ECG Interpretation Pre-Reading


  • This page was created to help avoid common pitfalls for trainees when they are asked to interpret an ECG.
  • Trainees are encouraged to read this page prior to coming to ECG rounds.

Principles of ECG Interpretation

  • If you are asked to interpret an ECG, novices often jump to a particular diagnosis. 
  • Expert readers repeatedly urge learners to avoid doing this.
  • The approach mirrors other areas of medicine, where senior physicians often ask trainees to work through the problem, rather than jump to a diagnosis.
  • The approach is always:
    1. Step 1 – Describe What You See
      • (i.e. “I see a fast rhythm with a narrow QRS”)
    2. Step 2 – Frame the problem
      • (i.e. “This is a narrow QRS tachycardia”)
    3. Step 3 – Formulate Differential Diagnosis
      • (i.e. “Differential includes atrial tachycardia, atrial flutter, …”)
    4. Step 4 – Use an approach to above problem to narrow the differential diagnosis
      (You can even arrange most to least likely)
      • “The tachycardia initiates abruptly on a PAC, which rules out sinus tachycardia.  P-waves are negative in the inferior leads, suggesting retrograde conduction or a low-atrial focus.  The VA interval is very short < 80ms, and the P-wave is fused with the terminal portion of the QRS.  The most likely diagnosis is AVNRT, other items on the differential include orthodromic AVRT, low atrial tachycardia, and atrial flutter.”

Frame the Problem

  • Framing the problem is the most important step because it allows you to develop and use an approach
  • Here are examples of common arrhythmia problems that you can develop an approach to:
    • Narrow QRS Tachycardia
    • Wide QRS Tachycardia
    • Non-Conducted P-waves
    • Extra QRS complexes
    • Bradycardia with a narrow/wide QRS
  • Here are some problems for devices:
    • Lack of Capture
    • Lack of Output
    • Pacing too Fast
    • Inappropriate ICD shock
  • Once you learn the common arrhythmia problems, you can develop an approach to each one of them. 
  • For novice learners, we recommend starting with the most common ones (above list is arranged most common to least)

The Approach To a Problem

  • Algorithmic Approach:
    • The most common approach used by clinicians
    • Published in form of flow-charts that require memorization
    • This works most of the time, but has many drawbacks.
    • Many experts caution against “blindly following an algorithm”
  • Mechanistic Approach:
    • Recommended by many expert electrophysiologists
    • Requires a deeper understanding of the arrhythmia mechanism
    • Theoretically more accurate because it involves starting with a differential diagnosis, and narrowing it using core mechanistic principles.
  • Many learners would often prefer to memorize an algorithm that they can use for rapid diagnosis.  This may be a faster way to learn and improve performance on an exam.  A good analogy is a game of chess: one can often memorize high-yield moves and starting positions, but it will only get you so far.
  • The mechanistic approach is preferred by many experts because it is dynamic, employs deductive reasoning, leads to insightful discussions and often turns arrhythmia diagnosis into fun puzzles. 
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