Rhythm Recognition and ECG Interpretation
Use a fixed sequence, every time
Rate → regularity → P waves → PR interval → QRS width → relationship of P to QRS. Candidates lose points by jumping straight to a diagnosis. Estimate rate three ways: 300 ÷ number of large boxes between R waves, 1500 ÷ small boxes, or count QRS complexes in a 6-second strip × 10. Normal PR is 0.12–0.20 s (3–5 small boxes); normal QRS is under 0.12 s (3 small boxes).
The rhythms that matter
- Sinus bradycardia (<50/min in the ACLS algorithm) and sinus tachycardia (fast, regular, one P per QRS).
- Narrow-complex tachycardia: SVT/AVNRT (regular, no visible P), atrial fibrillation (irregularly irregular, no P), atrial flutter (sawtooth, atrial rate ~300).
- Wide-complex tachycardia: ventricular tachycardia (regular, wide, monomorphic or polymorphic), SVT with aberrancy, pre-excited AF, hyperkalemia.
- Arrest rhythms: VF (chaotic, no discrete QRS), pulseless VT (fast, wide, regular), PEA (organized rhythm with no palpable pulse), asystole (no electrical activity).
Traps in questions
Fine VF mistaken for asystole - always confirm with a second lead, check gain and connections. A regular wide-complex tachycardia is treated as VT until proven otherwise; never give verapamil or adenosine to an undifferentiated wide irregular rhythm. 2:1 flutter hides its second P wave in the T wave. Artifact from chest compressions can mimic VF.
Rule of thumb
Rate = 300 ÷ large boxes. If the QRS is wide and the patient is unstable, the diagnosis is VT and the treatment is electricity.