant before the dropped beat. Confirm rhythm in more than one lead when possible.