Ever been in this situation?

You’re monitoring a sedation, and you pop your patient on an ECG.

They seem stable, but bradycardic - not unexpected, given that you’ve sedated them.

Their blood pressure is normal, though, so you’re not worried… and then you look at their ECG, and suddenly see that they’re dropping QRS complexes.

Atrioventricular block (AV block) is one of the most common bradyarrhythmias in anaesthetised or sedated patients. But how do we know what kind of AV block our patient has - and more importantly, when do we need to worry?

Today we’ll look at the 3 types of AV block - first, second, and third degree. We’ll break down what they are, what changes we see to our trace, why they happen, and how each is managed.

You’ll leave today’s post feeling more confident at recognising these arrhythmias, and understanding what they mean for your nursing care.

So what is AV block?

To recap our electrical pathways through the heart, the electrical impulse begins at our sinoatrial (SA) node, spreads to the atrioventricular (AV) node, across the bundle of His and the left and right bundle branches, and then throughout the Purkinje fibres.

The depolarization of the atria occurs as the impulse travels from the SA to the AV node, resulting in atrial contraction. And as the impulse transfers from our AV node to the bundle of His, branches and Purkinje fibres, we see the rest of our complex. 

AV block occurs when the impulse is not correctly transferred through the AV node, to the ventricles.

There are several types of AV block, with each resulting in a different ECG trace:

• First-degree (aka prolonged conduction)

• Second-degree (aka intermittent conduction)

• Third-degree (aka complete heart block)

First-degree AV block

First-degree block is also known as prolonged conduction. In this form of AV block, the electrical impulse is still transmitted through the AV node, but more slowly.

This results in a prolonged P-R interval (the gap between the P wave and the beginning of the QRS complex).

Why do we see this?

Because as the electrical impulse travels through the AV node, there is a brief pause in electrical activity - so our ECG returns to baseline. This is the P-R interval. If this conduction is prolonged, the baseline period will also be longer.

What signs do we see?

Thankfully, none! This is usually a rhythm that is generally picked up incidentally, e.g. under anaesthesia.

Second-degree AV block

Second-degree AV block is also known as intermittent conduction. In this arrhythmia, occasional electrical impulses are not transmitted through either the AV node, bundle of His, or left and right bundle branches.

This is one of the more common arrhythmias we see under anaesthetic or sedation, e.g. in patients receiving alpha-2 adrenergic agonists. We usually see this as occasional P waves that are not followed by a QRS complex.

There are several different types of second-degree AV block, including Mobitz type I, Mobitz type II, and high-grade.

Mobitz type I

This is also known as the Wenckebach phenomenon. In this form of second-degree AV block, the P-R intervals progressively lengthen, until a P wave without a QRS complex is seen.

Mobitz type II

In this form of AV block, the P-R intervals do not progressively lengthen, instead remaining static ahead of the dropped complex.

High-grade second-degree AV block

A second-degree AV block is characterised as a high grade when they have a P: QRS ratio of at least 2:1 (ie. they have at least 2 P waves for every QRS complex seen). This produces an extremely slow ventricular rate

So what signs do we see in these patients?

The answer really is that it depends on the type of second-degree AV block they have.

Every patient will have an absent S1 and S2 heart sound, and absent arterial pulse during the block (the dropped QRS complex) itself, but the impact this has on our patient will be variable depending on the frequency of these dropped complexes.

Patients with type I second-degree AV block generally require no treatment. Type II second-degree block is at a higher risk of progressing to a more severe (high-grade) form, so requires closer monitoring, but no specific treatment.

Patients with high-grade second-degree AV block often present with clinical signs matching more severe forms of AV block, such as syncope, and are at a higher risk of sudden death.

Third-degree AV block

Complete heart block (aka third-degree AV block) is seen when none of the electrical impulses are conducted from the atria to the ventricles.

These patients have atrial-ventricular dissociation, and we see a separate atrial and ventricular rhythm on our ECG.

The P waves (the atrial rhythm) occur more rapidly, whereas a slower ventricular escape rhythm results in slower, abnormal QRS complexes on the ECG. These ventricular escape rhythms either originate from subsidiary pacemaker cells in the AV node (known as nodal escape beats), or from the Purkinje fibres within the ventricles themselves (known as ventricular escape beats).

What signs do we see with third-degree block?

These patients generally have regular, but slow, heart and pulse rates. The rate is also not usually increased by any stimulation (eg. exercise or excitement) or medications (eg. atropine).

Jugular pulsations can also be seen, as occasionally the atria will contract whilst the ventricles are contracting - due to that dissociation between atrial and ventricular activity.

These patients generally have a history of weakness, exercise intolerance and syncope/collapse episodes associated with their arrhythmia.

What causes AV block?

The causes vary depending on the type of AV block present.

Both first and second-degree AV block can be seen due to non-cardiac causes. For example, secondary to anaesthesia, sedation or high vagal tone (seen in brachycephalic patients, or patients with GI, ocular, respiratory or neurological diseases).

First-degree and type I second-degree AV block can be considered normal/expected findings in these circumstances. Other forms of second-degree block and all cases of third-degree block are always considered abnormal.

Other causes of second and third-degree AV block include fibrosis, neoplasia or injury to the AV node, affecting impulse conduction. Rarely, electrolyte abnormalities and increased vagal tone can also cause these more severe forms of AV block.

So how do we treat it?

Where a specific underlying cause has been identified, we aim to treat that - in most cases, though, this is unfortunately not possible.

Second-degree AV block

For cases of high-grade second-degree AV block, we occasionally treat with positive chronotropic medications - drugs that increase conduction through the AV node, in turn increasing heart rate. These include theophylline and terbutaline. However, where clinical signs are present, more aggressive treatment with pacemaker implantation is usually required.

Third-degree AV block

In dogs with third-degree AV block, the only effective treatment is pacemaker implantation. Dogs are at risk of sudden death, and so regardless of whether clinical signs are present, if this arrhythmia is noted, a pacemaker should be implanted.

Cats are a little different - they are usually not clinical for this arrhythmia and generally do not require treatment unless clinical signs are present. 

What does this mean for our nursing?

So now we know how to spot AV block, and what it means for our patients - but what does this mean for us as nurses?

The most common time we’ll see this is when monitoring patients under anaesthesia - and this is usually first-degree, or type I second-degree AV block.

The first thing to ask yourself when you note these arrhythmias is “What is your patient doing?”

Look at their perfusion status, checking for:

• Changes in pulse quality

• Changes in blood pressure

• Changes in heart rate

• Progressive changes to their ECG trace

If everything is stable and your patient is not showing any specific clinical signs, then continue to monitor these parameters closely.

So there you have it - an overview of first, second and third-degree AV block! Remember, examine that P-QRS association to determine the type of AV block you have, then ask yourself:

• Is my patient stable?

• Do they have clinical signs?

And always go back to your basics - pulses, blood pressure, mentation and perfusion.

Do you see these arrhythmias often? Drop me a DM on Instagram and let me know!

References

1. Kittleson, M. D. 2023. 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#v76144521

Pace, C. 2013. Common arrhythmias: the importance of ECG interpretation [Online] The Veterinary Nurse. Available from: https://www.theveterinarynurse.com/review/article/common-arrhythmias-the-importance-of-ecg-interpretation