**1. Normal Complexes in the Frontal Leads**

Depolarization of the heart produces different P_QRS_T complexes in the six frontal leads. The frontal plane hexaxial system (FPHS) can explain these differences.

*Depolarization of the Atria in the Frontal Leads*

Under normal conditions the electrical activation of the atria

- starts in the sinoatrial node (SAN) region of the right atrium
- extends leftwards into the left atrium
- extends downwards to involve the rest of the right atrium, including the AVN (located in the lower part of the right atrium)

In the FPHS the multiple dipole vectors of atrial depolarization can be represented by a (single) mean atrial depolarization vector. This vector starts at the centre of the circle formed by the intersection of the axes of the FPHS and travels in the direction of the axis of Lead II (Figure 1). The mean atrial depolarization vector produces P waves in the frontal leads.

The size and direction of the P wave in each frontal lead depends on the direction of the depolarization vector relative to the lead axis. The P wave is below the isoelectric line (i.e. inverted) in Lead aVR because the mean atrial depolarization vector is travelling away from the positive pole of aVR. The P wave in Lead aVR is also termed a “negative” P wave. The P wave is above the isoelectric line in Leads I, II, III and aVF (i.e. upright) because the mean atrial depolarization vector is traveling towards the positive pole of all these leads. The P wave in these leads is a “positive” P wave. The size of the upright P wave is greatest in the frontal lead whose lead axis is most parallel to the direction of the mean atrial depolarization vector; in Figure 1 this is Lead II. The mean atrial depolarization vector is perpendicular to the axis of Lead aVL, so the P wave is not seen in this lead.

Features of Normal P Waves in the Frontal Plane Leads

- P wave is inverted in Lead aVR
- P wave is upright in Lead I, Lead II, Lead III and Lead aVF
- P wave amplitude is greatest in Lead II

*Depolarization of the Ventricles in the Frontal Leads*

Ventricular depolarization begins almost simultaneously on the subendocardial surfaces of both ventricles, and multiple dipole vectors pass from the inside of the ventricle to the outside of the ventricle. These dipole vectors produce the QRS complex. The link between these dipole vectors and the QRS complex is explained using the concept of “spatial vector electrocardiography” (Grant R.P. Spatial Vector Electrocardiography. A method for calculating the spatial electrical vectors of the heart from conventional leads. Circulation. 1950; 2: 676 - 695).

The ventricles are three dimensional structures, and are depolarized by dipole vectors (shown as arrow heads) that travel in different directions (Panel A of Figure 2).

The depolarization spreads from one region of the ventricle to another depending on the distribution of the Purkinje network. Vectors from different regions of the heart dominate the electrical field at different intervals of the QRS cycle.

This produces a sequence of different dipole vector origins and directions, as shown by the numbered arrows in Panel B of Figure 2.

All the vectors are assumed to originate from a single point at the centre or zero point of the electrical field. Because the instantaneous QRS vector has different directions and magnitudes from instant to instant during a QRS cycle, its end describes an irregular ellipse around the zero point during the cycle. This ellipse is called the QRS vectorcardiogram or QRS loop (Figure 3).

*Ventricular Depolarization in the Frontal Plane Hexaxial System (FPHS)*

The six frontal leadsform a circular FPHS. The FPHS is more than just a circle with the lead axes as its spokes; it is also a circular coordinate system that can describe the position and direction of depolarization vectors. We will now describe normal ventricular depolarization using the FPHS.

We start with a simplified version of the QRS loop shown in Figure 3. This has three sequential depolarization vectors (Vector 1, Vector 2 and Vector 3) that arise from the centre of the FPHS and travel in different directions (Figure 4).

The lead axes define a circular coordinate system in the frontal plane. The positive pole of Lead 1 is the starting point (0 degrees) of the 360 degrees found in a circle. The axis of Lead 1 divides theFPHS into a upper half-circle and a lower half-circle.

The 180 degrees in the upper half-circle are measured counter-clockwise from the positive pole of Lead 1 (0 degrees) to the negative pole of Lead 1 (-180 degrees). The positive pole of Lead aVL is thus located at -30 degrees and the positive pole of Lead aVR at -150 degrees.

The 180 degrees in the lower half-circle are measured clock-clockwise from the positive pole of Lead 1 (0 degrees) to the negative pole of Lead 1 (+180 degrees). Note that the negative pole of Lead 1 is ±180 degrees. The negative pole of Lead aVR (labelled -aVR) is located at +30 degrees. The positive pole of Lead II is at +60 degrees, the positive pole of Lead aVF is at +90 degrees and the positive pole of Lead III is at +120 degrees.

The direction of the three vectors are described as follows:

- Vector 1: (the mean depolarization vector of the interventricular septum) has a direction of +135 degrees
- Vector 2: (the mean depolarization vector of the ventricles) has a direction of +20 degrees
- Vector 3: (the mean depolarization vector of the base of the ventricles) has a direction of about -115 degrees

*Frontal Plane Hexaxial System (FPHS) & the QRS Vectorcardiogram*

Figure 5 shows the QRS loop that we derived previously (see Figure 3) arising from the centre of the FPHS. Three instant depolarization vectors(1, 2 and 3) indicate the main directions of the QRS loop.

The QRS complexes that are produced by the vectors in Lead aVR and Lead II are shown in Figure 5.

We can simplify the QRS vectorcardiogram we used in Figure 5 to form a “penultimate” narrow elliptical loop with a vector traveling from the centre of the FPHS and a vector returning to the centre.

The “ultimate” simplification is to reduce the vector loop to a line.

The QRS complexes in the frontal leads produced by the simplified QRS depolarisation loop are shown in Figure 7.

**Introduction to the Frontal QRS Axis**

We can use the FPHS to determine the direction of the mean ventricular depolarization vector (MVDV) from the size and direction of the QRS complexes in the frontal leads of the ECG. We will discuss this in more detail later, but for now we will present a simple example of the method.

Complexes from the frontal leads of a ECG are put next to the positive pole of their lead axis (Figure 8). We then asses the size and direction (positive or negative) of the QRS complexes.

The QRS complex in Lead aVLis very small, suggesting that the MVDV is at 90 degrees to the axis of Lead aVL. The QRS complexes in Leads I, II, III and aVF are all upright (positive) indicating that the MVDV is traveling toward the positive pole of these lead axes. The maximal QRS amplitude is in Lead II, so the MVDV is traveling along the line of the axis of Lead II. The direction of the MVDV in this ECG is thus about +60 degrees.

The range of the normal MVDV is between -30 degrees and +120 degrees. A shorter term for the “range of direction of the MVDV” is "frontal QRS axis", which is often shortened to “axis".

**Cabrera Arrangement of Frontal Plane Leads**

The FPHS can be modified by changing the location of the QRS complex recorded at aVR. Inverting this complex shows how this QRS shape would be displayed by the negative pole of the Lead aVR axis (labelled as - aVR in the FPHS).

Placing the inverted aVR complex at -aVR means that the frontal leads are now displayed in sequence: aVL, I, -aVR, II, aVF and III. The lead axes in this sequence are each separated by 30 degrees. This alternative frontal lead sequence is called the Cabrera lead order, and is useful in studying the frontal QRS axis.

**2. Normal Complexes in the PrecordialLeads**

Depolarization of the heart produces P_QRS_T complexes with different shapes in the six precordial leads. The horizontal plane hexaxial system (HPHS) can explain these differences.

*Ventricular Depolarization in the Horizontal Plane*

The principles are the same as those described for the frontal plane hexaxial system. Ventricular depolarize by a sequence of instantaneous vectors (Figure 10):

- septal depolarization (a depolarization vector travels from the left ventricular surface of the septum to the right [vector 1A], and a smaller vector from the right ventricular surface of the septum to the left [vector 1B])
- depolarization of the apex (vector 2)
- depolarization of the free wall of the left ventricle (vectors 3B, 4B and 5B)
- depolarization of the free wall of the right ventricle (vectors 3A, 4A and 5A)
- depolarization of the base of the heart (vectors 6R and 6L)

Depolarization of the right ventricular wall and the left ventricular wall occur almost simultaneously.

Ventricular depolarization in the HPHS can be simplified as shown in Figure 11.

Vector 1 is the septal depolarization vector, and vector 2 is the mean depolarization vector of the ventricular walls (dominated by the large depolarization vector of the left ventricle). These two vectors produce a rS complex in Lead V1 and a qR complex in Lead V6.

The normal progression in the shape of the QRS complexes between Lead V1 and Lead V6 is shown in Figure 12.