ECG Interpretation Crash Course

Pre-Procedure Steps


  • Paper strip
  • Electrocardiogram machine
  • Graph caliper

Procedure Steps (Use a systemic approach)

  • Identify the P waves, QRS, T or U wave
  • Assess the ST segments and R wave progression
  • Identify the axis, lead 1 and AVF
  • Note that P wave is equal to atrial contraction
    Identify any bundle branch block with a wide QRS complex with at least 3 small squares.
  • Note for any infarction or hypertrophy (Check lead V1-V6 for T-wave inversion).
  • Atrial Fibrillation Medical Condition



  • Assess/count the heart rate. Note that 1 big box = 0.2 seconds, 5 big boxes = 1 second & 30 big boxes = 6 seconds

Assess patient’s heart rate (two methods)

6-Second (for regular & irregular rhythm)

  • Count the number of QRS complexes in 6 seconds (i.e., 30 large boxes) and multiply by 10 to determine the rhythm.
Figure 7.2 Count the number of boxes between two R waves.
  • Count the number of boxes between two R waves and divide into 300 (this approach is best for regular rates). (Figure 7.2)

The Triplet Rule

  • Another approach to rapidly determine the heart rate is to memorize the numbers 300, 150, 100, 75, 60 and 50.
  • Count from the first R wave that peaks from a heavy black line which is the start line. Count off 5 boxes starting with 300, then next 5 boxes with 150, 100, 75, 60 and 50 respectively to the next R. (Figure 7.3)


Figure 7.3 Count the next box from first R wave 300, 150,100 & 75).


Determine the heart rhythm

  • Use your graph caliper, measure the distance between the R waves and compare to ensure the ventricular rhythm is regular, Then, compare the distance between the P-to-P waves to ensure the distance is the same and to ensure the atrial rhythm is regular.
  • P wave
    Measure the P-wave to ensure they are present and precede each QRS complex. The P wave should be less than 2.5mm in height and < 0.11 second in duration. Absent P waves signified that they must originate from either the AV node or the ventricles and not the SA node.
Figure 7.4 PR Interval

PR interval

Measure the PR interval to ensure it is normal (i.e., 0.12-20 seconds) (Figure 7.4). A PR interval greater than 0.20 seconds shows that there is a delay as the impulse passes through the AV node. A PR interval less than o.12 shows that the impulse may have originated from an area closer to the AV node or AV junction or passed through an abnormal pathway.

QRS complex

Assess the QRS complex to determine if they are wide or narrow. A wide QRS complex is greater than 0.12 second and suggests that the impulse originated from ventricles. A narrow PRS complex suggests that the impulse originated from above the AV node.


  • Identify the QRS complex for normal, right or left axis deviation.
  • Locate isoelectric QRS in a limb lead of axis quadrant.
  • Identify the axis rotation in chest leads in the horizontal plane and locate isoelectric QRS.


  • Identify any Q waves, inverted T waves or ST segment elevation or depression.


  • Identify any hypertrophy in V1 lead.
  • Check P wave for atrial hypertrophy
  • Check R wave for right ventricular hypertrophy and R wave height in left ventricular hypertrophy.
  • Identify S wave in lead V1

Post Procedure Steps

  • Interpret the patient’s cardiac rhythm, treat the underlying cause or refer to a cardiologist for further evaluation and treatment.


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