Skills Laboratory: How to determine and interpret the mean electrical axis

Don't be intimidated by this important ECG measurement. This cardiologist describes four methods for easily calculating the mean electrical axis and discusses what the abnormalities you may find signify.
Jan 01, 2006

You can easily detect most arrhythmias on physical examination, but you'll need an electrocardiogram (ECG) to identify an arrhythmia's exact nature. Obtain an ECG anytime a patient has an arrhythmia that isn't associated with respiration, has irregular pulses, or has a history suggestive of an arrhythmia, such as a collapse episode. To diagnose arrhythmias, a single lead II ECG tracing is generally all you need. However, to detect changes in the mean electrical axis, a multilead ECG is necessary.

Identifying abnormalities in the mean electrical axis helps you diagnose conduction abnormalities, detect ventricular hypertrophy, and differentiate the origin of arrhythmias. Further, a multilead ECG will help you rule out an artifact as the cause of electrocardiographic abnormalities seen in a single-lead tracing. Although thoracic radiography and echocardiography have higher sensitivity for diagnosing chamber enlargement, an abnormal ECG may signify that additional diagnostic tests are indicated. Additionally, an intraventricular conduction disturbance can only be diagnosed with an ECG.

Genesis of the ECG

An ECG is a surface recording of the net or average electrical activity of the myocardium.1 Atrial depolarization is represented by the P wave, and ventricular depolarization is represented by the QRS complex. Electrical activity of the conduction system (atrioventricular node, bundle branches, and Purkinje fibers) is not detectable by the surface ECG and occurs during the flat P-R segment.

Myocardial cells are polarized during the resting state, with the inside of the cell negatively charged relative to the outside. Depolarization results from ion shifts that cause the outside of the cell to become negatively charged relative to the inside. These ion shifts move across the myocardium in an orderly wave front. Normal depolarization of the heart occurs in a fairly regular sequence, which provides the familiar ECG tracing.

Figure 1. A frontal plane diagram of a hexaxial lead system (I, II, III, avR, avL, avF) superimposed over a short-axis view of the heart (RV = right ventricle, LV = left ventricle). The double-lined arc from 0 to +160 degrees indicates the normal mean electrical axis in cats. The solid bar arc from +40 to +100 degrees indicates the normal mean electrical axis in dogs.
The standard multilead system used in veterinary medicine is the hexaxial lead system formed by six limb leads (Figure 1).2 Leads I, II, and III are bipolar leads, meaning that they have a positive pole and a negative pole. Leads aVR (augmented voltage right arm), aVL (augmented voltage left arm), and aVF (augmented voltage left foot) are known as unipolar leads. Unipolar leads have only a positive pole; the activity of the positive pole is compared with the average of two other leads. By convention, the label for a lead is placed at the position of its positive pole on the ECG lead diagram.

Each ECG lead records the electrical activity of the heart relative to that lead. A wave of depolarization that is traveling toward the positive pole of a lead will result in a positive deflection in that lead. Conversely, a wave of depolarization that is traveling away from a positive lead will result in a negative deflection in the lead. ECG leads are positioned around the heart at different angles, which is why the waveform recorded by each lead appears different.