Chapter 2
The electricity of the heart

Electrical coordination of atrioventricular contraction

For the heart to function efficiently as a 'circulatory pump', it must have a coordinated contraction: the two atria contracting and passing blood into the two ventricles, followed by contraction of the ventricles, pumping blood into the aorta and pulmonary artery; that is, there must be a coordinated atrioventricular (AV) contraction. In order for the cardiac muscle cells to contract, they must first receive an electrical stimulus. It is this electrical activity that is detected by an ECG.

The electrical stimulus must first depolarise the two atria. Then, after an appropriate time interval, it must depolarise the two ventricles. The heart must then repolarise (and 'refill') in time for the next stimulus and contraction. Additionally, it must repeatedly do so, increasing in rate with an increase in demand and conversely slowing at rest.

Formation of the normal P-QRS-T complex

All cells within the heart have the potential to generate their own electrical activity; however, the sinoatrial (SA) node is the fastest part of the electrical circuit to do so and is therefore the 'rate controller', termed the pacemaker. The sinus node rate is, therefore, the dominant pacemaker (over the other cells in the heart) by being the fastest and by a mechanism termed overdrive suppression. The rate of the SA node is influenced by the balance in the autonomic tone, that is, the sympathetic (increases rate) and parasympathetic (decreases rate) systems.

The electrical discharge for each cardiac cycle (Fig. 2.1) starts in the SA node. Depolarisation spreads through the atrial muscle cells. The depolarisation wave then spreads through the AV node; however, it does so at a relatively slower rate, creating a delay. Conduction passes through the AV ring (from the atria into the ventricles) through a narrow pathway called the bundle of His. This then divides in the ventricular septum into left and right bundle branches (going to the left and right ventricles). The left bundle branch divides further into anterior and posterior fascicles. The conduction tissue spreads into the myocardium as very fine branches called Purkinje fibres.

Figure 2.1 Illustration of the heart's electrical circuit. SA - sinoatrial; AV - atrioventricular; RA - right atrium; LA - left atrium.

Formation of the P wave

The SA node is therefore the start of the electrical depolarisation wave. This depolarisation wave spreads through the atria (somewhat like the ripples in water created by dropping a stone into it). As the parts of the atria nearest to the SA node are depolarised (Fig. 2.2), it creates an electrical potential difference between the depolarised atria and the parts that are not yet depolarised (i.e. still in a resting state).

Figure 2.2 Illustration of partial depolarisation of the atria and formation of the P wave. The shaded area represents the depolarised myocardial cells; the arrows represent the direction in which the depolarisation wave travels. RA - right atrium; LA - left atrium; RV - right ventricle; LV - left ventricle; SAN - sinoatrial node; AVN - atrioventricular node.

If negative (-ve) and positive (+ve) electrodes were placed approximately in line with those shown in the diagram (Fig. 2.2), then this would result in the voltmeter (i.e. the ECG machine) detecting the depolarisation wave travelling from the SA node, across the atria, in the general direction of the +ve electrode. On the ECG recording, all positive deflections are displayed as an upward (i.e. positive) deflection on the ECG paper, and negative deflections are displayed downwards. The atrial depolarisation wave, therefore, creates an upward excursion of the stylus on the ECG paper.

When the whole of the atria become depolarised, then there is no longer an electrical potential difference, thus, the stylus returns to its idle position - referred to as the baseline. The brief upward deflection of the stylus on the ECG paper creates the P wave, representing the atrial electrical activity (Fig. 2.3). The muscle mass of the atria is fairly small, thus, the electrical changes associated with depolarisation are also small.

Figure 2.3 Illustration of complete depolarisation of the atria and formation of the P wave. RA - right atrium; LA - left atrium; RV - right ventricle; LV - left ventricle.

The P-R interval

During the course of atrial depolarisation, the depolarisation wave also depolarises the AV node. The speed at which the electrical depolarisation wave travels through the AV node is deliberately slow so that ventricular contraction will be correctly coordinated following atrial contraction. Once the depolarisation wave passes through the AV node, it travels very rapidly through the specialised conduction tissues of the ventricles, that is, the bundle of His, the left and right bundle branches and Purkinje fibres.

The formation of the QRS complex

The Q waves

Initially the first part of the ventricles to depolarise is the ventricular septum, with a small depolarisation wave that travels in a direction away from the +ve electrode (Fig. 2.4). This creates a small downward, or negative, deflection on the ECG paper - termed the Q wave.

Figure 2.4 Illustration of depolarisation of the ventricular septum and formation of the Q wave. RA - right atrium; LA - left atrium; RV - right ventricle; LV - left ventricle.

The R wave

Subsequently the bulk of the ventricular myocardium is depolarised. This creates a depolarisation wave that travels towards the +ve electrode (Fig. 2.5). As it is a large mass of muscle tissue, it usually creates a large deflection - this is termed the R wave.

Figure 2.5 Illustration of depolarisation of the bulk of the ventricular myocardium and formation of the R wave. RA - right atrium; LA - left atrium; RV - right ventricle; LV - left ventricle.

The S wave

Following depolarisation of the majority of the ventricles, the only remaining parts are basilar portions. This creates a depolarisation wave that travels away from the +ve electrode and is a small mass of tissue (Fig. 2.6). Thus, this creates a small negative deflection on the ECG paper - the S wave.

Figure 2.6 Illustration of depolarisation of the basilar portions of the ventricles and formation of the S wave. RA - right atrium; LA - left atrium; RV - right ventricle; LV - left ventricle.

Nomenclature of the QRS complex

The different parts of the QRS complex are strictly and arbitrarily labelled as follows:

• The first downward deflection is called the Q wave; it always precedes the R wave.
• Any upward deflection is called the R wave; it may or may not be preceded by a Q wave.
• Any downward deflection after an R wave is called an S wave, regardless of whether there is a Q wave or not.

However, this fairly rigid terminology becomes confusing when the shapes of ECG complexes vary and become complicated. Therefore, in this book, we will think of the 'QRS complex' as a whole, rather than try to recognise its individual parts.

Note

While the different parts of the QRS waveform can be identified, it is often easier to think of the 'whole ventricular depolarisation waveform' as the QRS complex. This will avoid any confusion over the correct and proper naming of the different parts of the QRS complex.

The T wave

Following complete depolarisation (and contraction) of the ventricles, they then repolarise in time for the next stimulus. This phase of repolarisation creates a potential difference across the ventricular myocardium, until it is completely repolarised. This results in a deflection from the baseline - termed the T wave (Fig. 2.7).

Figure 2.7 Illustration of complete depolarisation and repolarisation of the ventricles and completion of the P-QRS-T complex. RA - right atrium; LA - left atrium; RV - right ventricle; LV - left ventricle.

The T wave in dogs and cats is very variable, and it can be negative or positive or even biphasic (i.e. a combination of both). This is because repolarisation of the myocardium in small animals is slightly random, unlike in humans, for example, where repolarisation is very organised and always results in a positive T wave. Thus, the diagnostic value obtainable from the abnormalities in the T wave of small animals is very limited, unlike the useful features of the abnormal T waveforms seen in humans.

The repolarisation wave of the atria (Ta) is rarely recognised on a surface ECG, as it is very small and is usually hidden within the QRS...