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An ECG or electrocardiogram is a way of evaluating the electrical activity of the heart. It is regularly used and can detect many heart abnormalities at the bedside. However, it takes time to learn how to interpret in and a systematic approach is helpful. ECG physics is more complicated and is on a seperate page.



If you want more detail than this, see ECG physics but this is what you can probably get away with in terms of physiology. The ECG is split into big boxes, each made of 5x5 little boxes. Each little box is 1mm (big box is 5mm). Going across the page, you have time, down the page you electrical activity.

In terms of time:

  • 1 little box (1mm): 0.04ms
  • 1 big box (5mm): 0.2s

In terms of electrical activity:

  • 1 little box (1mm): 0.1mV
  • 1 big box (5mm): 0.5mV
This animation is concurrent with the actual electrical activity of the heart

These numbers come in useful later. Essentially, the normal ECG looks like that on the right.

As you can see each little peak and trough represents a part of the normal conduction cycle. The only ones that aren't labelled are the Q and S waves. Q waves are not always present depend on which ECG lead you look at (it is complicated but in simple terms, it is the beginning of ventricular depolarisation). S waves occur at the end of ventricular depolarisation - essentially the electrical activity goes a little past zero on its ways down, before going back up.


The numbers

These are the bits you can apply numbers to.


This is relatively easy to measure. As long as the rhythm is regular, you can simply divide the number of big boxes between each QRS complex by 300. The distance between each QRS is often referred to as the RR interval. So:

Rate = 300/no. of big boxes in RR interval


To assess this, look at the rhythm strip (the long one at the bottom). Grab a piece of paper, aligning the edge with the rhythm strip. Mark on the edge where each QRS complex is. Move the piece of paper along. If the rhythm is regular, the markings on the edge of the paper will align with QRS complexes further down the rhythm strip. If they don't, the rhythm is irregular.


If you learn nothing else about axis deviation, learn this diagram

There are lots of clever explanations in the understanding of axis devation. You will get none of those here (though check out ECG physics for a better explanation). When MedRevise stops being rubbish, I'll upload the actually very useful photo on the right. The key is to look at the limb leads (leads I, II, III).

In left axis deviation, if lead II is indeterminate, it is physiological change; if not, it is pathological. Yes, there are other combinations but these are the most common and will get you through most grillings by a consultant and most exams. Theoretically at the end, you should be able to vaguely judge how deviated something is - visit ECG physics if you're interested.

Final piece of advice:

  • Left is Leaving (leads I & III are pointing away from each other or "leaving" though remember that II needs to be negative as well)
  • Right is Reaching (leads I & III are pointing towards each other or "reaching" regardless of what lead II is doing)

The anatomy

These are the different bits of each complex.

P wave

Generally, this should be 1x1 little boxes in size i.e. last <0.04s and be <0.1mV. P wave abnormalities signify atrial pathology. The best lead to check out P wave abnormalities is V1:

  • >0.04s/1 little box wide & >+0.1mV/1 little box high - right atrial hypertrophy
  • >0.04s/1 little box wide & <-0.1mV/1 little box below - left atrial hypertrophy

PR interval

This the distance between the beginning of the P wave to the beginning of the QRS complex. Generally the PR interval should 0.12-0.2s/3-5 little boxes.

QRS complex

There are about a millions things that can go wrong with the QRS complex. But it basically comes down to two things:

  • Duration - 0.08-0.1s/2-2.5 little boxes. A wide QRS complex can signify bundle branch block or Wolff-Parkinson-White (look for ↓PR interval). Remember that broad QRS complex & tachycardia = broad-complex tachycardia which are almost invariably bad.
  • Q waves - to be considered normal: found in III & aVR; <0.04/1 little box wide; and <⅓ QRS complex height. They are abnormal anywhere else and can indicate infarction if present in V1, V2 OR V3.

QT interval

The QT interval should <1/2 RR interval. A prolongation is a risk factor for ventricular tachyarrhythmias and sudden death, making it very important that you notice it. It can be caused by: drugs - haloperidol, methadone, sotalol, amiodarone; pathology - hypothyroidism, genetic.

You may also see a short QT interval (<0.3/approx. 8 little boxes) in hypercalcaemia.

ST segment

This is where you will see MIs, essentially. By an large:

  • ST depression - angina
  • ST elevation - MI

Don't forget to look at it, particularly in people with chest pain. You can also tell where an infarction has taken place by looking at which lead the abnormality is: anterior wall (left anterior descending) - V1, V2; lateral wall (circumflex) - V3, V4; inferior wall (circumflex/right coronary/both) - V5, V6, I, aVL.

T wave

T waves should be the same direction as the QRS's largest deflection (usually the R wave). If not, it is inverted. The following are possible pathologies with abnormal T waves.

U wave

A small wave following the T wave that is usually not present. If it is > T wave in size, it is suggestive of hypokalaemia.