Electrical Axis

Electrical axis refers to the general direction of the wave of excitation as it moves through the heart. In the normal heart, the flow of electrical forces originates in the SA node,
spreads throughout atrial tissue, passes through the AV node, and moves throughout the ventricles. This flow of forces is normally downward and to the left, a pattern known as normal axis.

The ventricles make up the largest muscle mass of the heart and therefore make the most significant contribution to the determination of the direction of the flow of forces in the heart. For this reason, the QRS complex is examined when deciding the electrical axis.

A quick way to estimate the axis of the heart is to examine the direction of the QRS complex in leads I and aVF (Fig. 17-33). A QRS complex that is mainly upright in both leads represents a normal axis. A QRS complex that is upright in lead I and downward in lead aVF represents left axis deviation. A QRS complex that is downward in lead I and upright in lead aVF represents right axis deviation. A QRS complex that is downward in leads I and aVF is uncommon and represents indeterminate axis.

The direction of the flow of forces in the heart can change as a result of an anatomical shift of the heart in the chest wall. An anatomical shift may occur in very obese patients or in patients with large abdominal tumors or abdominal ascites. Left axis deviation can be caused by LBBB, left ventricular enlargement, or inferior wall MI. Right axis deviation can be caused by RBBB, right ventricular enlargement, or an anterior wall MI.

Patients with an axis shift are asymptomatic. The only way an axis shift can be detected is through a 12-lead ECG. The axis shift usually represents some underlying abnormality, and treatment is directed at the underlying cause.