For Emergencies:
ECG’s are one of those things I often get asked about, they seem to cause some confusion and consternation with people so I thought I’d write a quick blog going through some of the basics. To be as helpful as possible, I’ve divided it into 2 parts. In the first I’ll talk about how the electrics of the heart work and what we are actually looking for in an ECG. Then, in the second part we will look at some of the common ECGs we see and how to recognise them. Also some of the basic therapies.
So, where do we start with ECG’s? Well, an ECG is what gives us an insight into the conduction system of the heart. (Fig 1) By getting an idea of how it all works we can maybe discover where it may be going wrong. I always think it’s a good idea to have an awareness of the anatomy of our system so we’ll look at that first.
The impulse all starts with the SinoAtrial (SA) node which sits at the top of the right atrium, this is made up of numerous specialised cells which create the basic rhythm of the heart. It has an intrinsic rate of about 140 beats in the dog. There are then 3 pathways that take the electrical signal to the AtrioVentricular (AV) node. There are also various pathways that spread the impulse through both atria. The AV node is situated in the floor of the right atrium and is the only point of electrical contact between the atria and ventricles. Otherwise they are separated by a fibrous sheet which allows no other electrical activity through. The bundle of His arises from the AV node and then divides into a right and left bundle which go to innervate the two ventricles. The AV node consists of similar cells to the SA node and also has an intrinsic rate. This rate is normally slower than the SA node and so is overridden by it.
The building blocks of our ECG’s are the action potentials (AP) that produce the electrical signal through the cardiac muscle causing it to contract and produce our heartbeat. The ECG records electrical activity from the body surface and registers the differences in electrical potential generated by the heart. The signal recorded is determined by action potentials generated by millions of individual cells and their sequence of activation as it moves from the SA node, through the atria to the AV node and then through the ventricles.
Each cell produces an AP which then travels through conduction system to all the cardiac muscle cells. Each AP is produced by the movement of mainly Calcium, Potassium and Sodium ions in and out of the cells which causes an initial rapid increase in voltage followed by a plateau and finally tailing off.
These signals are transmitted to the surface of the body where our ECG leads pick them up and create the tracing we recognise. An ECG is an amalgamation of all the electrical signals.
Fig 1. Outline of Cardiac Conduction System
So, knowing what the ECG is all about, why do we need one? The principle reason for performing an ECG is to identify and diagnose arrhythmias. There may or may not be clinical signs with it, but if you hear something on auscultation that is not a regular rhythm an ECG should be performed. We can also use ECG’s to monitor for any changes during a general anaesthetic or in a critical care situation. Finally, as long as the animal is laid in right lateral recumbency, we can get some indication of cardiac enlargement from an ECG. Now that there is much more access to cardiac ultrasound this technique is not really used much more. However it is useful to know and it can given some pointers when assassin heart disease.
Parts of an ECG
The normal ECG is made up of the parts seen above. We designate them using the letters P, Q, R, S and T.
Using lead II as our standard, the P wave is the first deflection we see and is associated with atrial depolarisation and the contraction of both atria. Q is the first negative deflection. R the next positive deflection and S the next negative deflection. The QRS complex is associated with ventricular depolarisation and contraction of both ventricles. T is the deflection that comes after the QRS complex, usually in the same direction, and is ventricular repolarisation. There is a atrial repolarisation wave but it is small and normally hidden in the QRS complex.
PR is the distance between the beginning of the P wave and the beginning of the QRS complex, it is the time taken for the impulse to travel from atria to ventricles. ST is the area between the beginning of S and beginning of T and QT is beginning of Q and end of T wave.
Reading an ECG trace
There are 8 steps I use to read the ECG to help discover what’s going on. I tend to do them in the order below, but it’s not vitally important.
- What is the rate – Fast or slow (<60 in dog, 120 in cat and >160 in dog and 200 in cat)
- Does the rhythm look relatively normal – look at the R-R intervals
- Are there any very obvious large or strange complexes
- Is there a P for every QRS
- Is there a QRS for every P
- Do all the QRS complexes look the same
- Is there a regular rhythm – if irregular, is it regularly irregular or irregularly irregular
- Are there any extra beats seen.
You should get used to going through this list for every ECG tracing you look at, That will give you a good appreciation of what is normal which makes it easier to spot the abnormal. Getting to know what normal looks like is one of the most important things when starting to read ECG’s, then, when you are used to normal stepping through this list helps you narrow down where the issue actually is. In part 2 we shall have a look at the normal ECG’s we get and some of the most common abnormal ones, along with some treatments available.
