The cardiac axis is the average direction of the flow of depolarization throughout the heart. This section centers on describing the mean frontal QRS axis or ÅQRS_F.

When tracing QRS vectors, the general direction of the electrical current follows vector 2, which depicts flow moving inferiorly and laterally. When there is left ventricular hypertrophy, the vector shifts to the left, forming a left axis deviation. The vector shifts right, creating a right axis deviation, when there is significant right ventricular hypertrophy. 

The Einthoven triangle is an imaginary inverted equilateral triangle that depicts the six limb leads. The two shoulders and the pubis form the three points of the triangle. The center of this triangle is the summation of all potentials.

The Einthoven Triangle

The Cabrera circle (see Figure 12) is a hex axial reference system composed of the limb leads of a 12-lead ECG. 

Both the Einthoven triangle and Cabrera circle help the clinician calculate the direction of the mean frontal QRS axis, as depicted in Figure 13. Assessing axis deviation helps the clinician determine where the apex of the left ventricle lies. If the left ventricle is more negative than −30° (or −30° to ±150°) then there is left axis deviation. Patients with left ventricular hypertrophy present with left axis deviation on the 12-lead ECG. 

If the left ventricle lies more positive than +90°, right axis deviation is present. If the left ventricle sits at +120° to ±150°, then extreme right axis deviation is present on the ECG, and the clinician has evidence to support the diagnosis of right ventricular hypertrophy.

Cabrera Circle Integrated With Einthoven Triangle

Cabrera Circle With Normal Axis and Axes Deviations

Procedures to Determine the Mean Frontal Axis

The following explanation uses the ECG strip in Figure 14.

STEP 1: Locate the isoelectric vector. To determine the mean frontal QRS axis (ÅQRS_F), the clinician looks for the isoelectric vector in the limb leads aVR, aVL, aVF, I, II, and III. The isoelectric vector is the QRS complex that is most equiphasic, i.e., where the R and S waves are almost equal. 

In Figure 14, the isoelectric vector is limb lead I and represents almost equal depolarization toward and away from this limb lead.

STEP 2: Find the direction of the main axis. The ÅQRS_F  vector should be located 90° perpendicular (positive or negative) from the isoelectric vector lead I. On the Cabrera circle, lead aVF is perpendicular to lead I.

STEP 3: Determine if the axis is located at the positive or negative direction. To determine if the axis is positive or negative, the clinician should look at the lead that is perpendicular to the isoelectric vector. In this case, it is lead aVF.  

On the ECG tracing in Figure 14, aVF has a positive deflection, so this likely represents an axis at +90°. As further confirmation, a positive deflection is present in leads II and III, and a negative deflection is present in aVR and aVL. This information confirms that the direction is toward the inferior and lateral aspect of the heart. Therefore, the axis must be located at +90°.

STEP 4: Adjust the axis. Axis adjustments are only required if the isoelectric vector is not equiphasic.

Since the isoelectric vector is more positive than negative, the axis must move toward it. A general rule-of-thumb is to add or subtract 15° when adjusting the axis in situations that the isoelectric vector is not equiphasic.

STEP 5: Calculate the new axis. The estimated adjustment amount is then added or subtracted from the axis. The calculated ÅQRS_F  is +90°− 15°= 75°.

aVF Positive Deflection ECG

Basics of Cardiac Axis Screengrab

Exercise:

1. Determine the mean QRS frontal axis in the ECG image below.

QRS Frontal Axis Determination ECG

Explanation:

In the example above, the isoelectric vector is located in lead aVR. It is more negative than positive. Perpendicular to lead aVR is  lead III, which exhibits a positive deflection. Therefore, the axis should be calculated at +120°. However, since the isoelectric vector is more negative, the axis moves away from aVR by about −15 degrees. The ÅQRS_F  is now estimated at +105°, which can confirm a diagnosis of right axis deviation.

2. Determine the mean QRS frontal axis in the ECG image below.

Left Axis Deviation ECG

Explanation:

The ECG tracing above exhibits the isoelectric vector at lead aVR, and perpendicular to it is lead III, which exhibits a negative deflection. Therefore, the ÅQRS_F is calculated at −60°, depicting a left axis deviation.

Perils and Pitfalls

Caution is needed when evaluating the cardiac axis for patients with a left bundle branch block (LBBB). LBBB obscures the entire ventricular excitation where the true reading of the QRS complex is impossible. Any calculation of ÅQRS_F will be misleading, and the ECG data cannot be used to diagnose it. 

When evaluating patients with a right bundle branch block (RBBB), the only useful data for calculating ÅQRS_F  are seen in the first 60–70 milliseconds. The impulses received by the left and right ventricles occur one after the other, and the impulses of concern with RBBB should come only from the left ventricle. 

ÅQRS_F is always an approximate value.