Have you ever been in this situation…

You’re listening to your patient and you know something doesn’t quite sound right. You’re not sure what, exactly - but something has got that nurse intuition tingling.

So you decide you’re going to pop your patient on an ECG to see what’s what… and then you look at your monitor, and realise you’re not quite sure exactly what you’re looking at. You know it isn’t normal, but other than that…

We’ve all been there, right? I certainly have!

Well, the good news is that you don’t need to know the ins and outs of every arrhythmia that you see - knowing what’s normal (so that you can know what isn’t) is the best first step!

But actually, we can get a lot of information about our patient’s ECG trace - and what could be going on with their arrhythmia - by asking ourselves 3 simple questions.

And if you want more information on ECGs…

Before we dive into exactly what those questions are, I’m so excited to let you know that the medical nursing academy waiting list is officially open - because (at the time of writing this) we’re opening up some more academy spots in just 2 weeks!

And what does this have to do with ECGs, I hear you ask?! Well, I’m planning something special (and of course ECG related) in the academy in September, so if you’re wanting to learn more, you’ll want to make sure that you’re inside when those doors open later this month…

Pop your email address in the box above to be the first to grab a spot!

So how can we start to interpret our patient’s ECG?

In most cases, you can get a good overview of your patient’s ECG in 3 steps. There is one exception to this rule - it’s CPR. But don’t worry, because, at the end of today’s post, we’ll chat about how to interpret CPR rhythms!

What’s the rate?

Is your patient’s heart rate bradycardic, normal, or tachycardic?

If they have bradycardia, they’re likely to have one of the following:

• Sinus bradycardia

• An escape rhythm

• Atrioventricular (AV) block

• Sinoatrial block/arrest

• Atrial standstill

If they have a normal heart rate, you’re potentially looking at:

• Normal sinus rhythm

• Sinus arrhythmia

• Atrial fibrillation

• AV block

• Ventricular premature complexes (VPCs)

• Atrial premature complexes (APCs)

And lastly, if they’re tachycardic, they may have:

• Sinus tachycardia

• Atrial fibrillation/flutter

• Ventricular tachycardia

• VPCs

• APCs

What’s the relationship between the P and the QRS?

A normal sinus complex consists of a P wave, a PR interval, and then the QRS complex (and then the ST interval and T wave).

So this means that there must be a P wave for every QRS complex and a QRS complex for every P wave.

If there is not a QRS complex for every P wave, we’re potentially dealing with:

• AV block (second or third-degree)

• Atrial flutter

And if there isn’t a P wave for every QRS complex, then it might be dealing with:

• Atrial fibrillation

• Atrial standstill

• Ventricular tachycardia

• VPCs

• Supraventricular tachycardia

• APCs

• Escape complexes

So that is there is not a normal relationship between P and QRS - but what about if there is a P wave for each QRS complex?

Well, the next question we ask ourselves is how are they related. Are they close together, and is this consistent?

If your answer is ‘yes’, then you may have:

• Sinus rhythm

• Sinus bradycardia

• Sinus arrhythmia

• Supraventricular tachycardia

• APCs

And if you don’t have a normal, consistent relationship between the P wave and QRS, your patient could have either 3rd-degree AV block or ventricular tachycardia.

What’s the morphology?

What does your waveform actually look like? Specifically, we’re interested in the QRS complex here. Is the complex tall and narrow? Or more wide and bizarre?

If your QRS complex is tall and narrow, there’s a good chance that it has originated from the atria (remember, the AV node sits at the bottom of our right atrium).

If, however, you have a wide and bizarre QRS complex, this is abnormal and likely to have originated in the ventricles.

Think about the type of QRS you see with a VPC - these are more of those wide, bizarre complexes you expect to see originating from the ventricles themselves.

Anything else?

There’s one other thing I like to ask myself when looking at my ECG trace. And that’s my rhythm:

• Regular (ie a repeating pattern)

• Irregular (where there is no discernible pattern)

• Or regularly irregular (where there are abnormalities that repeat?)

This might help me understand how worried I’m going to be about the arrhythmia.

Let’s look at an example here:

Say I’m monitoring a patient under anaesthetic and they start having some second-degree AC block. They have 5-6 normal complexes, then they have 2-3 P waves without a QRS, and this repeats.

I assess my trace and I know that:

• I have a relatively normal heart rate (affected by anaesthetic drugs to some extent)

• I have P waves with no QRS

• I have a regularly irregular rhythm

So now, what do I do? Well, I know that several anaesthetic drugs can cause this, and it’s not a super irregular rhythm. So I’m going to assess my patient, and as long as they didn’t have any abnormalities before their anaesthetic, and their blood pressure, pulse quality, etc is normal, I’m going to monitor them but not intervene.

I will, of course, mention it to the vet and be guided by them… but in general, we can panic a little less about arrhythmias like this!

But what about interpreting ECGs in CPR?

CPR is a whole other ball game when it comes to our ECG interpretation. There are 4 rhythms that we care about in CPR - these can be divided into shockable and non-shockable rhythms.

The Non-Shockable Rhythms

Non-shockable rhythms are those which cannot be converted to a normal rhythm using electrical defibrillation. The two non-shockable rhythms are asystole and pulseless electrical activity.

Asystole is a flat (or near-flat) line associated with no electrical activity in the heart.

Pulseless electrical activity is a trace which should be associated with a pulse but is not. It appears as regular, repeating complexes which might be sinus, but might take other forms. It is a slower rhythm (rate <200 beats per minute).

The Shockable Rhythms

Shockable rhythms are those which are responsive to electrical defibrillation. This temporarily stops the electrical activity in the heart, for normal pathways to be restored. 

The two shockable rhythms seen during cardiopulmonary resuscitation are pulseless ventricular tachycardia and ventricular fibrillation.

Pulseless ventricular tachycardia is a rapid rhythm with a rate of over 200 beats per minute. The complexes are regular and repeating, as seen below.

Ventricular fibrillation is seen as an erratic, wavy line. There are no discernable complexes and a heart rate cannot be measured. This rhythm is associated with chaotic, irregular ‘fluttering’ activity of the cardiac muscle, resulting in a lack of proper cardiac contraction.

So how do we tell them apart?

Again, you can do this by asking yourself a few simple questions:

1. Is my ECG a flat line?

If the answer’s yes, you’ve got asystole. If not, move on to question 2!

1. Are the complexes regular and repeating?

If the answer is no, you’ve got ventricular fibrillation. If not, move on to the last question below.

1. Is the heart rate less than 200?

If the answer is yes, you’ve got pulseless electrical activity. If the answer is no, then you’ve got pulseless ventricular tachycardia!

So there you have it! ECGs don’t need to be complicated - we can start breaking them down by looking at a few simple questions:

1. What’s the heart rate?

2. What’s the P and QRS relationship?

3. What does the QRS complex look like?

And in CPR, looking at whether you have a flat line, regular repeating complexes, and the rate of the rhythm you’re seeing.

So go out there now and start looking at those ECGs! And - if you want more info on troubleshooting ECGs, make sure you’re on the academy waitlist ready for doors to open in 2 weeks!

References

1. Fletcher, D. et al. 2012. RECOVER evidence and knowledge gap analysis on veterinary CPR part 7: Clinical guidelines. Journal of  Veterinary Emergency and Critical Care, 22 (S1), S102-S131.

2. Pace, C. 2023. Electrocardiographs for veterinary nurses. InFocus [Online], available from: https://www.veterinary-practice.com/article/electrocardiographs-veterinary-nurses

3. Sleeper, M. 2020. Reading ECGs in veterinary patients: an introduction [Online] DVM360. Available from: https://www.dvm360.com/view/reading-ecgs-in-veterinary-patients-an-introduction