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ECG Tutorial 2: The Electrocardiograph

ECG Tutorial 2: The Electrocardiograph

You may well be wondering what an mugshot has to do with an ECG! Well don't worry the answer is contained within...

In this ECG tutorial we will cover the fundamentals of the Electrocardiograph. In the last tutorial: ECG Tutorial 1: Anatomy & Electrical System we discussed the gross anatomy and electrical conductivity of the heart. If you are not familiar with content of that tutorial I would suggest that you go back and read over it again as this tutorial will build on that information.

We have omitted some information that while useful, is of more use to people studying and interpreting 12 Lead ECG's. This tutorial is primarily aimed at people learning 3 Lead ECG's.

ECG Vectors

We have already discussed the electrical system of the heart. As well as understanding about the rates of the different nodes and branches and cells, it is important to be able to calculate the direction and strenght of the electrical activity. That is where vectors come in! A vector represents amounts of energy and direction. 

Think of it this way:

Imagine you were having a tug-o-war with someone who was the same size, weight and strenght as you. 

b2ap3_thumbnail_vector1.jpg

 Because the vectors would be the same size (representing the same force) and opposite direction (as you are both pulling away from each other) the net result would be zero. Meaning the only way somebody is going to win is if the other get's tired, looses their grip or gives up. 

 You may well be thinking why do I need to know this? Well it's simple really. The video below shows the electrical activity of the heart during normal sinus rhythm. 

As we can see, the electrical impulses change direction as it moves through the normal electrical cycle. 

 

The Electrical Axis of the Heart

Lets imagine we had to add up all the millions of individual vectors in the ventricles during the a normal cardiac cycle. We would be there for quite a while adding, subtracting and taking into account direction change. If we did manage to calculate the figure it would be called the electrical axis of the ventricle. There is a P-wave vector, a T-Wave vector, and ST segment vector and a QRS vector. The ECG is simply a measurement of these vectors as they pass under an electrode. 

The Electrodes
b2ap3_thumbnail_ecg_waves.jpg

Electrodes are sensing devices that pickup the electrical activity occurring beneath them. As a positive electrical wave moves away from the electrode, the ECG records this as a negative wave. When a positive wave is moving towards an electrode the ECG records this as a positive wave. When the electrode is somewhere in the middle the the ECG records a positive deflection for the amount of energy moving towards it and a negative deflection for the amount of energy moving away from it.

Think of an ECG as like taking a single picture. On it's own it cant tell us a whole lot.

Think of this, one of the things that happens when somebody is arrested is they are photographed. Usually holding some sort of serial number and standing infront of a chart indicating their height. Once they are photographed from the front they are usually asked to turn to the side for another photogarph showing a side view. This can give us details that the first picture cant give such as the shape of the nose. It's important that if you are trying to detail and catalogue something that you get multiple views. 

Looking at the heart from different views can help built up a three dimensional image of the hearts electrical axis. 

b2ap3_thumbnail_3Lead_Placement.jpg

Knowing where to place the electrodes is just as important. We need to make sure we are looking at the heart from the right angle. 

You should place the electrodes as shown in the picture. You can place the electrodes on the shoulders or arms. It doesn't matter if they are placed on the shoulders or arms as long as they are 10cm from the heart. You should always follow your own organisations protocol with regards to electrode placement. 

 

b2ap3_thumbnail_axis.jpgWhen we have the electrodes placed on the body it gives us pictures of the electrical activity of the heart from different angles or perspectives. This is shown in the picture below. We can see the how the leads view the heart relative to each other. So we are already starting to build up our three dimensional picture of the electrical activity of the heart.

We now have three different views of the heart:

Lead I
Lead II
Lead III

This is as far we go with the different views of the heart. We now have 3 different perspectives to view the heart from. Everything we do from here on in will be based on looking at the heart through one of those views. 

From this picture you can see why, when people are learning about ECG's, they will usually start by looking at Lead II, as this lead is the one closest to the normal QRS vector which shows the electrical axis of the ventricles. And we already know why the ventricles are so important! 

Looking Forward

Remember we said the ECG was like a mugshot? Think of the person who has been arrested, we have already discussed why its important to get a picture from different angles. But now what we have to do is give those pictures some scale and reference. In the police station they do that by having a height chart behind the person. With an ECG we do it by printing the different views onto special ECG paper. In the next tutorial we will cover the ECG paper and what it shows us about the pictures we have just taken. 

Conclusion

We have covered quite a bit in this tutorial. If you have understood the information in this tutorial then you are well on your way to having a solid understanding of ECG's and the corrsponding electrical activity within the heart. It is the basics covered in this tutorial that make the foundation of ECG interpretation.

 

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ECG Tutorial 3: The Printout
ECG Tutorial 1: Anatomy & Electrical System

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