HOW TO READ AN ELECTROCARDIOGRAM
An electrocardiogram is a non-invasive medical procedure that is used to measure the electrical activity of the heart over a period of time using electrodes placed on the skin.
- It charts the electrical activity of the heart and shows it as a moving line on the screen or records it on a moving paper.
- It is primarily used to give information about the structure and function of the heart and detect cardiac problems such as heartbeat irregularities, arrhythmias, coronary heart disease, past heart attacks and other condition that disrupts the normal functioning of the heart.
WHO CAN DO AN ELECTROCARDIOGRAM INTERPRETATION
As the ECG test measures many different aspects of the heart’s function, interpreting the report requires trained medical practitioners to provide a correct diagnosis. A cardiologist or other medical specialists analyse the dips and peaks that is shown when the test has been performed and the trace recorded.
INTERPRETING AN ELECTROCARDIOGRAM
The three main parts of the electrocardiogram are the P wave, the QRS complex, and the T wave. They are generally referred to as the PQRST complex. The PQRST complex is a systematic method of interpreting rhythm on the ECG.
THE P WAVE
A normal heartbeat with a regular rhythm is generated by the SA node. The SA node is the heart’s pacemaker that determines how quickly the heart beats.
The P wave represents the electrical activity of the atria (atrial depolarization) when the SA node is activated. The atria are the two small chambers located at the top of the heart
The P wave is important because cardiac arrhythmias can be recognized by looking at the relationship between the P waves and the QRS complexes. The shape and duration of P waves may also be indicative of an atrial enlargement.
The normal duration of the P wave is about 0.08 to 0.10 seconds, covering 2 to 2.5 small squares horizontally on the ECG paper
The P-wave height is usually 2.55mm or less, covering 2.5 square vertically on the ECG paper
The P wave is usually round, not peaked, or pointy
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex.
It represents the time it takes the electrical impulse to spread through the atria down to the AV node, to the ventricular muscles.
The PR interval should measure 0.12 to 0.20 seconds, covering 3 to 5 small boxes on the ECG
If the PR interval is abnormally long (> 0.20 seconds), it may suggest atrioventricular delay (AV block)
THE QRS COMPLEXES
The QRS complexes represent the electrical activity of the ventricles (ventricular depolarization) and it has three components: Q, R, and S wave.
The ventricles are the two small chambers located at the bottom of the heart that pump blood throughout the body.
It is the tallest and the most visible portion of the trace and usually pointy like a thin tall triangle.
The first negative deflection of the complex is the Q wave
The first positive deflection of the complex is the R wave
The first negative deflection of the complex after the R wave is the S wave
The duration, amplitude and the shape of the QRS complex can be used to diagnose cardiac arrhythmias and other heart conditions
The QRS normally lasts 0.08 t0 0.12 seconds, covering at most 3 small boxes horizontally on the ECG.
THE T WAVE
The T wave represents the repolarization of the ventricles
The T wave should not be greater than 5mm in the limb leads, or less than 10mm in the chest leads.
It should be rounded and not pointed
T waves in the limb lead greater than 5mm and in the chest lead greater than 10mm may suggest hyperkalaemia
HOW TO DETERMINE HEART RATE
- If regular, heart rate can be calculated by counting the number of large squares present within one R – R interval and dividing the number by 300
- For example, if there are 3 large squares in an R-R interval: 300/3 = 100 bpm
- if irregular, the number of complexes on the rhythm strip is counted (each rhythm strip is 10 seconds long) and the number is multiplied by 6.
- For example, 10 complexes on a rhythm strip x 6 gives 60 bpm
- The adult heart rate when normal is between 60-100 bpm, if it is greater than 100, it indicates tachycardia and if it is less than 60, indicates bradycardia.
HOW TO DETERMINE HEART RHYTHM
Heart rhythm may be irregular or regular.
To determine heart rhythm, one R-R interval should be marked out on the piece of paper and moved along the rhythm strip to gauge if the other R-R intervals on the ECG are an equal distance apart.
As you move them along the rhythm strip, it can be seen if the PR interval changes, if QRS complexes are missing or if there is a complete dissociation between the two.
If P waves are absent and there is an irregular rhythm, it may indicate atrial fibrillation