Yes, QRS complexes are dropped in second-degree atrioventricular block
In second-degree AV block, one or more atrial impulses (P waves) fail to conduct to the ventricles, resulting in dropped QRS complexes. This is the defining characteristic that distinguishes second-degree from first-degree AV block 1.
Understanding the Two Types
Second-degree AV block manifests in two distinct patterns:
Mobitz Type I (Wenckebach)
- Progressive PR interval prolongation occurs before the dropped QRS
- The PR interval lengthens with each beat until a P wave fails to conduct
- Usually occurs at the AV node level
- Generally benign prognosis with uncommon progression to higher-degree block 1, 2
- Typically associated with narrow QRS complexes
Mobitz Type II
- Constant PR intervals before and after the dropped QRS (this is essential for diagnosis)
- The block occurs suddenly without warning PR prolongation
- Almost always occurs in the His-Purkinje system (infranodal) 1, 3, 4
- Wide QRS complexes are typical
- High risk of progression to complete heart block 1, 5
Critical Clinical Distinction
The QRS width provides crucial prognostic information:
- Narrow QRS + dropped beats: Usually Mobitz I, AV nodal location, generally benign 5
- Wide QRS + dropped beats: Suggests infranodal disease (His-Purkinje system), requires pacemaker even if asymptomatic 1, 5, 1
Important Caveats
2:1 AV block cannot be classified as Type I or Type II because there is only one PR interval visible before the blocked P wave—you need at least two consecutive conducted beats to assess for PR interval changes 3, 4, 6. In this situation, QRS width and clinical context help determine the block location.
Beware of mimics: Concealed His bundle extrasystoles and nonconducted premature atrial contractions can create patterns that resemble second-degree AV block but are not true conduction disease 7, 3, 8.
Management Implications
Pacemaker indications differ dramatically:
- Mobitz Type II: Permanent pacing recommended even without symptoms, particularly with wide QRS, due to high risk of progression to complete heart block 1, 5, 1
- Mobitz Type I: Pacing generally only indicated if symptomatic or if the block occurs below the AV node (demonstrated by wide QRS or electrophysiology study) 1
The presence of dropped QRS complexes with constant PR intervals (true Mobitz II) represents infranodal disease requiring urgent pacemaker evaluation to prevent sudden progression to complete heart block and potential sudden death 2, 3.