Atrioventricular block: Clinical sciences

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. 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. 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. 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. 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.