What is the relationship between Phenytoin (Dilantin) and Mobitz type II heart block?

From the Guidelines

Dilantin (phenytoin) should be used with caution in patients with Mobitz type II heart block due to its potential to worsen cardiac conduction abnormalities. Phenytoin has sodium channel blocking properties that can further depress cardiac conduction, potentially converting a Mobitz type I (Wenckebach) block to a more dangerous Mobitz type II or complete heart block. For patients with pre-existing heart block who require antiepileptic therapy, alternative medications such as levetiracetam (Keppra), lamotrigine, or valproic acid are generally safer options as they have minimal cardiac effects.

Key Considerations

• The 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay recommends permanent pacing in patients with acquired second-degree Mobitz type II atrioventricular block, high-grade atrioventricular block, or third-degree atrioventricular block not caused by reversible or physiologic causes, regardless of symptoms 1.
• The guideline also emphasizes the importance of shared decision-making and patient-centered care in treatment decisions for patients with bradycardia who have indications for pacemaker implantation 1.
• In patients with Mobitz type II heart block, it is crucial to monitor for worsening conduction abnormalities when initiating phenytoin therapy, and alternative medications should be considered to minimize the risk of adverse cardiac effects.

Management Approach

• If Dilantin must be used in a patient with Mobitz heart block, close cardiac monitoring is essential, including regular ECGs and potentially telemetry during initiation.
• The medication should be started at lower doses (such as 200-300 mg/day) and titrated slowly while monitoring for worsening conduction abnormalities.
• The risk is particularly high during intravenous administration, where the rate should not exceed 50 mg/minute.
• This caution stems from phenytoin's mechanism of action, which involves blocking voltage-gated sodium channels that are present in both neuronal and cardiac tissue, affecting both seizure activity and cardiac conduction.

From the Research

Relationship Between Phenytoin (Dilantin) and Mobitz Type II Heart Block

There is no direct evidence in the provided studies to establish a relationship between Phenytoin (Dilantin) and Mobitz type II heart block.

Mobitz Type II Heart Block Characteristics

• Mobitz type II second-degree atrioventricular block is characterized by periodic atrioventricular block with constant PR intervals in the conducted beats 2.
• The site of the block in Mobitz type II is almost always below the AV node, and it is more likely to progress to complete heart block and Stokes-Adams arrest 2.
• Mobitz type II AV block can be influenced by changes in parasympathetic and sympathetic tone, with drugs such as atropine, propranolol, and isoproterenol affecting the atrial pacing cycle length at which 1:1 His-Purkinje conduction occurs 3.

Mechanisms and Triggers

• Supernormal conduction can occur in cases of Mobitz type II atrioventricular block, where the block is followed by recovery of atrioventricular conduction at shorter coupling intervals without prolongation of the H1H2 and H2V2 intervals 4.
• Exercise can induce Mobitz type II AV block in patients with chronic bifascicular block, manifested clinically by paradoxical slowing of the heart rate and decreased blood pressure 5.
• Junctional extrasystoles can also contribute to the mechanism of Mobitz type II second-degree atrioventricular block, resulting in retrograde concealed conduction and prolonged local refractoriness in the AV node 6.