Management of Dropped QRS Complex on ECG
When an ECG shows a dropped QRS complex (a P wave not followed by a QRS), this represents second-degree atrioventricular (AV) block, and management depends critically on distinguishing between Mobitz Type I (Wenckebach) and Mobitz Type II patterns—the latter requires permanent pacemaker implantation even in asymptomatic patients due to high risk of progression to complete heart block. 1
Immediate Assessment Steps
1. Characterize the Type of AV Block
Examine the PR intervals before the dropped beat:
- Mobitz Type I (Wenckebach): Progressive prolongation of the PR interval before the dropped QRS, typically associated with a narrow QRS complex 1
- Mobitz Type II: Constant PR interval before the dropped QRS, typically associated with a wide QRS complex (indicating infranodal block) 1
2. Exclude Reversible Causes
Before any pacemaker decision, systematically rule out and correct: 1
- Electrolyte abnormalities (particularly hyperkalemia, hypomagnesemia)
- Medications (beta-blockers, calcium channel blockers, digoxin, antiarrhythmics)
- Acute myocardial infarction (particularly inferior MI affecting AV node)
- Lyme disease (acute phase—block typically resolves)
- Perioperative causes (hypothermia, inflammation near AV junction)
3. Assess for Symptoms
Evaluate for hemodynamic compromise: 1
- Syncope or presyncope
- Fatigue, malaise
- Signs of low cardiac output (hypotension, altered mental status)
- Symptoms of "pacemaker syndrome" (elevated ventricular filling pressures)
Management Algorithm
For Mobitz Type II Second-Degree AV Block:
Permanent pacemaker implantation is indicated even in asymptomatic patients (Class IIa recommendation), particularly when: 1
- Associated with fascicular block (bundle branch block pattern)
- Wide QRS complex present
- Risk of progression to complete heart block is high
This is a critical distinction: Mobitz Type II is a precursor to complete AV block and requires intervention regardless of symptoms. 1
For Mobitz Type I (Wenckebach):
Management is symptom-driven: 1
- Asymptomatic patients: Observation and monitoring; pacemaker generally not required
- Symptomatic patients: Consider permanent pacemaker if symptoms are clearly attributable to bradycardia
Special Circumstances Requiring Prophylactic Pacing:
Even if AV block is transient, permanent pacing is recommended in: 1
- Sarcoidosis (risk of disease progression)
- Amyloidosis (progressive conduction system involvement)
- Certain neuromuscular diseases (progressive nature)
Additional Diagnostic Considerations
Evaluate for Alternating Bundle Branch Block:
If the ECG shows evidence of block in all three fascicles (right bundle branch block in one lead, left bundle branch block in another), this indicates severe conduction system disease requiring pacemaker implantation. 1
Consider Advanced AV Block:
If two or more consecutive P waves fail to conduct, this represents advanced (high-degree) AV block and typically requires permanent pacing due to hemodynamic impairment. 1
Critical Pitfalls to Avoid
Do not confuse blocked premature atrial contractions (PACs) with AV block: Carefully examine T waves for premature P waves that may be blocked due to physiologic refractoriness rather than pathologic AV block. 1
Do not delay pacemaker implantation in Mobitz Type II: Unlike Mobitz Type I, Type II block is infranodal and carries high risk of sudden progression to complete heart block with potential for syncope or sudden death. 1
Do not assume hemodynamic stability indicates benign disease: Stable vital signs do not exclude the need for pacing in Mobitz Type II or advanced AV block. 1
In athletes or young patients, distinguish physiologic from pathologic findings: Mobitz Type I can be a normal variant in highly trained athletes, but Mobitz Type II is never physiologic. 1
Documentation and Follow-up
Obtain: 1
- 12-lead ECG during the episode
- Ambulatory ECG monitoring (Holter) if intermittent
- Correlation of symptoms with rhythm disturbances
- Echocardiogram to assess for structural heart disease and ventricular function