Interpreting Electrocardiogram (ECG)

ECG is one of the popular medical machines shown on TV. It is usually shown whenever an actor/actress is dying and the sound of beep is associated with death. ECG illustrates the electrical activity of the heart and can indicate various heart diseases.

ECG contains various squiggles (waves) that are hard to interpret for any beginner. I was overwhelmed by ECG throughout my cardiorespiratory rotation and it took me a while to understand the basic of ECG. There are various heart diseases and hence various ECG patterns for each disease.

The target audience for this post is for any medical students who are starting to learn about ECG and thus, the rest of this post may contain various medical jargons.

There are 5 important waves in ECG. P wave represents atrial contraction, QRS complex represents ventricle depolarisation and T wave is associated with ventricle repolarization.

This is what an ECG printout looks like. It shows 12 leads and one rhythm strip usually from Lead II. ECG look at the heart from 12 different angles and each angle has different shape of waves. There are 6 limb leads (I, II, III, aV_L, aV_R and aV_F) and 6 precordial leads (V₁ to V₆₎ and you should remember the names of each lead.

Image from ECG Library

Lead II, III and aV_F are called inferior leads since they look at inferior surface of the heart. Lead I, aV_L, V₅ and V₆ are lateral leads. Lead V₁ to V₄ are anterior leads.

Inferior leads are located at the bottom left hand corner of ECG printout and anterior leads are located in the middle. If you are able to remember this location, interpreting ECG will be easier and you might able to correlate any episodes of ischemic heart disease with the coronary arteries :).

One of my consultants came up with a good mnemonic for ECG. This mnemonic serves as a foundation and it will be a good starting point for any beginner with ECG.

Please remember that one small square in a standard ECG strip represents 0.04 s (40 ms) and one large square represents 0.2 s (200 ms).

R_ate R_hythm A_xis H_ypertrophy I_nterval M_orphology

1. Rate
To calculate the heart rate, we need to find the distance between one R wave and the subsequent R wave (RR interval). The formula for calculating the heart rate is:


Heart rate = 300 / RR interval.

In this formula, RR interval is the number of large squares. Therefore, if there are 5 large squares between one R wave to the subsequent R wave, the heart rate is 300/5 = 60 beats per minute (bpm). If RR interval is 3.5 large squares, heart rate is 300/3.5 = 86 bpm.


2. Rhythm
There are two main rhythms that we need to differentiate. The rhythms are either sinus rhythm (normal heart rhythm) or non-sinus rhythm. If there is normal P wave in ECG, this is most likely to be in sinus rhythm. If there is no P wave, it is more likely to be in non-sinus rhythm.


3. Axis
To determine heart axis, look for main wave deflections at Lead I and aV_F. Positive deflection is defined when the height of R wave is greater than the S wave and negative deflection when the height of S wave is greater than R wave.
Please refer to the diagram above. If the main wave deflection in Lead I is positive and lead aV_F is negative, the heart axis is left axis deviation. If both leads are negative, the heart axis is not able to be determined.

Please note that in the diagram above, positive for lead aV_F is at the bottom instead of pointing up.


4. Hypertrophy
This will only cover about left ventricle hypertrophy. There are three ways to determine whether left ventricle hypertrophy exist.

The first method is to calculate the height of S wave in V1 and R wave in V5. The second method is to use two different leads, S wave in V2 and R wave in V6. If the sum of both waves is greater than 7 large squares (35 small squares), the heart has left ventricle hypertrophy.

The third method is to calculate the height of R wave in lead aV_L. If the height of R wave is greater than 12 small squares, we can assume that the heart has left ventricle hypertrophy.


5. Interval
There are three main intervals that we are concerned for in ECG. The intervals are PR, QRS and QT. The normal range of PR interval is less than 1 large square (0.2s) and for QRS complex, is 3 small squares (0.12 s).

In term of QT interval, we need to determine whether QT is prolonged or not. If QT interval is more than half of RR interval, QT is prolonged.


6. Morphology
There are various morphology that we need to know but as for a beginner, we can start with P wave.

If P wave is 'tall, pointy, pulmonale', this indicate right atrial enlargement.

From ABC of clinical electrocardiography

If P wave is 'short, comfortable to sit on, mitrale', this indicate left atrial enlargement. Please refer to the diagram below.

From ABC of clinical electrocardiography

A journey of a thousand miles begins with a single step and hopefully this will help you to make the first step in understanding ECG.

A good book that I highly recommends to learn more about ECG is 'The ECG Made Easy' by John R Hampton. There are various excellent website resources for ECG. Among them are ECG Learning Center, ECG library and ECG tutorial.

BMJ has good resources on ECG. Here are the topics under ABC of Clinical Electrocardiography.

• Introduction. I - Leads, rate, rhythm, and cardiac axis
• Introduction. II - Basic terminology
• Broad complex tachycardia - Part I
• Broad complex tachycardia - Part II
• Junctional tachycardias
• Bradycardias and atrioventricular conduction block
• Myocardial ischaemia
• Paediatric electrocardiography
• Acute myocardial infarction - Part I
• Acute myocardial infarction - Part II
• Atrial arrhythmias

If you would like to test your ECG skills, ECG Wave-Maven will be good for you :).

UPDATE 4 April 2008 : Add more web resources for ECG.