The Electrical Conduction System of the Heart

The electrical “wiring” of the heart, and the way in which the wave of electrical excitation spreads through the heart muscle to induce a contraction, are both quite easy to understand. With a little background knowledge, you will be able to understand the majority of ECG changes!

First, you have to understand the formation and propagation of the electrical excitation on a purely anatomical level.

This diagram shows the two atria and the two ventricles, the interatrial and the interventricular septum which separates the atria and the ventricles, as well as the opening of the upper and lower vena cava into the right atrium.

The structures involved in the formation and propagation of the electrical excitation are:

The sinus node,

The right and left atria,

The atrioventricular node - or the AV node for short,

The bundle of HIS and the Tawara branches, and finally

The Purkinje fibers which transport the information into the ventricular myocardium.

Cardiac excitation starts in the sinus node. The sinus node is an accumulation of specialized myocardial cells located in the wall of the right atrium. These cells spontaneously depolarize, setting off the wave of excitation. The excitation spreads from cell to cell. The sinus node excites the adjacent atrial myocardium. First, the right atrium is depolarized. Then, 20 to 40 milliseconds later, the left atrium is depolarized. The excitation then reaches the AV node, which is located close to, or sometimes within, the septum. The AV node forms the electrical connection between the atria and the ventricles.

The AV node delays the spread of electrical excitation through the bundle of HIS to the Tawara branches. These high-speed lines then lead into both ventricles. Depolarization then spreads slowly through the ventricular myocardium.

During ventricular excitation, the atrium is already re-polarized and is thus ready to receive the next wave of excitation from the sinus node.

The cycle is now complete.

What is the correlation between these anatomical and physiological factors and the features observed on the ECG tracing? And how does this help me to understand my patient’s problem?

The ECG device registers differences between positive and negative charges.

A myocardial cell can have two electrical states. When the cell is at rest, it is "polarized", which means that its charge is negative intracellularly and positive extracellularly. When the cell is in the depolarized or excited state, its charge is positive intracellularly and negative extracellularly.

The electric field can be measured from the extracellular side of a myocardial cell. The lines and waves that you will finally see in the ECG originate from these measurements.

What we must remember when looking at an ECG is that:

- A polarized cell carries a positive charge.

- A depolarized cell carries a negative charge.

Now let’s consider what happens between two adjacent cells: if an electrical excitation meets the first cell, it is depolarized. The other cell remains polarized. A difference in charge has therefore arisen between the two cells:

one is already extracellularly negative, the other one is still positive.

A unit of two cells, --- one positively and one negatively-charged --- is considered as dipole. Two cells produce only a tiny difference in charge. Obviously, we need more than just one dipole to produce a heart beat.

The myocardium consists of many millions of cells. As explained already, the electrical excitement spreads from cell to cell, starting from the sinus node and then traveling to the right and left atria and the AV node, and then into the right and left ventricle via the high-speed electric tracts of the Tawara branches and the Purkinje fibers.

During this process, many cells have not yet been depolarized, and thus remain positively-charged extracellularly. The already excited—that is depolarized and negatively charged—cells form a migrating electric front in the direction of the positive charge. This front comprises all dipoles, which together form one large dipole. This dipole has a certain orientation in space, which corresponds to the direction of propagation of the electrical front. In mathematical terms, a line with direction in space is called a "vector".

So the vector points from the electronegative, that is, the already excited myocardium, to the electropositive, or the as yet non-excited myocardium. The main vector of the excitation of the heart is also called the electric axis of the heart. This is what is being referred to when we talk about the heart axis.

The ECG records voltage differences. There are two situations in which there is no difference in voltage between the myocardial cells: at rest, when the ventricular myocardium is positively charged; and at the time of maximum excitation, when the entire ventricular myocardium is negatively charged. So at these stages of the cardiac cycle, no electrical difference is present between the myocardial cells. Consequently, there is no vector. When the heart is in this state, the ECG records an isoelectric zero line.

When interpreting an ECG, why do we have to consider many different leads rather than just one?

Each patient has only one heart, and that heart has only one electrical axis. However, the heart is a three-dimensional structure. We therefore use different leads to ensure that we record every aspect of the formation, propagation, and regression of its electrical activity.

Depending on which aspect we want to focus on, different leads are of greater or lesser relevance. However, an ECG can only be fully understood and interpreted when all of the leads are considered.

For a better understanding of the differences between the ECG leads, let’s look at the following graph:

First, let’s look at the spread of depolarization through the ventricles, which is represented by the arrow running from left to right. This is represented on the ECG by what is known as the QRS complex.

To make things easier, let’s consider the propagation of excitation in the main bulk of the ventricular myocardium.This is indicated on the ECG by the R wave.

The side of the arrow marked with a minus is already excited and thus negatively charged. Excitation proceeds from left to right; that is, towards positively-charged cells that are thus not yet depolarized and still resting.

In general:

A positive amplitude indicates that depolarization is moving towards the respective lead.

When evaluating the propagation of excitation in the ventricular myocardium, we are particularly interested in the limb lead that runs as precisely along the electric heart axis as possible. This will register the highest R wave of all the limb leads.

The larger the angle at which a lead deviates from the heart axis, the smaller the R will be. If the lead runs perpendicular to the heart axis, there is no amplitude at all, meaning that the positive area under the R wave equals the negative area under the S wave and the Q wave. If the angle is greater than 90 degrees, the area under the curve will be negative. The lead that runs in exactly the opposite direction to the vector (that is, at an angle of 180°) records a downward deflection, in which the area under the curve corresponds to the maximum positive deflection in the opposite lead.

Interpreting an ECG is just like everything else in life: in the end, it’s not quite as simple as it is in theory. This is because depolarization does not spread out through the three-dimensional heart structure along a straight line that can be captured perfectly by a single ECG lead.

Before the ventricular apex is excited by the electric front via the Tawara branches and Purkinje fibers, small branches leave the major electrical highways relatively quickly after dividing into a left and a right Tawara branch. These small branches excite the septum and the papillary muscles. This excitation does not extend along the electrical heart axis to the left, but somewhat to the right and then back towards the heart’s base. The leads pointing in the direction of the electric heart axis—and thus away from this septum and papillary nerve excitation—will initially show a small negative deflection. Here, we refer to the physiological "septal Q". After reaching the apex, as indicated by the R wave, the heart base is depolarized. Again, there is a negative deflection, called the S wave, in leads running along the electric heart axis.

Under physiological conditions, the most recently depolarized myocardial cells re-polarize first.

For this reason, re-polarization in the ECG is also characterized by positive vectors, since during this phase, the most distal cells are positive (meaning that they are at rest again) while the more proximal cells are negatively charged, and thus still depolarized). For this reason, the vector continues to point in the direction of the apex during repolarization.