How do you differentiate and manage sinus bradycardia versus Mobitz type 1 (second-degree atrioventricular block) in patients with bradycardia, considering symptoms, heart rate, blood pressure, and past medical history?

Differentiating and Managing Sinus Bradycardia vs Mobitz Type 1

The key distinction is that sinus bradycardia shows a sinus rate <50 bpm with every P wave conducting to the ventricles (1:1 conduction), while Mobitz Type 1 shows progressive PR interval prolongation before a single nonconducted P wave with inconstant PR intervals. 1

ECG Differentiation Algorithm

Sinus Bradycardia Characteristics

• Sinus rate <50 bpm with normal P-wave axis and PR interval 1
• 1:1 atrioventricular conduction - every P wave conducts to the ventricles 1
• Constant PR intervals throughout the rhythm strip 1
• QRS complexes follow each P wave predictably 1

Mobitz Type 1 (Wenckebach) Characteristics

• P waves with constant rate (<100 bpm) where AV conduction is present but not 1:1 1
• Progressive PR interval prolongation before the blocked beat - this is the hallmark finding 1
• Periodic single nonconducted P wave with inconstant PR intervals before and after 1
• "Group beating" pattern on ECG due to the cyclical nature of conduction 1
• Block typically occurs at the AV node level (not infranodal) 1, 2

Clinical Context Assessment

Determine Symptom Attribution

Symptomatic bradycardia requires documented bradyarrhythmia directly responsible for syncope, presyncope, dizziness, lightheadedness, heart failure symptoms, or confusional states from cerebral hypoperfusion 1

Critical Timing Considerations

• Vagally mediated AV block can occur during sleep when parasympathetic tone increases, often asymptomatic 1
• Look for concomitant sinus node slowing (P-P prolongation) during episodes of vagal AV block 1
• Vasovagal syncope can cause bradycardia through sudden parasympathetic surge 1

Management Algorithm

For Sinus Bradycardia

Asymptomatic patients:

• No treatment required if physiologic (athletes, sleep) 3, 4
• Monitor for symptoms and reversible causes 4, 5

Symptomatic patients:

• Atropine 0.5 mg IV bolus every 3-5 minutes up to maximum 3 mg total dose for acute management 6, 3
• Caution: Doses <0.5 mg may paradoxically slow heart rate due to parasympathomimetic effects 2
• Identify and treat reversible causes (medications, electrolytes, hypothyroidism) 4, 5
• Permanent pacemaker indicated only if symptomatic sinus node dysfunction persists after reversible causes excluded 1, 3

For Mobitz Type 1

Asymptomatic patients with typical (AV nodal) Mobitz Type 1:

• Generally benign and does not require pacemaker 2
• Avoid AV nodal blocking agents (beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, adenosine) as they worsen AV nodal conduction 2
• Continuous cardiac monitoring if hospitalized 2

Symptomatic patients:

• Atropine 0.5 mg IV bolus may be therapeutic for AV nodal-level Mobitz Type 1 2, 6
• Assess hemodynamic status before any IV bolus administration 2
• Consider temporary pacing if atropine ineffective and symptoms persist 3, 5

Critical Pitfall: Infranodal Mobitz Type 1

Do not assume all Mobitz Type 1 is benign - rare infranodal cases exist with worse prognosis similar to Mobitz Type 2 2, 7

Red flags suggesting infranodal disease:

• Wide QRS complex (≥120 ms) suggests His-Purkinje disease 2
• Higher risk of progression to complete heart block 2, 7
• May require permanent pacemaker like Mobitz Type 2 7

Medication Safety Considerations

Contraindicated in Both Conditions

• Beta-blockers (esmolol, metoprolol, propranolol) contraindicated in AV block >first degree or SA node dysfunction unless pacemaker present 2
• Non-dihydropyridine calcium channel blockers (diltiazem, verapamil) carry same contraindication 2
• Digoxin slows AV nodal conduction and should be avoided 2
• Adenosine contraindicated in AV block >first degree without pacemaker 2

Safe Medications

Most IV medications without AV nodal blocking properties can be administered safely to Mobitz Type 1 patients 2

When to Escalate Care

Immediate Cardiology/EP Referral Needed

• Any symptomatic bradycardia unresponsive to atropine 3, 5
• Mobitz Type 1 with wide QRS (concern for infranodal disease) 2
• Progression to higher-grade AV block on monitoring 2, 5
• Hemodynamic instability requiring transcutaneous pacing 3, 5

Monitoring Requirements

Continuous cardiac monitoring essential when administering any IV bolus to patients with Mobitz Type 1 to detect progression to higher-grade block and assess hemodynamic compromise 2