What is the diagnosis for a sinus rhythm with dropped QRS (quantitative research system) complexes?

Sinus Rhythm with Dropped QRS: Second-Degree Atrioventricular Block

Sinus rhythm with dropped QRS complexes is second-degree atrioventricular (AV) block, where one or more atrial impulses (P waves) fail to conduct to the ventricles, resulting in absent QRS complexes after visible P waves. 1

Classification of Second-Degree AV Block

The specific type of second-degree AV block must be determined by examining the PR interval pattern before the dropped beat:

Mobitz Type I (Wenckebach)

• Progressive PR interval prolongation occurs before the dropped QRS complex 1
• The PR prolongation may be subtle in the last cardiac cycles before the blocked P wave 1
• The ECG shows "group beating" as a result of the dropped QRS complexes 1
• Block typically occurs at the AV node level, especially when the QRS complex is narrow 1
• Generally has a more benign prognosis and rarely progresses to complete heart block 1

Mobitz Type II

• The PR interval remains constant before and after the blocked P wave 1
• Block is usually located in the His-Purkinje system, particularly when the QRS is wide 1
• Carries significantly higher risk of progression to complete AV block 1
• More likely to cause symptoms including syncope 1

2:1 AV Block (Cannot Be Classified)

• When only 2:1 conduction is present, it cannot be definitively classified as Mobitz I or II 1
• Determining the level of block (nodal vs. infranodal) is critical and may require electrophysiologic study 1, 2

High-Grade (Advanced) AV Block

• Defined as ≥2 consecutive P waves at normal rate that fail to conduct without complete loss of AV conduction 1
• Generally considered to be intra- or infra-Hisian block 1
• Requires pacing treatment 1

Critical Diagnostic Considerations

QRS Width Determines Block Location

• Narrow QRS complex: Block is usually in the AV node 1
• Wide QRS complex: Block may be in the AV node OR His-Purkinje system—only His bundle electrogram can locate it precisely 1
• Type II block with wide QRS has particularly poor prognosis 1

Common Pitfall to Avoid

• Blocked atrial bigeminy can simulate sinus bradycardia 1
• Always search carefully for premature P waves that may be hidden in the preceding T wave 1
• The distinction is critical because blocked atrial bigeminy is benign while severe sinus bradycardia may indicate systemic illness 1

Clinical Implications and Management

Mobitz Type I with Narrow QRS

• Often asymptomatic and may be seen in healthy, active patients 1
• Pacing indications are controversial unless symptoms are present or block occurs below the AV node 1
• Some evidence suggests pacemaker implantation improves survival in elderly patients, even when asymptomatic, especially if occurring during daytime hours 1

Mobitz Type II or Wide QRS

• Progression to complete heart block is common 1
• Permanent pacing is recommended due to high risk of symptomatic progression 1
• Untreated chronic second-degree block below the His bundle has poor prognosis 1

Symptomatic Patients

• Any second-degree AV block causing syncope or near-syncope requires permanent pacing 1
• His-Purkinje block suspected as cause of symptoms warrants electrophysiologic study if not established by ECG 1

Emergency Management

• Atropine is unlikely to be effective for type II or high-grade AV block where the block is in non-nodal tissue 1
• Transcutaneous pacing should be initiated in unstable patients who do not respond to atropine 1
• Prepare for transvenous pacing as transcutaneous pacing is only a temporizing measure 1