What medications should be avoided in patients with 2nd degree heart block?

Medications to Avoid in Second-Degree Heart Block

Patients with second-degree heart block should avoid or use with extreme caution beta-blockers, non-dihydropyridine calcium channel blockers (verapamil, diltiazem), digoxin, Class I antiarrhythmics, and amiodarone, as these agents can worsen AV conduction and precipitate complete heart block. 1

Critical Medications to Avoid

Beta-Blockers

• Beta-blockers have negative dromotropic effects on the AV node and prolong AV nodal conduction time, making them particularly dangerous in pre-existing AV conduction abnormalities 1
• Concomitant therapy with beta-blockers and calcium channel blockers may result in additive negative effects on heart rate and AV conduction, with reports of excessive bradycardia and complete heart block 2
• The combination of verapamil and beta-blockers should be used only with extreme caution and close monitoring, as risks may outweigh benefits 2
• Metoprolol-induced AV block frequently persists or recurs (24 of 36 cases) even after drug discontinuation, whereas carvedilol-induced block typically resolves (21 of 24 cases) 3

Non-Dihydropyridine Calcium Channel Blockers

• Verapamil and diltiazem are contraindicated as they significantly impair AV nodal conduction 1, 2, 4
• Verapamil should be administered cautiously to patients with impaired hepatic function, with dose reductions to approximately 30% of normal dosing, and careful monitoring for abnormal PR interval prolongation 2
• Diltiazem increases exposure to ivabradine and may exacerbate bradycardia and conduction disturbances; concurrent use should be avoided 4
• The combination of calcium channel blockers with multiple AV nodal blocking agents carries particularly high risk of worsening AV block 1

Digoxin

• Digoxin should be avoided as it can further slow AV conduction and potentially worsen the block 1
• Digoxin-induced AV block usually improves (28 of 39 cases) after withdrawal of the drug 3
• When digoxin is coadministered with verapamil, serum digoxin levels may increase by 50-75% during the first week of therapy, potentially resulting in digitalis toxicity 2
• Patients with second-degree AV block at high risk of complete heart block (including those with digitalis intoxication) should use quinidine only with extreme caution 5

Antiarrhythmic Agents

• Class I antiarrhythmic agents (flecainide, propafenone) should be avoided as they may worsen conduction disorders 1
• Amiodarone should be used with extreme caution due to its potential to cause bradycardia and worsen AV block 1
• Quinidine should be used only with caution in patients at high risk of complete AV block, including those with second-degree AV block 5
• Disopyramide should not be administered within 48 hours before or 24 hours after verapamil administration 2
• Flecainide and verapamil may have additive effects on AV conduction, resulting in additive negative inotropic effect and prolongation of AV conduction 2

Other Medications

• Ivabradine is contraindicated in patients with second-degree AV block 1
• Tricyclic antidepressants should be used with caution as they can cause PR and QRS prolongation 1
• Certain antipsychotic medications (thioridazine, haloperidol) that prolong QT interval should be avoided 1

Clinical Context: Mobitz Type I vs Type II

Mobitz Type I (Wenckebach)

• Mobitz type I is characterized by progressive PR interval prolongation until a beat is not conducted, with block usually at the AV node level 1
• It is commonly associated with inferior wall myocardial infarction and usually resolves spontaneously or after reperfusion 1
• Despite its traditional "benign" reputation, chronic Mobitz type I block has similar prognosis to Mobitz type II block, with five-year survival of only 41% in unpaced patients versus 78% in paced patients 6

Mobitz Type II

• Mobitz type II block occurs almost always below the AV node (infranodal) and is more likely to progress to complete heart block and Stokes-Adams arrest 7
• Type II AV block must be distinguished from nonconducted premature atrial contractions and atrial tachycardia with block 7

Management Approach

Drug Discontinuation

• When medication-induced AV block occurs, the offending agent should be discontinued immediately 1
• Drug discontinuation leads to resolution of AV block in only 41% of cases initially, but 56% of these patients experience recurrence of AV block even in the absence of therapy 8
• AV block is commonly "related to drugs" but rarely "caused by drugs" - only 15% of patients with II or III degree AV block during therapy with beta-blockers, verapamil, or diltiazem have truly drug-induced block 8
• Roughly half of patients with drug-induced AV block ultimately require permanent pacemaker implantation 3

Acute Symptomatic Management

• If Mobitz type I AV block is symptomatic with hemodynamic compromise, atropine (0.5 mg IV every 3-5 minutes to maximum 2.5 mg) can be considered as first-line treatment 9, 1
• Atropine doses less than 0.5 mg may paradoxically result in further slowing of heart rate due to central reflex stimulation of the vagus or peripheral parasympathomimetic effects 9
• Atropine is the drug of choice for type I second-degree AV block, especially when complicating inferior myocardial infarction with symptoms of hypotension, ischemic discomfort, or ventricular arrhythmias 9
• Atropine is rarely useful for type II second-degree AV block and may increase the sinus rate while actually enhancing the block 9

Pacing Indications

• Temporary pacing may be indicated for medically refractory symptomatic or hemodynamically significant bradycardia 1
• Permanent pacing is generally not indicated for Mobitz type I AV block unless it is symptomatic and does not respond to medication discontinuation 1
• Pacemaker insertion is the treatment of choice for symptomatic bradycardia not responding promptly to atropine administration 9

Critical Pitfalls to Avoid

• Do not assume that drug discontinuation will permanently resolve AV block - 27% of patients with improved AV conduction after drug withdrawal experience recurrence despite continued absence of the culprit medication 3
• Do not combine multiple AV nodal blocking agents (e.g., beta-blocker plus calcium channel blocker) as this carries particularly high risk 1
• Do not use atropine doses less than 0.5 mg as this may paradoxically worsen bradycardia 9, 1
• Do not assume Mobitz type I block is benign - unpaced patients with chronic Mobitz type I have poor prognosis similar to Mobitz type II, with only 57% five-year survival 6
• Careful monitoring for abnormal PR interval prolongation is essential when any AV nodal blocking agent must be continued 2