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CRAT: Rhythm Analysis: Systematic Strip Interpretation and Core Rhythm Criteria

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Rhythm Analysis: Systematic Strip Interpretation and Core Rhythm Criteria

The Rhythm Analysis domain tests whether you can identify a rhythm from a monitor strip or 12-lead ECG and defend the label with measured criteria rather than pattern guessing. Expect items on sinus rhythms, atrial and junctional rhythms, AV blocks, premature beats, wide-complex tachycardia, and lethal rhythms such as ventricular fibrillation.

Use the same sequence every time

  • Rate: use the 6-second method (QRS complexes × 10) for irregular rhythms; use the 1500 method (1500 ÷ small boxes between R waves) or the 300-150-100-75-60-50-43 sequence for regular rhythms.
  • Regularity: compare R-R intervals; a variation greater than 0.12 s (three small boxes) is irregular.
  • P waves: present, uniform, upright in lead II, and one before each QRS in sinus rhythm.
  • PR interval: 0.12–0.20 s and constant.
  • QRS duration: narrow (<0.12 s) versus wide (≥0.12 s).
  • P-to-QRS relationship: is every P conducted, and does every QRS have a P wave?

Rhythm criteria that earn marks

Sinus bradycardia is <60 bpm and sinus tachycardia >100 bpm. Sinus arrhythmia shows phasic R-R variation with breathing but uniform P waves. Atrial flutter shows sawtooth flutter waves at an atrial rate near 300, with 2:1, 3:1, or 4:1 AV conduction. Atrial fibrillation is irregularly irregular with no distinct P waves. Multifocal atrial tachycardia is irregular with at least three P-wave morphologies and a rate >100. Junctional rhythm is 40–60 bpm with a narrow QRS and absent or retrograde P waves. Idioventricular rhythm is 20–40 bpm and wide. A PVC is wide, lacks a preceding P wave, and usually has a full compensatory pause. Ventricular tachycardia is three or more consecutive PVCs at >100 bpm.

AV blocks

First-degree block: PR >0.20 s, constant, with every P conducted. Mobitz I: PR lengthens progressively until a QRS is dropped. Mobitz II: PR stays constant and a QRS drops suddenly. Third-degree block: P waves and QRS complexes are dissociated, with the atrial rate faster than the escape rate.

Worked example

A regular narrow-complex strip has an R-R interval of about 10 small boxes and sawtooth waves between QRS complexes. 1500 ÷ 10 = 150 bpm ventricular rate. The flutter waves are about five small boxes apart, giving an atrial rate of 300 bpm. That is 2:1 conduction, so the answer is atrial flutter with 2:1 block-not sinus tachycardia, because sinus P waves would be discrete and upright, not sawtooth.

Traps

  • Labeling any irregular narrow-complex rhythm as atrial fibrillation; MAT and flutter with variable block also qualify.
  • Missing 2:1 flutter because one flutter wave is buried in each QRS or T wave.
  • Calling a wide-complex tachycardia VT without checking for fusion or capture beats, concordance, or a preceding P wave.
  • Confirming asystole from one lead without checking leads, gain, and a second lead.
  • Forgetting that junctional P waves may be inverted and can sit before, during, or after the QRS.