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Electrocardiogram (ECG) Interpretation

This guide can be used to read an ECG in a systematic manner


We begin by confirming the patient’s details and introducing the ECG for presentation.

  1. “This is a XX second 12-lead ECG trace of…” 
  2. Check Name & Age & Date of exam
  3. Are there presenting complaints written at the top?

Eg. “This is a 10 second 12-lead ECG trace of John Smith, a 60-year-old gentleman, taken 2 hours ago in the Emergency Department. It is noted that he experienced shortness of breath and chest pain at the time of examination”

Rate & Rhythm

Looking at the rate strip (usually lead 2, and located along the bottom of the trace): 

  1. Is the patient in sinus rhythm? – Are there P waves preceding every QRS complex? Are there QRS complexes after each P wave?
  2. What is the rate? Calculate by 300/number of large squares between each R wave, or the number of complexes over the strip x 6 (if it is a 10s strip). 
  3. Is the rate regular, or irregular (regularly irregular or irregularly irregular)? 
  • Notes: 
    • Narrow complex tachycardia – Normal QRS complex duration with an increased heart rate. 
    • Broad complex – extended QRS complex duration with a reduced heart rate 
      • Are they in sinus? If so, it is Ventricular based. If not, Atrial based.
      • Is it a regular rhythm (so Tachycardia) or irregular / not uniform (so Fibrillation)?  
    • Generally – if not in sinus, fast rate, and irregular = Atrial Fibrillation.  

The ECG axis demonstrates the direction of overall electrical activity within the heart. Normally, overall conduction would travel in downward right motion (from R atrium to L ventricle).

Calculating Axis Deviation using the Quadrant Method and Leads I and aVF 

  1. Look at Leads 1 and aVF. Are the QRS complexes Positive, Equiphasic or Negative?
  1. Using the quadrant method to determine overall axis deviation. 
    • Lead 1 – right half is for positive. 
    • Lead aVF – the bottom half is positive.  
  1. If Lead 1 is positive, then it will point towards the right. If aVF is positive, then it will point downwards.  
  2. The average of these 2 (where they overlap) determine axis. In the above diagram, light blue is Left deviation, Pink is right deviation. Dark blue is normal. 
  • Notes: 
    • If Left Axis Deviation – most likely CAD. 
    • If Right Axis Deviation – most likely chronic lung disease or PE 
P Wave & PR Interval
  • We have already observed the P wave during the rhythm check, 
  • P waves correlate to Atrial activity. Absence means loss of atrial activity. 
  • PR Interval should be between 120-200ms (3-5 small squares) 
  • A prolonged interval: 
    • Suggests the presence of atrioventricular delay (AV block
    • A fixed PR interval consistency over 200ms (5 squares) suggests First-Degree Heart Block. 
    • An increasing PR interval until QRS is dropped suggests Type 1 Second-Degree Heart Block 
    • A fixed PR interval with an intermittent drop of QRS suggests Type 2 Second-Degree Heart Block. 
    • Completely random PR intervals (P waves and QRS complexes with no association) is suggestive of Third-Degree Heart Block (complete AV block) 
  • Shortened PR interval: 
    • Can be normal (due to SAN closer to the AV node) 
    • Can indicate impulses are taking a shortcut to the ventricles (will have Delta wave) 
QRS Complex 
  1. Is the complex Narrow (<0.12s) or Broad (>0.12s) 
    • Narrow indicates well-organised conduction and synchronised ventricular depolarisation 
    • Broad indicates abnormal depolarisation. 
      • Can be a Bundle Branch Block, where depolarisation spreads slowly through ventricular myocardium from one ventricle to the other. 
  2. Are they short or tall
  3. Morphology 
    • Q waves 
      • Should be <25% of following R wave or <2mm in height and <40ms in duration. 
    • R and S waves 
      • Progression. In leas V1, the S wave should be dominant compared to the R wave. This should gradually shift so that at around V3 and V4, the R wave is larger than the S wave. 
      • Is the R waved “notched”? 
        • If it’s an M shape in V5/6 – Left Bundle Branch Block 
        • If it’s a W shape in V1 – Right Bundle Branch Block 
ST Segment 
  • A normal ST segment is an isoelectric line between the end of the S wave and the start of the T wave. 
  • Is there ST Elevation? 
    • Greater than 1mm in 2 or more limb leads, or >2mm in 2 or more chest leads 
    • Commonly caused by full-thickness myocardial infarction
  • Is there ST Depression? 
    • >0.5mm depression in 2 leads. 
    • Indicates myocardial ischaemia
T Waves 

T waves represent repolarisation of the ventricles 

  1. Are the waves tall? 
    • >5mm in limb leads and >10mm in chest leads 
    • Can be a sign of Hyperkalaemia or Hyperacute STEMI 
  2. Are the waves Inverted? 
    • They are normally inverted in V1 and Lead III 
    • In other leads, is pathogenic. 
      • Can be a sign of Bundle Branch Block, previous ischaemia, PE, general illness. 
U Waves 
  • Not normally seen. If so, often >0.5mm deflection after the T wave in V2 or V3. 
  • Classically U waves are seen in various electrolyte imbalances, hypothermia and secondary to antiarrhythmic therapy 
Recap and Summarise 
  • Don’t go back through the whole ECG, only mention significant changes that you have observed.
  • You should also present a diagnosis or differential diagnosis during the recap, if you have formed one.
    • Don’t worry if you can’t determine a diagnosis. It is likely that having a diagnosis is only a couple of marks. If you have analysed each wave and picked up on abnormalities, you will have a good standing to pass the OSCE station.

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