Step 3b: Calculate the electrical axis

Analysis and Interpretation of the Electrocardiogram

How to use this module

The 12 lead ECG

Waves and complexes

Intervals and segments

Approach to the ECG

Step 1: Determine the heart rate

Step 2: Measure important intervals

Step 3a: Calculate the electrical axis

Step 3b: Calculate the electrical axis

Step 3c: Calculate the electrical axis

Step 4: Evaluate the cardiac rhythm

Step 5: Inspect P waves for atrial enlargement

Step 6: Inspect QRS complexes for ventricular hypertrophy or low voltage

Step 7a: Inspect QRS complexes for bundle branch block or fascicular block

Step 7b: Inspect QRS complexes for bundle branch block or fascicular block

Step 7c: Inspect QRS complexes for bundle branch block or fascicular block

Step 7d: Inspect QRS complexes for bundle branch block or fascicular block

Step 7e: Inspect QRS complexes for bundle branch block or fascicular block

Step 8: Assess Q waves and determine significance

Step 9: Assess ST segments and T waves

Step 10: Measure QT interval for specific diagnoses

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Step 3b: Calculate the electrical axis

The mean QRS axis is oriented towards the lead with the greatest net QRS deflection. To calculate the net QRS deflection, add up the number of small squares that correspond to the height of the R wave (positive deflection), and subtract the number of small squares that correspond to the height of the Q and S waves (negative deflection).

In actual fact, the net QRS deflection can be approximated without resorting to counting squares. In the example shown here, one can easily see that the net deflection is slightly more positive than negative.

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