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ECG - Calculate the electrical axis
Related Subjects: |ECG Basics |ECG Axis |ECG Analysis |ECG LAD |ECG RAD |ECG Low voltage |ECG Pathological Q waves |ECG ST/T wave changes |ECG LBBB |ECG RBBB |ECG short PR |ECG Heart Block |ECG Asystole and P wave asystole |ECG QRS complex |ECG ST segment |ECG: QT interval |ECG: LVH |ECG RVH |ECG: Bundle branch blocks |ECG Dominant R wave in V1 |ECG Acute Coronary Syndrome |ECG Narrow complex tachycardia |ECG Ventricular fibrillation |ECG Regular Broad complex tachycardia |ECG Crib sheets
Calculate the electrical axis
The mean QRS axis refers to the average orientation of the heart's electrical activity. In most cases, an approximation of the axis will be sufficient for the ECG interpretation. There are many different approaches to axis determination, but this discussion will be limited to a simple technique that uses the leads I and aVF to calculate an approximate axis.
- Recall that the axis can be considered in terms of four quadrants, with lead I oriented at 0°, and aVF oriented at +90°. An ECG with the QRS axis oriented to the quadrant between 0° and 90° is said to be normal.
- An ECG with the QRS axis oriented to the quadrant between -1° and -90° is said to have left axis deviation.
- An ECG with the QRS oriented to the quadrant between +91° and 180° is said to have right axis deviation.
- An ECG with the QRS oriented to the quadrant between -91° and -180° is said to have an indeterminate axis because one cannot tell if it represents right or left axis deviation.
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).
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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.
Approximate the net QRS deflection for leads I and aVF. Remember that the mean QRS axis will be oriented towards the lead with the greatest positive net QRS deflection. If the net deflection is positive for both, the axis lies between leads I and aVF (0-90) and is therefore normal.
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