The two common causes of a right axis shift of the mean electrical axis are right ventricular hypertrophy (Figure 5) and right bundle branch block (RBBB). These two conditions are distinguished electrocardiographically by the duration of the QRS complex. Right ventricular hypertrophy is associated with a normal duration QRS complex, while RBBB results in a prolonged QRS complex duration because of slow myocyte-to-myocyte conduction. In dogs, RBBB is associated with a QRS complex duration of greater than 0.08 seconds (≥ 0.07 seconds in toy breeds). In cats, RBBB results in a QRS complex duration of greater than or equal to 0.06 seconds. Right ventricular hypertrophy can occur concurrently with RBBB, but it would be necessary to perform an echocardiogram to make this distinction.

Left bundle branch block (LBBB) does not usually cause a shift in the mean electrical axis. With LBBB, the left ventricle depolarizes by myocyte-to-myocyte conduction. As in the normal conduction system, this causes the left ventricle to predominate over the right. Electrocardiographically, LBBB is characterized by a normal mean electrical axis with a prolonged QRS complex duration (> 0.08 seconds in dogs [≥ 0.07 in toy breed dogs] and ≥ 0.06 seconds in cats).

The right bundle branch terminates in the Purkinje fibers, which distribute to the right ventricular myocardium. The left bundle branch bifurcates into two widely branching anterior and posterior fascicles, which further subdivide into Purkinje fibers. Left anterior fascicular block is the most common conduction abnormality in cats and results in a marked left axis deviation of the mean electrical axis (Figure 3). It can accompany any form of feline cardiomyopathy, though hypertrophic cardiomyopathy is the most common. Left anterior fascicular block can also be seen with hyperkalemia and hyperthyroidism. Left anterior fascicular block is uncommon in dogs.

Bundle branch and fascicular blocks are not hemodynamically significant and are generally not detectable on physical examination. However, they are frequently markers of underlying disease and signify the need to perform additional diagnostic tests to determine their cause.

Conclusion

Calculating the mean electrical axis is a simple and rapid diagnostic procedure that should be considered part of routine ECG analysis. Shifts in the mean electrical axis, in conjunction with radiographic changes, can help you diagnose cardiac chamber enlargement and conduction defects. So careful attention to the mean electrical axis may be particularly helpful in practices that do not have access to echocardiography. However, it is important to remember that cardiac chamber enlargement can occur without a concurrent shift in the mean electrical axis and that changes in the mean electrical axis occasionally occur in the absence of overt cardiac disease.

Deborah M. Fine, DVM, MS, DACVIM (cardiology)
Department of Veterinary Medicine and Surgery
College of Veterinary Medicine
University of Missouri
Columbia, MO 65211

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