Tachycardia and Heart Block: Coexistence on ECG
Yes, a patient with tachycardia can have heart block on ECG, as these conditions can coexist through various physiologic mechanisms.
Mechanisms of Coexistence
Tachycardia-dependent heart block can occur when increased heart rates lead to impaired conduction through diseased conduction tissue, resulting in Mobitz Type II or 2:1 second-degree AV block during periods of tachycardia 1
Paroxysmal atrial tachycardia with AV block can occur even in patients with structurally normal hearts, where rapid atrial rates exceed the refractory period of the AV node, causing functional block 2
Concealed junctional extrasystoles can produce ECG findings that mimic Mobitz type II second-degree AV block with narrow QRS complexes during tachycardia 3
Clinical Scenarios
Supraventricular tachycardia with block: Rapid atrial rates can overwhelm AV nodal conduction capacity, resulting in functional block despite tachycardia 4
Tachycardia-bradycardia syndrome: Patients may alternate between periods of tachycardia and bradycardia due to sinus node dysfunction, with AV block potentially manifesting during either phase 4
Drug effects: Medications that prolong AV conduction (such as digoxin, beta-blockers, calcium channel blockers) can induce both tachycardia and AV block simultaneously 2
Acute myocardial infarction: Patients may present with sinus tachycardia due to pain, anxiety or compensatory mechanisms while simultaneously developing AV block due to ischemic injury to the conduction system 5
Diagnostic Considerations
A 12-lead ECG is essential to confirm the presence of both tachycardia and heart block, with careful attention to P wave morphology and PR intervals 4
Ambulatory ECG monitoring is indicated in patients with structural heart disease and symptoms when there is high pre-test probability of identifying an arrhythmia responsible for syncope 4
Longer-term continuous monitoring may be necessary, as the median time to first detection of high-grade heart block is approximately 40 hours (IQR 10-118 hours) 6
Electrophysiologic studies may be considered in selected patients with second-degree AV block to determine the level of block and whether they may benefit from permanent pacing 4
Management Implications
Treatment should address both the tachycardia and the conduction disorder, with careful consideration of how treating one might affect the other 7
In patients with symptomatic AV block attributable to a known reversible cause that does not resolve despite treatment of the underlying cause, permanent pacing is recommended 4
For patients with acquired second-degree Mobitz type II AV block, high-grade AV block, or third-degree AV block not attributable to reversible causes, permanent pacing is recommended regardless of symptoms 4
In cases of tachycardia-dependent AV block, beta-blocker medication may be initiated to prevent high sinus rates during everyday activity, potentially alleviating symptoms 1
Important Caveats
The distinction between Mobitz type I and type II block is not merely descriptive; it has prognostic implications as type II is more likely to progress to complete heart block and Stokes-Adams arrest 5
Patients with bifascicular block and syncope warrant careful evaluation, as syncope may be due to transient third-degree AV block even if not captured during initial monitoring 4
Not all apparent AV blocks during tachycardia represent true AV nodal or infranodal disease; nonconducted premature atrial contractions can mimic second-degree AV block 5
Careful interpretation of the ECG is essential, as the site of block (intranodal vs. infranodal) has significant implications for prognosis and management decisions 4