By

Dr. Fibhaa Syed 
MBBS, FCPS, MRCP, SC Endocrinology
Assistant Professor of Medicine
Shaheed Zulfiqar Ali Bhutto Medical University,
Pakistan Institute of Medical Sciences


Following steps should be follow while interpreting ECG. Details may be found elsewhere, in this article brief outlines regarding ECG interpretation are shared, which were explained by Assistant Professor of Medicine, Dr. Fibhaa Syed during a Post-Graduate teaching session at Shaheed Zulfiqar Ali Bhutto Medical University, Islamabad Pakistan.

Please note Normal Criteria is shared here unless specified otherwise.

ECG Interpretation Steps

  • Identification

    • Name, 
    • Date and Time

  • Calibration 

    • Normally
      • Width = 5 squares (1 big square), (one small square = 0.04 seconds)
      • Height = 10 small squares (1 small square = 0.1mV, 10 small squares = 1mV

  • Rate

    • 300 divided by Number of of large squares between two QRS complex = Rate / min

  • Rhythm

    • Check in lead II. 
      • QRS should be preceded by the P Wave. 
      • Check if Regular or Irregular
      • Check if P wave present or not. 

  • P wave morphology 

    • Criteria of normality
      • Check in lead 2.
        • should not be broader than 3 small squares
        • should not be taller than 2.5 small squares. 
      • Check in V1 as well
        • Negative component should not exceed the positive component. 

  • PR interval

    • Normal: 3 - 5 small squares

  • Cardiac axis

  • Q waves

    • Abnormal when
      • Width is more than 1 small square, 
      • Depth is more than is 1/4th of the R wave. 

  • QRS complex

    • Duration
      • Should not be more than 2.5 small squares. 
    • Voltage : check height in aVL and aVF
      • aVL: not more than 13 small squares
      • aVF: not more than 20  small squares
      • Tall R waves V1
      • If height is more than 3-4 small squares it will termed as tall R wave in V1.
    • Progression of R wave 
      • Normal Progression if 
        • Usually V5 is the tallest 
        • V5 is tallest and V6 is smaller
        • V4 is taller than V5 which is taller than V6. 
        • If V4 is taller than V5 and V5 is also smaller than V6 this is abnormal. 
    • Transition zone, 
      • When 1st positive becomes the first negative, Normally in V3 or V4. 
    • QRS waves in chest leads. 
      • Voltage criteria, 
        • R wave should not be taller than 27 small squares
        • S wave should not be deeper than 30 small squares
        • and Sum of R and S should not be more than 40 small squares
    • Ventricular activation time 
      • Beginning of Q wave to the tip of R wave 
      • Check in V5 and V6 and it should not be more than 1 small square. 

  • T waves;

    • In Limb leads: 
      • should follow QRS complex, if is not following then calculate T wave axis and Compare T wave axis with cardiac axis, If difference is more than 45 then it is pathological. 
    • Check T waves in chest leads. 
      • Height should not be more than 2/3rd of the height of the R wave
      • Generally upright but;
        • In 30% inverted in V1
        • In 10% inverted in V1 and V2
        • and in 1% inverted in V1, V2 and V3 (west Indian heart) 
          • should have previous ECG present to compare in above 3 cases. 
        • Generally T wave inversion in V4, V5 and V6 is abnormal. 

  • ST changes: 

    • Elevation and depressions (details will be added later)

  • QT interval

    • Normal 14 small squares.

RBBB criteria:


  1. Broad QRS > 2.5 small squares
  2. Secondary R wave in V1
    • Additional Criteria
      • ST depression 
      • T wave inversions in V1 to V4
      • Deep S wave in V1 and aVL

LBBB Criteria


  1. Broad QRS > 3 small squares
  2. Absence of secondary R wave in V1
  3. Normal Axis
  4. ST-T changes in V5 V6, lead 1 and aVL. 

Left ventricular hypertrophy


  1. Tallest R wave > 20 small squares
  2. Deepest S wave > 30 small squares 
  3. Sum of both more than 40 small squares
  4. Ventricular activation time > 1 small square 


Right ventricular hypertrophy 


  1. Frontal axis is more than 90
  2. Dominant R wave in V1 
  3. Tall R Wave in V1, that is greater than  3 - 4 small squares. 

Left axis deviation causes


  1. left anterior hemiblock
  2. LBBB
  3. Hyerkalemia

Right axis deviation causes


  1. RVH
  2. left posterior hemiblock 
  3. inferior wall MI. 

Disclaimer:
Article is under review and is not final. Errors may be present.

Special thanks to Prof. Dr. Rauf Niazi. 
and Dr. Salman for providing the missing data. 

Dr. Adil Ramzan
MBBS, MD Internal Medicine Resident,
Shaheed Zulfiqar Ali Bhutto Medical University
Pakistan Institute of Medical Sciences. 

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