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Lesson 1: ECG Basics

  • Tooba Alwani
  • Feb 27
  • 2 min read

Updated: Mar 27



Summary of Learning Points

  • The general approach to every ECG: Start with rate and rhythm. Then evaluate P-waves, PR intervals, Axis and QRS complex, QT interval, and lastly look for any Q waves and ST changes. 

  • Rate can be identified by the box method (counting boxes between RR intervals) or counting the total number of complexes in the rhythm strip and multiplying by 6. 

  • Rhythm: Identify if your rhythm is regular or irregular and narrow or wide complex. Differentiate between atrial (is it sinus?) and ventricular rhythm. 

  • P waves: look in lead II and V1. Tall p-wave >2.5mm is RA problem, P-wave with a double hump in lead II or negative component in V1 is an LA problem 

  • PR interval should be between 3 little boxes to 1 big box (0.12 to 0.2 seconds) 

  • Axis: Always check lead I, II and aVF. 

  • When sinus rhythm is present with intact AV conduction wide QRS >3 little boxes (120 ms) suggests a bundle branch morphology. Check leads V1, V6 and I to identify if it’s R or L! 

  • Cornell criteria is the most sensitive criteria for LVH, combine the product of the S wave in V3 + R wave aVL. 

  • Normal QT interval is less than 500 ms. Normally QT interval is less than half the R-R interval. 

  • To identify ST changes always compare to TP segment and assess shape (concave up, down or flat) 

  • Pathological Q waves are wider and deeper than benign Q waves and indicate infarcted myocardium 

  • You can consistently see T wave inversions in aVR and V1 and often find T wave inversions in lead III and V2. 

  • Peaked T waves are often seen in hyperkalemia, differential also includes acute ischemia. 


Practice ECG

Answer

Rate: ~ 150BPM (ventricular), atrial rate 300 

Rhythm: Aflutter 2:1 

P waves: None, flutter waves 

PR interval: N/a 

Axis: Left axis deviation (negative in II, positive in I) 

QRS: Narrow, meets LVH criteria (sum of S wave in V3 + R wave in avL >28mm) 

ST/T changes: Non-specific in distal precordial and lateral leads 

Q waves: Inferior leads 

QT: <500ms 

Interpretation: Aflutter with 2:1 block, LVH with associated non-specific ST/T changes, possible prior inferior wall MI


 
 
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