Electrocardiograms

Hi :) This post will deal with topic four of our Veterinary Physiology 1 unit, that is the Cardiovascular System. In this post I'll be discussing how an electrocardiogram records the electrical activity of the heart. 

The Spread Of Depolarisation of the Heart

This occurs in a series of 8 steps:

1. Depolarisation initiated at SA node
2. Depolarisation spreads through atria
3. AV nodal delay (~0.1sec)
4. Depolarisation travels down bundle of His and bundle branches
5. Depolarisation spreads through septum
6. Depolarisation spreads through bulk of ventricles (inside-out)
7. Depolarisation completed at base of left ventricle
8. Repolarisation spreads through ventricles (outside-in)

The Electrocardiogram (ECG)
The ECG records the depolarisation and repolarisation from electrodes in specific locations on the skin surface. It is the most frequently used clinical tool for the assessment of cardiac electrical dysfunction. 

Before we start talking about how ECGs work, you need to know about dipoles. A dipole occurs when a positive and negative charge is separated by a short distance. When the heart is at rest, no dipole is created. However, when the heart is stimulated the muscle fibres depolarise as the charge travels from one end of the muscle fibre to the other, this creates a dipole. When the fibre is fully depolarised no dipole is created because the charge is the same across the whole fibre. When the fibre repolarises, we get a dipole again because one part of the fibre becomes negative with respect to the other part of the fibre - different charges are separated by a small distance. 

Now, when we consider the whole heart working in the body, multiple adjacent muscle fibres are activated together. The activation of each fibre creates a dipole oriented in the direction of the activation. This produces what's known as an activation front in which the sum of all the dipoles can be represented as a single dipole. The overall strength and direction of the dipole is represented as a vector. The electrical field created in and around the heart varies at different points during the cardiac cycle. This electrical field passes through various structures until it reaches the skin where it can be detected by electrodes placed at specific sites.

Depolarisation which travels towards the positive recording electrode will give a positive (upwards) deflection on the ECG and vice versa. The larger the mass of heart muscle involved, the bigger the deflection on the ECG recording will be. If there is no movement of depolarisation or repolarisation the ECG trace will return to zero. 

Parts of an ECG trace:

An ECG Trace

I am going to refer to the diagram above which shows the various parts of an ECG trace and relate them to the steps of depolarisation of the heart mentioned earlier.


Initiation (Step 1):



In this step the SA node depolarises, however, the mass of tissue is too small to cause a deflection on the ECG. This refers to the horizontal section of the trace, just before the PR interval.


Atrial Depolarisation (Step 2): P wave

Depolarisation spreads through the atria from right to left, producing a positive charge on the positive recording electrode, resulting in a positive deflection. When the atria is full polarised, the trace returns to zero (baseline). This refers to the first bump in the trace shown above. 

AV Nodal Delay (Step 3 and 4):PR Segment


The electrical impulse travels rapidly through the bundle of His and enters the bundle branches. The mass of depolarised tissue is too small to cause a deflection on the ECG


Early Ventricular Depolarisation (step 5): Q Wave:


Depolarisation spreads from left to right through the septum. This produces a small negative charge on the recording electrode which causes a negative deflection on the ECG.


Ventricular Depolarisation (Step 6): R Wave


Left and right bundle branches conduct APs (Action Potentials) to ventricular apex. From there, Purkinje fibres carry APs up the interior walls of both ventricles. Depolarisation spreads through the bulk of the ventricular muscle from inside to outside. This produces a positive charge on the positive electrode which leads to a positive deflection on the ECG. Because the ventricles have a high mass a large deflection is produced. 

It is important to note that although a mean vector directed towards the apex of the heart is generated in small animals, in large animals the vector is generated towards the base of the heart. This causes a negative deflection to be seen in horse and other ungulates. 


Late Ventricular Depolarisation (step 7): S Wave


The last part of the ventricles to depolarise is the base of the left ventricle. This produces a negative charge on the positive electrode creating a small deflection on the ECG. Because this is a small mass of tissue a small deflection is made. After the S wave, the ECG returns to baseline and stays there for some time. This is because all cells throughout both ventricles are uniformly at the plateau of their AP and so no dipoles exist.


Ventricular Repolarisation (Step 8):T Wave

The repolarisation occurs from sites that were most recently depolarised to sites that were the first to be depolarised (from inside to outside). This causes a positive charge on the positive electrode which leads to a positive deflection on the ECG. A large mass of tissue is involved but the repolarisation is asynchronous so only a small deflection is produced.  


The Six Lead ECG


In veterinary medicine, a 6 lead ECG is used. Electrodes are placed on the left forelimb, right forelimb and left hindlimb. This makes a triangle around the animal's heart. The voltage in the left forelimb compared to the right forelimb is called lead I. The voltage between the left hindlimb compared to the right forelimb is called lead II. The voltage of the left hindlimb compared to the left forelimb is called lead III. These are known as standard limb leads and provide three different "angles" for viewing the depolarisation and repolarisation of the heart. 

Three additional "views" are provided by the augmented unipolar limb leads (aVr, aVl and aVf). aVr measures the voltage from the right forelimb electrode compared with the average voltage from the other two electrodes. aVl and aVf measure the voltages from the left forelimb and left hindlimb electrodes compared with the average voltage from the other two electrodes.  


That's it for this post, see you next time :)