Atrioventricular block: Clinical sciences

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Introduction0:00–0:46

Atrioventricular block, or AV block for short, is a conduction disturbance that occurs when an atrial impulse gets delayed or completely blocked at some point along the electrical conduction system of the heart.
This conduction disturbance can occur within the AV node, the bundle of His, or bundle branches; and can be caused by either reversible causes, such as increased vagal tone; or irreversible causes, like fibrosis of the heart.
AV block presents with bradycardia, which can range from asymptomatic and benign, to severe and life-threatening. Now, based on the severity of the AV block, we can classify it as either first-, second-, or third-degree.If your patient presents with a chief concern suggesting an atrioventricular block, perform an ABCDE assessment to determine if the patient is unstable or stable.

Unstable Patient0:46–1:49

If they’re unstable and there’s a detectable pulse, follow the ACLS guidelines for Bradycardia with a Pulse. You should stabilize their airway, breathing, and circulation.
Next, obtain IV access and put your patient on continuous vital sign monitoring including heart rate, blood pressure, and pulse oximetry, as well as cardiac telemetry.
Finally, if needed, provide supplemental oxygen to maintain oxygen saturation.Now, here’s a high-yield fact! A first step in the acute management of an unstable patient with bradycardia and a pulse includes the administration of atropine, and if the bradycardia and hemodynamic instability persist, begin transcutaneous pacing and consider an infusion of epinephrine or dopamine.Now, let’s go back to the ABCDE assessment and look at stable patients.

Stable Patient1:49–2:38

In this case, obtain a focused history and physical examination. Your patient may report lightheadedness or fatigue, as well as shortness of breath with exertion or even angina or syncope.
Additionally, the physical exam will reveal a heart rate below 60 beats per minute. At this point, you can diagnose bradycardia, so order a 12-lead electrocardiogram, or ECG for short.
Now, if the ECG demonstrates a regular rhythm, where a P wave precedes every QRS complex, and the PR interval and P wave morphology are normal; you should consider alternative diagnoses, such as sinus bradycardia or chronotropic incompetence.Let's go back and look at the ECG again.

First-degree AV block2:38–3:11

If the ECG findings reveal a prolonged PR interval greater than 200 milliseconds and that P wave precedes every QRS complex, diagnose first-degree AV block.
Despite its name, this type of AV block is not a true heart block, but rather a conduction delay within the AV node, which results in a prolonged PR interval.
Typically, first-degree AV block is considered benign, so management primarily relies on observation.Now back to the ECG.

Second-degree AV block3:11–4:36

If some, but not all, of the P waves are followed by a QRS complex, diagnose a second-degree AV block. This means that the conduction between the atrium and ventricles is only occurring intermittently.
Your next step is to determine the type of second-degree AV block you’re dealing with, so you’ll need to assess the pattern of P wave conduction.
Here’s a clinical pearl to keep in mind! When a patient presents with any type of AV nodal block, you should assess for an underlying cause, since treatment of the condition often resolves the issue.
Some important physiologic and reversible causes of AV block include increased vagal tone, as seen in endurance athletes and individuals with obstructive sleep apnea; ischemia, like in an inferior wall myocardial infarction or MI; and medications that can slow cardiac conduction, like digoxin and beta blockers.
Additionally, electrolyte abnormalities, like hypokalemia and hyperkalemia, can cause AV block. On the flip side, important irreversible causes of AV block include fibrosis and protein deposition from cardiac sarcoidosis or amyloidosis, which are associated with the destruction of the cardiac conduction system.
Now, second-degree AV block comes in different flavors. If there’s a progressive lengthening of the PR interval followed by a non-conducted P wave and a missing QRS, diagnose Mobitz type I, which is also known as the Wenckebach phenomenon.

Mobitz type I4:36–6:05

Most patients with Mobitz type I don’t require treatment and can simply be observed, but if the symptoms are severe and persistent, you can use ambulatory ECG monitoring to determine whether the patient’s symptoms correlate with the AV block.
If symptoms are related to the AV block, consider a permanent pacemaker. Here’s a clinical pearl for you!
A pacemaker is a small device that is implanted into a patient’s chest, below the clavicle, in order to maintain a normal heart rate and rhythm.
A pacemaker can be used temporarily, when the heart is expected to recover, such as in a medication overdose; or it can be implanted permanently for irreversible AV block.
The pacemaker consists of a small box containing a battery, as well as insulated wires, known as leads. Leads are placed into a vein and guided into the heart, with the ends of the leads touching the wall of the atrium or ventricle.
When indicated, the pacemaker generates an electrical pulse, which travels down the wires to the atrium or ventricle and stimulates the heart to contract.
Alright, let’s go over our next flavor of second-degree AV block. If the ECG reveals an unpredictable pattern of non-conducted P-waves, and the interval between P waves remains constant, diagnose Mobitz type II.

Mobitz type II6:05–7:19

This type usually occurs below the level of the AV node, in the His-Purkinje system. These patients are at risk for progression to complete heart block and usually require pacemaker placement, regardless of the severity of symptoms.
Now, here’s another clinical pearl! Mobitz type II block is rarely seen in individuals without underlying structural heart disease, so you should always order a transthoracic echocardiogram, or TTE for short, to assess the underlying cause.
The most common causes include MI, and conditions affecting the heart muscle walls, such as cardiomyopathies. Less common causes include endocarditis, as well as infiltrative conditions, such as amyloidosis.
If TTE reveals no underlying cause, the AV block could be due to progressive damage to the cardiac conduction system due to fibrosis of the heart tissue.Finally, let’s go all the way back to our ECG one last time.

Third-degree AV block7:19–8:46

If ECG findings reveal complete AV dissociation, meaning that P waves have no relation to the QRS complex, you can diagnose a third-degree AV block, also called complete heart block.
In a third-degree heart block, the atrium and the ventricles work independently, and the atrial rate is faster than the ventricular rate.
These patients are at risk of asystole if the ventricles do not generate their electrical activity, which is often referred to as an escape rhythm.
Finally, management is primarily based on treating the underlying cause, so a transthoracic echocardiogram should also be performed to evaluate for structural heart disease, infiltrative cardiomyopathy, or endocarditis.
Additionally, persistent third-degree heart block requires placement of a permanent pacemaker, regardless of the severity of symptoms.
And here's one last high-yield fact! Lyme carditis, a manifestation of Lyme disease, can present with first-, second-, or third-degree AV block, which is typically transient and resolves with appropriate antibiotic treatment of the underlying infection.
Other less common infectious causes of AV block include viral myocarditis, toxoplasmosis, and Chagas disease.Alright, as a quick recap...

Review8:46–9:34

Atrioventricular block is a conduction disturbance that occurs when an atrial impulse gets delayed or completely blocked at some point along the electrical conduction system of the heart.
Based on ECG findings, AV blocks can be classified as either first-degree blocks; second-degree blocks, which can be further classified as Mobitz type I and II; and third-degree blocks.
First-degree AV blocks are considered benign, so management primarily relies on observation. Similarly, second-degree Mobitz type I blocks don’t require treatment and can simply be observed, but if symptoms are severe and persistent, consider ambulatory ECG monitoring and pacemaker placement.
Finally, second-degree Mobitz type II blocks and third-degree blocks always