For Emergencies:
In our last blog we looked at the basic structure of the conduction system in the heart, how that created the ECG we look at and the basic structure of our ECG – the P-QRS-T complexes. This time, we are going to have a look at some normal ECGs and then some common abnormal ones. We will also touch a little on treatments for them. Just to recap before we go on, the 8 steps to consider when reading your ECG’s. By applying these every time you look at an ECG, it will make interpretation easier.
- 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.
Normal ECGS –
i) Sinus Rhythm –
In this normal trace you can see there is a P wave before every QRS and every P wave is followed by a QRS. The distance between each QRS complex is relatively regular too. The rate will fall between 60-160 for a dog or 120-200 for a cat. All the complexes look pretty much the same too.
ii) Sinus Bradycardia –
This trace shows sinus bradycardia. Apart from the rate being less than 60 in the dog and 120 in the cat, it looks exactly the same as sinus rhythm. Although this could be pathogenic, you can see it in really fit dogs or when very relaxed. It can also be seen with some sedatives like medetomidine.
iii) Sinus arrhythmia –
Sinus arrhythmia is a common finding on ECG and is often one that can confuse. With this, as you can see above, there seems to be a regular change in the rate between fast and slow. This change coincides with breathing, with an increase on inspiration and slowing on expiration. The complexes all look normal. This occurs due to pressure changes in the chest from breathing that affect the vagus nerve. In the dog this is considered normal, unless it is exaggerated. In the cat it is never normal. It can be associated with pulmonary disease.
The first three traces we’ve seen can be considered as normal findings on an ECG. It is worth getting used to what they look like as we better recognise the abnormal when we have a clear idea of what the normal looks like.
Abnormal traces can be roughly divided into 3 categories:
- Supraventricular rhythms originate from above the ventricles. These are generally due to issues in Atrial tissue or the AV node.
- Ventricular rhythms which originate from the ventricular tissue.
- Conduction issues which are all about the electrical signal not getting where It’s meant to go.
iv) Supraventricular premature contractions
These are ectopic or extra bears that originate prematurely from atrial or nodal tissue.
There are a few points to bear in mind to recognise it –
- The beat comes earlier than expected;
- The QRS complex looks similar to others (i.e. narrow);
- The P wave usually looks a little different to the normal ones;
- There is usually a non compensatory pause (i.e. the distance between the premature beat and next P wave is the same as previous normal P-P distance.
- When there are Multiple premature complexes in a row we call this Supraventricular tachycardia (3 or more).
- This ECG is usually an indication of Atrial disease. (enlargement)and generally only needs treating if it is causing haemodynamic issues.
v) Atrial fibrillation
This is a common occurrence in dogs with advanced heart disease. It usually happens because the atria become very enlarged at this stage of the disease. It can also occur in giant breed dogs without structural heart disease, this is called lone Afib and it carries a better prognosis.
On the trace you can see completely disorganised atrial depolarisations and the rate can be running at 200-1200 bpm. You can just see an irregular baseline and no P waves. There are only a percentage of stimuli allowed through the AV node as it would otherwise be fatal having the ventricles contract at the same rate. The complexes are still narrow as they are originating from the AV node. Atrial fibrillation is a rapid irregularly irregular rate and auscultating it is often compared to hearing ‘Sneakers in the Tumble drier’.
This does require treatment as it can rapidly deteriorate. A combination of Digoxin and diltiazem are the drugs of choice. The aim is to reduce the ventricular rate not conversion. It is a poor prognostic indicator when the animal is in congestive heart failure.
vi) Ventricular premature complexes
In this rhythm the complexes are instigated in the ventricles. Like SVPCs they appear early, but this time they will have a wide and bizarre QRS complex, there is also a compensatory pause (i.e. the distance between preceding and proceeding waves is equivalent to 2 normal complexes.) This problem is caused by either diseased ventricular tissue setting up an ectopic foci or underlying systemic disease causing cardiac irritation e.g. GDV; Splenic tumour; sepsis. If there are more than 3 complexes, we will classify this as Ventricular tachycardia.
When making this diagnosis you need to be aware of false flags as there are a couple of situations that can mimic them – (1) changes in the QRS complex due to cardiomegaly and an axis shift; (2) intraventricular conduction disturbances within the bundle branches which can lead to wider QRS complexes; (3) abrupt motion or other artefacts; (4) the wide but nonpremature QRS complexes of ventricular escape beats; and (5) QRS morphology changes caused by severe hyperkalemia.
There are also two specific diseases to be aware of that may be expressed through multiple VPCs – Right Ventricular Arrhythmogenic Cardiomyopathy in boxers and sudden death in German Shepherd dogs.
Treatment will be based on the underlying cause and clinical symptoms.
vii) Ventricular Tachycardia
When we see 3 or more VPCs it will be classed as ventricular tachycardia, however, it can also appear like this image which shows sustained ventricular tachycardia.
The causes are the same as VPCs and clinically it can cause weakness, syncope, hypoxic/ anoxic seizures. How much an issue it causes depends on the underlying rate and how long it is sustained for.
It can cause serious haemodynamic effects and therefore does need treatment. If the dog has collapsed and the rate is very fast, it is an emergency. You then should ensure there is not a major underlying cause as many treatments themselves can be proarrhythmogenic.
Generally lidocaine bolus at 2-3mg/kg is the initial treatment of choice.
Be aware of an accelerated idioventricular rhythm. It looks similar and is run of VPC’s however, is at slower rate, usually between 70-160 bpm. It is fairly benign, but needs careful monitoring.
viii) Ventricular fibrillation
This is generally a terminal event. There is a chaotic baseline with no set rhythm or complexes seen. It usually requires electrical defibrillation to have any chance of conversion.
You don’t want to see this in practice.
ix) 1st degree AV Block
AV blocks occur when something interferes with the conduction of the signal from the Atria to the Ventricles, the problem is usually in the area of AV node.
There are three categories of AV block seen first, second and third degree.
1st degree AV block as seen here, is generally not an issue. With this you will see a longer than normal PR interval. You do need to be aware of it as it can progress to more serious blocks and cause bradycardia.
x) 2nd degree AV block
In 2nd degree AV block, not all the P waves are conducted. There are 2 types described, I and II. In type I the PR interval gets longer and longer until a block occurs. With type II, (seen in image above) there are regular P waves without a following QRS complex. It will be in a set pattern such as 2:1; 3:1 etc.
Type I is usually benign whereas type II can generally be more serious. The issues seen are caused by the level of bradycardia. Syncope, weakness, exercise intolerance and very occasionally seizures can be seen. If treatment is required then a pacemaker is treatment of choice.
xi) 3rd degree AV Block
In 3rd degree AV block, none of the P waves are conducted. It can occasionally look like they are but is just where the P waves will randomly appear in what looks like the correct placing occasionally. The QRS complex can be wider than the normal QRS complex as it is an escape rhythm and generated from ventricular foci.
This condition will usually cause clinical symptoms due to a pronounced bradycardia. The treatment of choice is a pacemaker.
xii) Bundle Branch blocks.
Where there is some disturbance to the conduction of electrical signal into the ventricles you can have a block to either the right or left bundle branch conduction system. It will produce a wider QRS complex as the signal takes longer to propagate in the relevant ventricle. Right blocks are generally negative in Lead II and Left are positive.
They are generally an indication of changes to the ventricles. It is important to distinguish them from VPCs as they can look similar. The main differences are – There is a P wave before each complex; There is a fixed PR interval; The QRS complexes are consistent throughout.
RBBB
LBBB
Conclusion
I’ve tried to go through the majority of changes that you may see when recording an ECG, they are more and also variations of the above but just getting used to what these look like is always a good starting point. The secret to learning how to read ECGs is to look at them regularly, have a good understanding of what normal looks like and then use basic principles to see what is not normal.
Good luck in your learning, and as always we are always here to give advice if you get stuck.
