Electrical System of The Heart

Here we are looking at Learning objective 10. Identify the SA, AV nodes and interventricular septum and appreciate their significance.

A human heart has the capability of generating its own impulse, thereby triggering a contraction. This impulse is propagated through a conduction system which consists of the sinoatrial node (SA node), the atrioventricular node (AV node), the atrioventricular bundle (bundle of His), the atrioventricular bundle branches and the purkinje cells.

Figure showing the heart and conducting cells

Sinoatrial node

The sinoatrial node consists of a collection of conducting cells which are situated in the superior and posterior walls of the right atrium close to the superior vena cava. It initiates the normal electrical pattern of the heart which leads to muscle contraction and determines heart rate, hence why its called the pacemaker.

This electrical pattern travels from the sinoatrial node to the atrial myocardial cells and the atrioventricular node, through the internodal pathways. This takes approximately 50 milliseconds. Another pathway is responsible for conducting the electrical activity from the right atrium to the left atrium and this is called Bachmann's bundle.

Both pathways lead to the electrical impulse reaching the atrioventricular septum. As this happens, the connective tissue of cardiac skeleton prevents the impulse from travelling to the ventricular myocardial cells, except through the atrioventricular node. This propagation of electrical activity leads to depolarisation in the same direction. Therefore, as the atrial myocardial cells become depolarised they contract, pumping blood into the ventricles (steps 1-3 in the figure below).

Atrioventricular node

The atrioventricular node is also a collection of conducting cells but these cells are located in the inferior portion of the right atrium. Its purpose is to propagate the electrical impulse to the bundle of His and eventually ventricles. There is a delay between the propagation of electrical impulses between the AV node and the bundle of His (step 3 in the figure below). This pause is important to the proper functioning of the heart because it allows the atrial myocardial cells to complete their contraction, which leads to the pumping of blood into the ventricles before the electrical signal is transmitted to the ventricular myocardial cells.

Bundle of His and Bundle Branches

Arising from the bundle of His are two bundle branches- left and right bundle branches which supply their corresponding ventricle.

The right bundle branch is also found in the moderator band and supplies the right papillary muscles, making each papillary muscle contract simultaneously before that of all the ventricular myocardial cells. This allows the chordae tendinae to develop tension before ventricular contraction occurs. The bundle branches descend to the apex to connect with the purkinje fibres (step 4 in the figure below). This propagation takes 25 milliseconds. Note however, that there is no moderator band in the left side of the heart.

Purkinje fibres

The purkinje fibres travel from the apex of the heart towards the atrioventricular septum and the base of the heart. They propagate the electrical impulse to the muscles of the ventricles (step 5 in the figure below). This propagation begins from the apex of the heart, causing ventricular contraction to begin from this same region and it travels upward to the base of the heart.

The contraction of the heart involves a torsion movement which maximises the amount of blood which is then pumped out of the heart from the ventricles through the aorta and pulmonary trunk. The total time taken from the generation of electrical impulse in the SA node till the depolarisation of the ventricles is 225 milliseconds.

Figure showing the steps in the propagation of electrical activity in the heart

Clinic corner

The heart can get damaged over the course of life in some patients, while others might have congenital heart defects. One of the several tests carried out to diagnose heart conditions is an electrocardiogram (ECG).

The purpose of an ECG is to observe the electrical activity of the heart. This test is carried out by placing sensors on the patient's skin which will detect electrical activity each time the heart beats.

ECGs can be ordered if a patient is presenting with symptoms such as;

Chest pain
Palpitations
Shortness of breath
Dizziness

ECGs are used to detect conditions such as

Arrhythmias
Heart attacks
Coronary heart disease

Patients diagnosed with heart conditions are then monitored over time with a series of ECGs. Watch the video on the right hand side of this page to see how an acutal ECG test is carried out.

The ECG consists of waves, intervals and a complex, each of which represent different parts of the cardiac cycle.

P wave: Represents atrial contraction and is small due to the small muscle mass of the atria.

QRS complex: Represents ventricular contraction and has a large amplitude due to the large muscle mass of the ventricles.

T wave: Represents ventricular repolarisation.

PR interval: Represents the time taken for the spread of electrical activity to the ventricles. It is the distance between the start of the P wave and the start of the QRS complex.

PR segment: Represents the time taken for the spread of electrical activity from the AV node, to the bundle of His, the bundle branches then the ventricular muscles. It is the distance between the end of the P wave and the start of the QRS complex. It has no deflection, in other words it is isoelectric.

ST segment: Is the distance between the end of the S wave and the start of the T wave. It is also isoelectric.

QT interval: Represents the time between ventricular depolarisation and repolarisation. It is the distance between the start of the QRS complex and the end of the T wave.

Understanding each feature of an ECG trace is important in diagnosing heart diseases, because each of them are affected in different conditions. Below is a list of heart conditions and how they affect each feature of an ECG trace.

PR interval:

Shortens in the presence of ventricular pre-excitation.
Increases in the presence of first degree heart block.

PR segment:

Is not isoelectric in the presence of atrial injury.

QRS complex:

Its duration increases in the presence of ventricular escape beats or ventricular hypertrophy.

QT interval:

Shortens in patients with hyperkalaemia and hyperthyroidism.
Increases in patients with hypokalaemia and coronary heart disease. A consequence of having a long QT interval is torsade de pointes, which is a form of ventricular tachycardia that may lead to sudden cardiac death.

ST segment:

Depression might indicate myocardial ischaemia.
Elevation indicates ventricular aneurysm.

A typical ECG trace showing the waves and segments

Image credits: National Institutes of Health (USA) 2017, Lumen Learning 2012, Anthony Atkielski 2007- All Image Usage Complies with CC BY-SA 3.0

Youtube video: British Heart Foundation 2014.

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