In med school we were taught about the Einthoven’s triangle. For an ECG purist, its the best explanation – however to workup and analyze an ECG its not the easiest. So instead of the triangle lets look at the easiest construct that we can use to analyze and interpret ECGs. Most of you already know this – but I would like to refresh it here.
An ECG lead is basically a view to the heart’s electrical activity from a given angle. This view represent the summation of all electrical activity of the heart as seen from that angle at a given moment.
We have 12 leads which look at the heart and 6 of them are arranged in the horizontal plane (the limb leads) and the other 6 around the chest (precordial leads)
Any electrical activity coming towards the lead (the view) will lead to a positive deflection and any activity going away will lead to a negative deflection
These two are the most useful constructs to work out an ECG. Although its highly simplified, it actually works out well to analyze the ECG. Lets see how the normal ECG looks using this setup.
As you can see most of the ECG can be easily worked out having in mind where the lead is looking at the heart.
It is also very important to keep in mind, that certain things on the ECG cannot be explained by this model – thats because we have really simplified the complex 3D shape and position of the heart.
As electrophysiologists the distinct advantage that we have is – our knowledge of cardiac structural anatomy – so we need to put little details of anatomy to the picture to understand some things happening in the ECG.
For example, in the normal ECG, one cannot explain tiny Q waves in inferior leads using this picture – because we have conveniently forgotten the outflow tract septum – which is depolarized upwards. However when we do factor it, it becomes readily apparent.
Therefore application of relevant anatomy is crucial in analyzing certain ECGs.
In the relevant sections, these are discussed (e.g. identification of location of accessory pathways, analyzing P wave origin in atrial tachycardia, analyzing QRS complexes to locate VT etc….
In this section, we will see how to localize things to the correct site of an ventricular arrhythmia focus by working out the ECG – not by pattern recognition. This is a fundamental concept to master as very often we need to know which chamber to go after to sort of out EP problems. Once we do this, we can easily figure out the main bundle branch block types – would be discussed later.
Using the frontal plane
In the frontal plane (limb leads), one common zone of reference is the inferior leads (II, II, aVF). If all of these leads are positive, that implies something coming form above. The commonest example, being – ouflow tract ventricular ectopics. These originate in the ouflow tracts (i.e. near below the pulmonic and aortic valve) and are directly coming down towards the main body of the ventricle – therefore all of these leads would be positive. In contrast, if an ectopic is coming from the base (bum) of the heart, they would be traveling upwards to the main body of the ventricle – thus inscribing negative waves on the inferior leads.
This top -bottom differentiation using inferior leads should be the first step. Once that has been done, the other leads can be used to fine tune the location. if aVR and aVL are both equally positive, then a very midline focus in the bottom is likely and the the inferior leads too should be equally negative. For stuff coming from the top it would be the opposite. If the lead III is more negative compared to the other two, then something from the RV base may be coming up and correspondingly, aVL should be more positive than aVR.
What about stuff from the middle of the ventricles?. If there is a focus on the left lateral wall (e.g. after a left lateral MI) leads I and aVL will be negative. Depending on the direction, lead III may be positive. In contrast a septal focus would make lead I positive as the wave would be traveling leftwards.
Lets look at some examples how we can use the limb leads to see where impulses are coming from
Now lets look at the precordial leads and see how they help with the ECG diagnosis. For EP, the main use of the precordial leads are to assess the left / right of a arrhythmia focus – i.e whether its coming from a left side chamber or right side chamber.
The most crucial fact to remember is that in the standard 12 lead ECG, V1 is the only precordial lead looking at the heart from the right side.
Therefore dominant positive deflection on V1 implies that something is coming towards it – from the septum or the left side
Conversely, V6 is at the other end and any dominant positive deflection on it implies that something is coming towards it – from the right side or the septum.
Now for a moment forget spontaneously firing foci. Just imaging that there is a heart with the left bundle branch not working. Can you workout the ECG?
Now lets try to figure out fascicular blocks. This area “Sounds difficult” and there is lot of confusion – but once you understand the principles, it is very simple to work it out.
First let’s look at monofascicular blocks (blockage of single fascicle on left branch)
Therefore bifascicular blocks are either:
- Unexplained left axis deviation and RBBB (LAHB + RBBB)
- Unexplained right axis deviation and RBBB (LPHB + RBBB)
The clue to the fascicular block is the axis deviation – which needs to be unexplained by other causes – e.g. like ventricular hypertrophy as these are common explained causes of axis deviation rather than a true fascicular block.
Accepted true Trifascicular block is either:
- 3rd degree AV block + RBBB + LAHB
- 3rd degree AV block + RBBB + LPHB
Note : the common clinically referenced tri-fascicular block – RBBB + First degree AV Block + LAD is a misnomer and should not be used (as this is a bifascicular block with AV nodal block – not true trifascicular block)
Sometime it is not very clear and there are other ECG features to look for in fascicular blocks – I don’t want to put them here as it would be confusing and my goal above is to make clear how to work out the ECG. The best clinical ECG reference on the internet to learn these are : Life in the Fast Lane (ECG)