What is the cause of an alternate missed QRS (quantitative R-to-S interval) complex?

Alternating Missed QRS Complex: Causes and Clinical Significance

Primary Cause

Alternating missed QRS complexes most commonly indicate second-degree atrioventricular (AV) block, where intermittent failure of atrial impulses to conduct to the ventricles occurs in a pattern. 1 The specific pattern and QRS morphology determine the anatomical location and clinical urgency of the conduction disturbance.

Anatomical Classification by QRS Morphology

Narrow QRS Complex (AV Nodal Block)

• When alternating missed QRS complexes occur with a narrow QRS and progressively prolonging PR intervals before the blocked beat, this represents Mobitz Type I (Wenckebach) second-degree AV block, with the conduction delay occurring in the AV node. 1
• This pattern typically has a benign prognosis and rarely progresses to complete heart block. 1
• The block is usually reversible and may be drug-related or related to increased vagal tone. 1

Wide QRS Complex (Infranodal Block)

• When alternating missed QRS complexes occur with a constant PR interval and wide QRS complex, this represents Mobitz Type II second-degree AV block, indicating disease in the His-Purkinje system below the AV node. 1
• This pattern carries significant risk for progression to complete heart block and sudden cardiac arrest. 1, 2
• Permanent pacemaker implantation is recommended even in asymptomatic patients with Type II block, particularly when associated with fascicular block. 1

Special Pattern: Alternating Bundle Branch Block

A critical and life-threatening cause of alternating missed QRS complexes is alternating bundle branch block, where QRS morphology alternates between left bundle branch block (LBBB) and right bundle branch block (RBBB) patterns, often with intermittent nonconducted P waves. 1

• This pattern indicates severe bilateral bundle branch disease with unstable conduction in both fascicles. 1
• Permanent pacing is mandated (Class I recommendation) due to extremely high risk of sudden-onset complete heart block with slow or absent ventricular escape rhythm. 1
• The case example from American Family Physician describes a 79-year-old woman showing LBBB morphology until a nonconducted P wave occurs, followed by RBBB morphology on the subsequent conducted beat—this patient developed complete heart block requiring pacemaker implantation. 1

2:1 AV Block Pattern

• When every other P wave fails to conduct (2:1 pattern), the anatomical site cannot be definitively determined from surface ECG alone, as there is only one PR interval to examine before the blocked P wave. 3
• A narrow QRS with 2:1 block suggests AV nodal disease (80% of cases), while a wide QRS with 2:1 block suggests His-Purkinje disease (80% of cases outside acute MI). 3
• In sustained 2:1 or 3:1 AV block with wide QRS complex, the block occurs in the His-Purkinje system in 80% of cases. 3

Advanced/High-Grade AV Block

• Advanced AV block refers to blockage of two or more consecutive P waves, representing a higher degree of conduction failure. 1
• This pattern indicates severe conduction system disease requiring urgent evaluation for permanent pacing. 1

Reversible Causes to Exclude

Before attributing alternating missed QRS complexes to permanent conduction disease, exclude:

• Electrolyte abnormalities (particularly hyperkalemia) 1
• Drug toxicity (beta-blockers, calcium channel blockers, digoxin, antiarrhythmics) 1
• Acute myocardial infarction with transient ischemic block 2
• Perioperative block from hypothermia or surgical inflammation near the AV junction 1
• Acute Lyme disease (may resolve spontaneously) 1

Progressive Conditions Requiring Prophylactic Pacing

Even if AV block is transient, prophylactic pacing is recommended in conditions with expected disease progression: 1

• Sarcoidosis 1
• Amyloidosis 1
• Neuromuscular diseases (Kearns-Sayre syndrome, Emery-Dreifuss muscular dystrophy) 1

Clinical Decision Algorithm

1. Assess QRS width on conducted beats:

   • Narrow QRS → likely AV nodal block (better prognosis)
   • Wide QRS → likely infranodal block (worse prognosis, requires pacing)

2. Examine PR interval pattern:

   • Progressive PR prolongation → Type I (Wenckebach), usually benign
   • Constant PR interval → Type II, requires pacemaker

3. Check for alternating QRS morphology:

   • Alternating LBBB/RBBB → immediate pacemaker (Class I)

4. Exclude reversible causes before permanent pacing decision 1

5. For symptomatic patients with syncope and bundle branch block, obtain His bundle electrophysiology study if HV interval ≥70 ms or infranodal block is found, permanent pacing is indicated. 1

Critical Pitfall

Do not administer atropine to patients with suspected His-Purkinje disease (wide QRS with 2:1 block), as this may paradoxically increase the degree of AV block by accelerating the atrial rate beyond the capacity of diseased infranodal tissue to conduct. 3