M04.04.028 Reading an ECG

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Learning Objective

After completing this section, learners will be able to systematically interpret a standard ECG, assess rate and rhythm, evaluate waveforms and intervals, and estimate the mean electrical axis (MEA) using the quadrant method.

Step 1: Rate and Rhythm

Rhythm:

  • Examine the R-R intervals.
  • Steady rhythm: R waves occur at regular intervals.
  • Unsteady rhythm: R waves occur irregularly.

Heart Rate Estimation (using heavy vertical lines / big boxes):

  • If the R wave falls between heavy lines, the rate is between the corresponding bpm values.
  • Example: R-R interval spans 4 big boxes → HR ≈ 75 bpm.
Boxes between R waves Heart Rate (bpm)
1 300
2 150
3 100
4 75
5 60
6 50

 

Step 2: Waves

  • Check P wave, QRS complex, T wave
  • Ensure all are present and morphologically normal

Step 3: PR Interval

  • Measure from the start of the P wave to the start of the QRS complex
  • Normal range: 120–200 msec (~3–5 small boxes)
  • Assess consistency across multiple cycles

Step 4: Estimate the Mean Electrical Axis (MEA) – Quadrant Method

  • MEA: net direction of ventricular depolarization
  • Use lead I and aVF to determine quadrant:
Lead I Lead aVF MEA Interpretation
+ + 0° to 90° Normal
+ – 0° to –90° Left axis deviation
– + 90° to 180° Right axis deviation
– – – Extreme axis deviation

Causes of Axis Deviations:

Left axis deviation:

  • Left ventricular hypertrophy or dilation
  • Conduction defects in the left ventricle
  • Right-sided MI (unless RV dilates)

Right axis deviation:

  • Right ventricular hypertrophy or dilation
  • Conduction defects in the right ventricle or the posterior left bundle branch
  • Left-sided MI (unless LV dilates)

Note: Quadrant method provides an approximate MEA, suitable for early learning.

Activity

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