ECG 101

Reading Time: 2 minutes
Picture of Alyx Presler
Alyx Presler
Emergency department nurse turned to the CVICU dark side. Revels in the airway, FOAMed, hiking, and hanging out with a black lab named Blue Gill.

The Pre-brief

Reading 12-lead electrocardiograms (ECG/EKGs) is a skill found in various nursing and healthcare specialties. It can be tricky, but this is meant to be a basic walkthrough on reading ECGs. 

Let’s start with the anatomy of an ECG!

Small boxes (1mm) = 0.04 seconds

Large box (5mm) = 0.2 seconds

Large boxes X 5 = 1 second

There are 50 large boxes per standard EKG print out, 10 seconds total.

P wave – Represents right and left atrial depolarization/contraction

QRS Complex – Represents right and left ventricular depolarization/contraction

T wave – Represents ventricular repolarization/electrical recovery

U wave – Represents after-repolarization of ventricles, rarely present >95 bpm

Normal Intervals (important ones in bold): 

P wave: 0.08 – 0.11s

PR interval: 0.12 – 0.20s

Q wave: <0.04s 

QRS complex: 0.07 – 0.10s

ST segment: 0.08 – 0.12s

T wave: 0.10 – 0.25s 

QT interval: 0.36 – 0.44s

QTc (rate-corrected QT interval): <0.44s in men and <0.46s in women

What is sinus rhythm?

  1. P wave before EVERY QRS complex
  2. Positive/upright P wave in leads I and II
  3. Negative/inverted P wave in lead aVR
  4. P waves are round and uniformly shaped throughout the ECG
  5. Fixed and within normal measurement PR intervals

When interpreting an ECG, it is important to assess each one strategically and the same way each time. Here are the steps to take when walking through an ECG:

  1. Rate – Normal 60 – 100 bpm

Bradycardia <60 bpm

Tachycardia >100 bpm

  1. Rhythm – Normal sinus rhythm (see above)? Sinus bradycardia/tachycardia?
  2. Regular or irregular? Map out R-R and P-P for ventricular and atrial regularity, respectively.
  3. Is the QRS wide or narrow?
  4. Is the PR interval within 0.12-0.20s? 
  • A prolonged PR indicates a delay through the AV node and thereby a heart block, and an abnormally narrow PR can be associated with Wolff Parkinson White (WPW) Syndrome.
  1. ST-segment – elevated, depressed, or isoelectric?
  2. T wave upright and following each QRS? Any biphasic or inverted T waves?

The Debrief:

  • Know the definition of sinus rhythms
  • Follow the same algorithm every time you assess an ECG. Make it habitual!
  • Walk yourself through as many ECGs as possible, practice makes improvement.


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