The top 4 ventricular arrhythmias in dogs and cats explained
Let me share a story from much earlier in my career…
I had recently qualified. I trained in a practice that didn’t have an ECG. I saw mostly stable patients, except for the odd splenectomy or GDV, and we transferred them to our larger hospital for post-operative monitoring (because they had things like ECGs, alongside 24/7 staffing).
So when I made the jump, 6 months later, into a referral hospital, I had a lot of learning to do.
One day I was monitoring a patient’s sedation. It was for a post-op x-ray case on an orthopaedic patient. He had the usual medetomidine and butorphanol sedation, and I was left with the patient in a side room whilst the x-ray room was prepared. I had my stethoscope and was keeping a close eye on him, when suddenly things started sounding REALLY weird.
I couldn’t tell you what was going on with that patient’s heart, but it was certainly fast, and certainly NOT normal.
I panicked and had absolutely no idea what to do at that moment (except shout for help, which I did. I think the patient probably had a run of ventricular tachycardia, but it had resolved by the time we got him on an ECG).
Now, with the benefit of hindsight and more experience, I know I’d get that patient on an ECG without even questioning it and see what I was dealing with… but back then, I had never really used them, so didn’t feel confident with them.
But the only way to get more confident with them is to START USING THEM. And it doesn’t matter if you don’t know what you’re looking at - as long as you know what’s normal, you’ll understand when something’s not right.
And the more you use them, the more you’ll understand the different arrhythmias we see.
In today’s post, we’re going to talk through the top 4 ventricular abnormalities you’ll see on an ECG - ventricular premature contractions, accelerated idioventricular rhythm, ventricular tachycardia and ventricular fibrillation.
You’ll leave today’s post feeling more confident about what each of these is, how it happens, and how we tend to manage them… so you don’t end up in the situation I was in, not knowing what to do next.
PS. If you want more information on ECGs, I’m hosting an event inside the Medical Nursing Academy that you’ll NOT want to miss! To save your spot, get your name on the waiting list below because doors are opening in just one week!
Let’s revisit the heart’s normal electrical pathways for a second…
In a normal patient, the electrical impulses are transmitted from the sino-atrial node to the atrioventricular node, across the bundle of His and to the Purkinje fibres.
From the Purkinje fibres, the impulse spreads across the fast, conductive cardiac myocytes in the ventricles, causing them to contract in a coordinated, controlled way.
But what happens in a ventricular arrhythmia? Usually, the electrical impulses originate abnormally from within the ventricles themselves, bypassing the normal conductive pathways we’ve just described.
So let’s look at these arrhythmias…
Ventricular premature contractions (VPCs)
VPCs are extra, irregular beats originating from the ventricles. These beats interrupt our patient’s normal sinus rhythm.
We can see these singly, or in couplets (two in a row), triplets (three in a row), or runs (four or more VPCs in a row).
VPCs appear on our ECG as a wide, bizarre QRS complex. This is because the usual Purkinje fibres that cause ventricular contraction are bypassed when VPCs occur, with the electrical impulses instead being transmitted through other, slower, cardiac myocytes responsible for the relaxation of the heart.
Because of their premature timing, they are often not preceded by a P wave (sometimes a P wave may incidentally occur around that time, but not in association with the VPC) and they are followed by a much shorter diastole phase, and therefore a shorter TQ interval.
VPCs can be seen in patients with cardiac disease, but also in non-cardiac cases, for example:
Secondary to anaesthetic agents
Secondary to liver or splenic disease
So what do we do about a VPC? Well, if you’re only seeing a few in a row, the patient is not significantly tachycardic and their pulse quality and/or blood pressure is normal, we tend to monitor them.
We only begin to consider specific treatment when we’re seeing lots of VPC runs in a row, with a rapid heart rate.
Accelerated idioventricular rhythm (AIVR)
AIVR is defined as four or more VPCs in a row, with a heart rate higher than an escape rhythm, but less than ventricular tachycardia (more on that in a minute). It generally is a rate somewhere between 60-120 beats per minute but can be up to 160 beats/minute.
Because the heart rate in AIVR is similar to the patient’s normal (sinus rhythm) heart rate, the two will often compete to be the ‘pacemaker’ or dominant rhythm. This means that we typically see runs of VPCs interspersed with sinus complexes on our patient’s ECG.
Like VPCs, AIVR can be seen both due to cardiac and non-cardiac causes, for example:
Abdominal diseases such as pancreatitis, splenic disease
Trauma
Abdominal surgery (e.g. following GDV correction)
Ischaemia (lack of blood flow/perfusion, for example to the cardiac myocytes)
IMHA
So what about treatment for AIVR? The good news is, much like VPCs, as long as the patient is haemodynamically stable, no treatment is needed. So keep an eye on their blood pressure, pulse quality and mentation, and assess the ECG regularly for changes to their rhythm, or increases in their heart rate. Usually, you’ll find that as their underlying systemic disease is treated, the AIVR resolves.
If the rate increases and our patient tips into ventricular tachycardia, that’s another story, and treatment will be required.
Ventricular tachycardia (VTach/VT)
VT is a life-threatening arrhythmia consisting of runs of VPCs without associated P waves, at a high rate (>160 beats/minute).
It can occur in patients with primary cardiac disease such as dilated cardiomyopathy or right ventricular cardiomyopathy, or secondary to non-cardiac diseases.
There are several changes which increase the risk of sudden death with ventricular tachycardia, including:
Increasing heart rate
R-on-T phenomenon (where there is no diastole period so no normal T wave; instead, the T wave goes straight into the R wave of the next complex)
Torsade de pointes (an abnormality resulting in rapid VT where the complexes reverse polarity, resulting in a twisting effect on the ECG. This rhythm is at risk of turning into ventricular fibrillation)
Ventricular tachycardia is also often associated with poor perfusion as the fast rate does not allow for sufficient refilling of the heart, and the abnormal complexes may not pump blood around the body as effectively. When assessing these patients, look for pulse deficits (where the pulse rate and heart rate do not match) and assess blood pressure, as hypotension is a risk.
So we know that VT is one of our ventricular arrhythmias requiring urgent drug treatment. This is generally achieved with lidocaine initially, with boluses given +/- a CRI administered as required.
Other treatments used to manage VT include sotalol, mexiletine, and amiodarone.
Ventricular fibrillation
The last ventricular arrhythmia we’re going to look at today is ventricular fibrillation (VF or VFib). This rhythm is associated with cardiac arrest and requires emergency assessment and management.
In VF, the electrical activity within the ventricles is disorganised, resulting in the ventricles ‘quivering’ rather than contracting.
This means there is no ventricular contraction, no heartbeat, and therefore no cardiac output.
These patients will present in cardiac arrest and require CPR, so basic life support must be initiated whilst advanced monitoring (including an ECG) is placed, and the arrhythmia identified.
Once you’ve identified that your patient is in VF, the only effective treatment is electrical defibrillation - where we pause the electrical activity in the heart, allowing it to ‘reset’ and hopefully convert to a normal (sinus) rhythm.
So there you have it! 4 of the top ventricular arrhythmias we encounter in our patients, and how they are managed. I really hope that you don’t end up having to deal with a ventricular tachycardia, or worse, a fibrillation case - but if you do, I hope that you now feel more confident in what to look for in these patients, and how to effectively treat them!
Luckily for us as VNs, most of the arrhythmias we’ll see will be VPCs or AIVR cases. So in these patients, go back to basics and assess your patient first, looking at their:
Pulse quality
Presence of pulse deficits
Blood pressure
Mentation
Overall status
And ask yourself - what is worrying you about the patient? Trust your gut. If you think the only abnormality is their VPCs but they’re otherwise fine, don’t panic. Instead, alert the vet, keep a close eye on them, and watch for any change in their ECG trace or heart rate.
Do you deal with many arrhythmias in practice? How do you feel about interpreting ECGs? Drop me a DM on Instagram and let me know!
PS. Don’t forget, if you want to work through examples of arrhythmias like these, make sure you’re on the academy waiting list ready for our ECG event next month!
References
Estrada, AH. and Jones, A. 2014. Top 5 arrhythmias in dogs and cats [Online] Clinician’s Brief. Available from: https://www.cliniciansbrief.com/article/top-5-arrhythmias-dogs-cats
Kavanagh, J. 2023. What are the five things you should know about companion animal cardiology? [Online] InFocus. Available from: https://www.veterinary-practice.com/article/five-things-to-know-about-companion-animal-cardiology
Kittleson, MD. 2023. Heart disease: conduction abnormalities in dogs and cats [Online] MSD Veterinary Manual. Available from: https://www.msdvetmanual.com/circulatory-system/heart-disease-conduction-abnormalities-in-dogs-and-cats/heart-disease-conduction-abnormalities-in-dogs-and-cats
Ohad, DG. 2020. Ventricular premature complexes (VPCs) [Online] Beecardia. Available from: https://beecardia.com/blog/ventricular-premature-complexes-vpcs
