One determines axis from the six limb leads only. These include the bipolar electrodes of Einthoven (I, II, and III) plus the augmented limb leads which can be thought of as bipolar electrodes with an intermediate orientation relative to leads I, II, and III. The six limb leads together constitute the “hexaxial reference system.” A bipolar electrode provides for a positive, negative, or isoelectric deflection on the recording paper, depending on the orientation of the electrode and the direction of the electrical activity (Fig. 2.1).

When electrical activity is going towards the positive pole of a bipolar electrode, a positive or upright deflection is recorded on the graph paper (see Fig. 2.1a). When electrical activity is going towards the negative pole, a negative or downward deflection is recorded (see Fig. 2.1b). When electrical activity is perpendicular to the lead, no deflection is recorded (see Fig. 2.1c). Because the heart is of more than one dimension and the direction of electrical activity is not always exactly in the same direction, no totally flat bipolar record is possible. The equivalent of the flat bipolar record in electrocardiography is the “isoelectric” lead, in which an equal upward and downward deflection is recorded (see Fig. 2.1d). It must be noted that the average direction of electrical activity in Fig. 2.1d is the same as in Fig. 2.1c, i.e., the average direction of electrical activity in both Fig. 2.1c and Fig. 2.1d is perpendicular to the orientation of the electrode. It should also be noted that the magnitude of the deflection is judged by the area above or below the deflection, not the mere height or depth of the deflection. Applying this concept to each of the limb leads means that if the net deflection (positive area vs. negative area) of the wave is positive, the axis is on the positive side of the perpendicular to that lead.

The six limb leads are arranged such that their intersections equally divide the circle of the frontal plane into 30° sectors (Fig. 2.2). The positive side of each lead is labelled with the lead’s identifier. As related to the expression of axis, horizontal towards the patient’s left is arbitrarily designated as 0°, with positive extending downward (clockwise) and negative extending upward (counterclockwise) from left horizontal. Whenever axis is reported, the report must include either “+” or “−,” except for 0° and 180°. A normal axis is between 0° and +90°, although some authors believe that the normal axis can actually extend as far to the left as −30°. Right axis deviation (RAD) is between +90° and 180°, left axis deviation (LAD) is between 0° and −90°, and either “extreme” right axis or “extreme” LAD is between 180° and +270°, or −90° and 180°, respectively.

With these fundamental concepts in mind, determination of axis can be easy and quick. There are three steps in determining axis (Table 2.1).

Step One: Examine leads I and aVF and see if the QRS deflections are net positive, net negative, or isoelectric. With this information, one can immediately determine the axis or, more usually, in which quadrant the axis is located—normal axis, RAD, LAD, or extreme axis deviation (Table 2.2). This is just a quick application of the positive vs. negative net deflection concept in the horizontal and vertical limb leads (I and aVF, respectively). If either I or aVF are isoelectric, then the axis is perpendicular to that lead and in the direction dictated by the other lead. Specifically, if I is isoelectric and aVF is positive, the axis is +90°. If aVF is isoelectric and I is positive, the axis is 0°. If I is isoelectric and aVF is negative, the axis is −90°. If aVF is isoelectric and I is negative, the axis is 180°.

Step Three: If there is not an isoelectric lead, then one interpolates. Interpolation is within a 30° sector between two perpendiculars to leads. The correct 30° sector is determined by the net deflection of the waves in the limb leads (Table 2.3). This means that the leads closest to isoelectric are determined, and the relative positivity or negativity of the leads is examined to assess how close to isoelectric those leads are. The closer the lead is to isoelectric, the closer the axis is to the perpendicular of that lead. There should be no more than 5–15° interobserver or intraobserver variability in reading axis, and axis is conventionally reported by humans to the nearest 5° (computers are programmed to report axis to the nearest 1°).