ECGs are pretty simple once you get the hang of it. And I’m pretty far from getting the hang of it. But here’s a quick run through of what I do know.
We first need to understand the electrical pathway of the heart. ECGs trace the signals that run through your heart as the excitation travels down this pathway. Essentially, the signal is first generated in the SA node. This signal then travels through the atria while at the same time going towards the AV node via the internodal pathways. The signal then makes it way through the Bundle of His, down the Purkinje fibres, and finally through the ventricles.
12 Lead ECGs
This is really a misnomer. There aren’t 12 leads in a 12 lead ECG. In fact, there are only 10. They run in order from V1-V6, RL, RA, LL, LA. The RL line is sometimes called the ground line. RL goes on the right leg, RA to the right arm, and so on. The V1 to V6 leads are placed on the chest. Here is a diagram showing their placement.
V1 and V2 are placed on either side of the sternum in the 4th intercostal space. We then place V4 in the mid clavicular line in the 5th intercostal space. And V3 roughly in between V2 and V4. V5 goes in the anterior axillary line, following a rough line you draw from the leads already place. And V6 is once again on that curve in the mid axillary line.
You then hit the button and off you go! there’s no saying how to work an ECG machine as each one works differently. Your hospital might have more than one machine which makes for lots of fun times trying to get the machines to work.
ECGs can be ruined by patients moving, talking, or even just the simple misplacement of leads. Always check that you’ve attached them correctly of you’ll get a horrifying reading.
Understanding the ECG
When reading an ECG, remember that each small square is equivalent to 0.04s. And each bigger box is 0.2s. Therefore, 30 bigger boxes makes 6 seconds. To calculate rate, count the number of QRS complexes in 30 boxes and multiply by 10 to get your rate per minute.
PR Interval is rather oddly named, since it measures the length from the start of the P wave to the beginning of Q. This shows the time taken for the excitation to spread from the SA node, through the atrial muscle and the AV node, down the bundle of His and into the ventricular muscle. This, normally, should take 0.12 to 0.2 seconds (or 3-5 small squares).
The QRS complex measures how long the excitation takes to spread through the ventricles, causing depolarization (not contraction). This should take 0.12s (or 3 squares).
ST segment is the bit between the S wave and the T wave. This bit can be elevated (in ischemia) or depressed (in infarction). We can only say it is an elevation or depression is there is a deviation from the baseline of more than 2 squares vertically.
Axis deviation happens most commonly during hypertrophy of either side of the heart. For instance, during Right Axis Deviation, the right ventricle is hypertrophied, causing the heart to be shifted slightly and causing a negative QRS in Lead I. An easy way to remember this is to look at Lead I and aVF. If the QRS complexes are “reaching” towards one another, it’s a Right Axis Deviation. If they are “leaving” one another, it’s a Left Axis Deviation. LL, RR. Easy.
We don’t usually see a U wave. And the presence of one might or might not be pathological.
Steps to do when looking at an ECG:
- Rate
- Is it regular or irregular
- Is there any axis deviation
- Is there a P wave
- What is the PR interval
- What is the QRS interval
- Is there an ST segment elevation/depression
Rate can also tell you about any pathology that is going on. For instance, a normal sinus rate is 60-100. The heart contains its own natural cardiac pacemakers, each firing at its own rate.
- SA Node - 60-100
- AV Node – 40-60
- Bundle of His/Purkinje fibres – 20-40
Usually in a normally functioning heart, the SA node fires and sends the signal down the pathway before the other pacemakers have time to fire as well, thereby overriding them.
Important to know is also which part of the heart you’re looking at. This table shows where each lead is giving you a window to. Identifying where an infarct is is very important.
Now there are a ton of ways to go about interpreting an ECG. and there are a ton of different ECGs as well. Having some sort of systematic approach would be your safest option when first starting out, and ensure you don’t miss anything. Here’s something I got from my attachment at Sale Hospital last year. Please don’t take it without asking first. It’s sort of copyrighted.
And there you have it. A short introduction to ECGs.
