Second Degree Heart Block

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Welcome to BoardsMD. It is our mission to help you succeed on your Step 2 CK exam, and in your career as a doctor. Here is our overview of Second Degree Heart Block.

Type: Mobitz I (Winckebach)

Most commonly, patients with Mobitz I (Wenckebach) second-degree heart block are going to be asymptomatic
They may also present in the context of myocardial ischemia
Medications that slowed down conduction at the AV node, including 𝛃-blockers, calcium channel blockers (CCBs), and Digoxin may also tip off a patient into Mobitz I.

Pathophysiology

Pathophysiology is most commonly going to be at the AV node
could be triggered by ischemia of the right coronary artery as the right coronary artery actually supplies the AV node
we can also see Mobitz I in athletes including runners, as they have extremely high vagal tone at baseline, and the vagus nerve is ultimately going to slow down conduction at this AV node.

Diagnosis

EKG is going to show progressive PR lengthening until a beat is dropped
The PR interval will then reset to its original length
These patients typically will also have a narrow QRS complex

Management

These patients are going to be asymptomatic
If these patients are indeed asymptomatic, then no intervention is going to be required.
Ultimately Mobitz I carries a very, very low risk of progression to complete heart block or third-degree heart block and therefore no intervention is generally going to be needed in these patients.

Type: Mobitz II

These patients also most commonly will be asymptomatic
They may present in the context of myocardial ischemia
There are medications that can ultimately slow down conduction at the AV node in tip-off a patient into Mobitz II, including 𝛃-blockers, calcium channel blockers, and Digoxin.

Pathophysiology

Fibrosis of the conduction system
This can be seen in patients who have a history of myocardial infarction, as death of the myocardium could ultimately lead over time to fibrosis, and therefore inhibition of the conduction system
Classically Mobitz II, unlike Mobitz I is going to occur below the AV node.

Diagnosis

The result of this fibrosis below the AV node in patients with Mobitz II is that these patients will have unexpected dropped beats on their EKG
Additionally - and very high yield to distinguish Mobitz II for Mobitz I (Winckebach), is that in Mobitz II, these patients will have no change in their PR interval
Mobitz II tends to be more commonly symptomatic and has a greater risk of progression to complete heart block than Mobitz I.

Management

Because of this risk of progression to complete heart block, when we have Have a patient with Mobitz II, it does not matter whether the patient is symptomatic or asymptomatic as a pacemaker is going to be indicated regardless
This can be achieved via a double lead pacemaker as pictured below with leads in both the right atrium and the right ventricle.
Or we can use a single lead pacemaker with a single lead present in the right ventricle.
Therefore this distinction shows that in Mobitz I, generally, we are not going to need to perform any intervention rather than Mobitz II we are going to need to put in a pacemaker is an extremely important distinction to keep in mind.

Mobitz I vs. Mobitz II

In addition to the different EKGs between Mobitz I and Mobitz II, as well as the fact that we are going to manage patients with Mobitz II with the use of a pacemaker regardless of symptoms. There are also some differences between these two types of second-degree heart block when it comes to their responses to exercise and atropine. Recall from your physiology that for both exercise and atropine, these are going to lead to an increase in heart rate through inhibition of vagal activity. Therefore, their overall result is going to be an increase in conduction through the AV node. Because Mobitz I is generally a problem with conduction through the AV node, both exercise and atropine are actually going to improve symptoms as well as EKG findings in patients with Mobitz I. However, Mobitz II involves a problem that occurs below the AV node, and therefore exercise and atropine and are actually going to worsen or have no effect on Mobitz II.

Vagal Maneuvers

This difference between Mobitz I and Mobitz II is also reflected in how these different types of heart blocks respond to vagal maneuvers. As we know vagal maneuvers increase conduction through the vagal nerve and therefore are going to lead to decreased conduction through the AV node. Vagal maneuvers include the Valsalva maneuver as pictured below,

carotid sinus massage, as well as the ocular cardiac reflex. In the case of all of these vagal maneuvers, there's going to be increased conduction through the vagal nerve, and therefore this is going to lead to slowed conduction through the AV node, thus exacerbating Mobitz I. However, this will have the exact opposite effect on Mobitz II, which occurs below the AV node. And therefore anything that leads to increase vagal activity such as vagal maneuvers is going to worsen Mobitz I. However, anything that decreases vagal activity, such as exercise or atropine is going to improve Mobitz I. However, the opposite effects will be seen for Mobitz II.

Third Degree Heart Block

Patients present with Presyncope/Syncope, as well as Angina, Dyspnea, and Fatigue
Can be set off by several reversible causes, including myocardial ischemia, increased vagal tone, Metabolic arrangements,
as well as the actual genetic causes, such as AV blocking medications, including 𝛃-blockers and calcium channel blockers.

Heart Block III

Pathophysiology

Failure of transmission from the atria to the ventricles
Ventricle starts to perform independent conduction that is independent from the conduction through the atria – Escape rhythm

Diagnosis

Failure of transmission from the atria to the ventricles.
No impulse conduction from the atria to the ventricles.
Ventricular rate is slower than the atrial rate

Management

Evaluate for reversible causes
If patient is symptomatic and has third degree heart block, they're absolutely going to need a pacemaker
Make sure in these patients that we are not giving AV blocking medications, such as 𝛃-blockers or calcium channel blockers
If this condition is left untreated, these patients could ultimately go on to progress to develop Asystole, Arrhythmias, and even death.

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