Electrocardiography and Rhythm Recognition
Build every rhythm interpretation on a systematic method: rate, rhythm regularity, P waves, PR interval, QRS width, and relationship of P to QRS. Know the normal values: PR 0.12–0.20 s, QRS under 0.12 s, QT rate-dependent. Understand which lead views which wall and why the 12-lead is organized into limb and precordial leads.
You must recognize sinus rhythms, atrial fibrillation and flutter, supraventricular tachycardia, premature atrial and ventricular complexes, junctional rhythms, and the AV blocks (first degree, second degree type I and II, third degree). Know ventricular tachycardia and ventricular fibrillation as wide-complex or chaotic lethal rhythms, and identify asystole and pulseless electrical activity as non-shockable.
Traps: calling any wide-complex tachycardia "SVT"; missing that a regular narrow-complex tachycardia with hidden P waves may be SVT; confusing Mobitz I (progressively lengthening PR) with Mobitz II (constant PR with dropped beats); and labeling artifact as a rhythm. Another frequent error is measuring rate from an irregular rhythm using the 300 method instead of counting complexes over a time interval.
Rule of thumb: "Narrow and regular with P before every QRS = sinus or SVT; wide and fast = treat as VT until proven otherwise." For irregular rhythms, count R waves in a 6-second strip and multiply by 10.
Worked example: a strip shows a QRS every 4 large boxes. 300 ÷ 4 = 75 bpm. If P waves precede each QRS with a constant PR, it is normal sinus rhythm at about 75 bpm.