What are the causes of Mobitz Type 1 (Second-Degree Atrioventricular (AV) Block) block with intermittent First-Degree (1st degree) AV block?

Causes of Mobitz Type 1 Block with Intermittent 1st Degree AV Block

Mobitz Type 1 (Wenckebach) block with intermittent first-degree AV block is most commonly caused by increased vagal tone, medications (particularly beta-blockers, calcium channel blockers, and digoxin), or myocardial ischemia affecting the AV node. 1

Pathophysiological Understanding

Mobitz Type 1 block is characterized by progressive PR prolongation until a P wave fails to conduct, while first-degree AV block shows a consistently prolonged PR interval (>200 ms). When these occur together, it typically indicates:

• The block is located at the AV node rather than below it 1
• The conduction system is experiencing variable degrees of impairment 1

Common Causes

Physiologic/Vagal Causes

• Increased vagal tone 1
• Sleep or obstructive sleep apnea 1
• High-level athletic conditioning 1
• Neurocardiogenic mechanisms 1

Medication-Related Causes

• Beta blockers (metoprolol, propranolol, etc.) 1
• Calcium channel blockers (verapamil, diltiazem) 1
• Digoxin 1
• Antiarrhythmic drugs 1

Cardiac Causes

• Acute myocardial infarction (particularly inferior MI) 1, 2
• Myocarditis 1
• Coronary ischemia without infarction (unstable angina, variant angina) 1
• Valvular heart disease 1
• Cardiac surgery (especially valve surgery) 1
• Transcatheter aortic valve replacement (TAVR) 1

Infectious/Inflammatory Causes

• Lyme carditis 1
• Bacterial endocarditis with perivalvar abscess 1
• Acute rheumatic fever 1
• Chagas disease 1
• Cardiac sarcoidosis 1

Metabolic/Endocrine Causes

• Acid-base disorders 1
• Thyroid disease (both hypothyroidism and hyperthyroidism) 1
• Adrenal disease (pheochromocytoma, hypoaldosteronism) 1

Infiltrative/Degenerative Causes

• Amyloidosis 1
• Lev's and Lenegre's diseases (progressive cardiac conduction disease) 1
• Rheumatologic diseases (systemic sclerosis, SLE, RA) 1

Clinical Significance and Progression

The combination of Mobitz Type 1 block with intermittent first-degree AV block has important clinical implications:

• Generally considered more benign than Mobitz Type II block 1
• Usually transient and responsive to autonomic manipulation 1
• May respond to atropine administration 1
• Can progress to higher-degree blocks in some cases, particularly in the setting of acute myocardial infarction 2
• May be exacerbated by induced hypertension in susceptible individuals 3

Diagnostic Approach

When evaluating a patient with Mobitz Type 1 block and intermittent first-degree AV block:

• Obtain a 12-lead ECG to confirm the rhythm 1
• Review medication history, focusing on AV nodal blocking agents 1
• Consider cardiac monitoring to assess for progression to higher-degree blocks 1
• Evaluate for underlying structural heart disease or acute coronary syndrome 1, 2
• In selected cases, exercise testing may help identify exercise-induced worsening of conduction abnormalities 1

Common Pitfalls and Caveats

• Don't confuse this with Mobitz Type II block, which occurs below the AV node and carries a worse prognosis 1
• Be aware that 2:1 AV block cannot be classified as Mobitz I or II by surface ECG alone 1
• Remember that vagally-mediated AV block typically improves with exercise or atropine 1
• Consider that some cases previously thought benign may actually represent early manifestations of progressive conduction system disease 4, 5
• Recognize that treatment with pure alpha-1 agonists in patients with first-degree AV block can precipitate progression to Mobitz Type I block due to reflex vagal activation 3