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Tuesday, March 25, 2008

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, aVL, aVR and aVF) and 6 precordial leads (V1 to V6) and you should remember the names of each lead.

Image from ECG Library

Lead II, III and aVF are called inferior leads since they look at inferior surface of the heart. Lead I, aVL, 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).

Rate Rhythm Axis Hypertrophy Interval Morphology
  1. Rate
  2. 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.

  3. Rhythm
  4. 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.

  5. Axis
  6. To determine heart axis, look for main wave deflections at Lead I and aVF. 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 aVF 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 aVF is at the bottom instead of pointing up.

  7. Hypertrophy
  8. 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 aVL. If the height of R wave is greater than 12 small squares, we can assume that the heart has left ventricle hypertrophy.

  9. Interval
  10. 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.

  11. Morphology
  12. 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.

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


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.
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.

4 comments:

Anonymous said...

salam. again my reincarnation :) I learnt something today. Thanks. ECG does typically give headaches on my current cardiorespiratory fitness sem. :D but u help me sort it out. Ex-kisas ke awak ni?

Hafiz.

hafiz238 said...

I'm glad this brief note help someone :). My high school was MCKK not Kisas though.

Roy said...

Thanks for such an informatory blog post. I was wondering if you teach interpreting EKGs and if yes, there is a way to contact you?

Hafiz said...

Hi Roy,

Thanks for leaving a comment. Unfortunately I didn't teach EKG and this post was written when I was a student few years ago.