Rate dependent Bundle Branch

Block

• Rate-related bundle branch block conduction is present when the QRS complex during a tachycardia is wider (usually >0.12 seconds) when compared to the QRS duration during a slower sinus rhythm

• Rate-related aberration is the result of slowed and delayed conduction within the bundle of His or the right or left bundle branches

• Intrinsic disease

• Antiarrhythmic drugs

• Hyperkalemia

• If the rate of impulse stimulation is faster than the ability of that part of the conduction system to have repolarized from the prior impulse, the impulse may fail to conduct through part of the His Purkinje system

• Heart rate at which BBB appears is known as critical heart rate

• At faster rates - tachycardia dependent BBB

• At slower rates - Bradycardia dependent BBB

• Aberrancy may also be seen when a single atrial or junctional premature beat arrives at an RR interval (or rate) that is faster than the conduction system can conduct

• Impulse encroaches on the relative refractory period of the previous beat

• Block is most often in the right bundle, which generally has a longer refractory period than the left bundle at similar heart rates

• RBBB configuration is the most common morphology for the aberrated QRS complex.

• TACHYCARDIA-DEPENDENT, IN PHASE 3 ABERRANCY, OR PHASE 3 ABERRATION

• Resulting from the occurrence of impaired intraventricular conduction as the heart attains a specific critical rate

• The appearance and disappearance often depends on very small changes in cycle length

BRADYCARDIA-DEPENDENT, PHASE 4 ABERRANCY

• Occurrence of impaired intraventricular conduction after long pauses or slowing of the heart to a critical rate

• Due to a gradual loss transmembrane resting potential during a prolonged diastole with excitation from a less negative take-off potenial

• Unexpected since there is adequate time for the bundle to recover

• Bundles are serving as pacemaker tissue and manifest spontaneous automaticity. This pacemaker tissue is no longer suppressed by stimuli from upper pacemakers when the cycle length is very prolonged, leading to generation of an impulse from one of the bundles and manifesting a block pattern in the other bundle.

• Heart rate is slowed as a result of enhanced vagal tone, which also leads to impaired or slowed conduction through a bundle.

• there is disease in the bundle that leads to a spontaneous increase in phase 4 depolarization and sodium channel inactivation, preventing conduction down the bundle until it can be rese t

• Rate dependent LBBB

• Prior to Angiography, believed to be manifestation of CAD

• Wayne et al - exercise induced LBBB associated with CAD(1983)

• Riera et all - exercise induced LBBB in patients with CAD versus no CAD

• No CAD - better in comparison with patients with CAD

• Exercise Induced RBBB

• Uncommon(0.29%) and benign

• Stein et all (8047 patients)

• Exercise induced LBBB

• Greater cardiovascular risk

• When it occurs <140/min chance of obstructive CAD rises

Ashman phenomenon

• Gouaux-Ashman phenomenon or Ashman phenomenon is an intraventricular conduction abnormality restricted to the His-Purkinje system, caused by a change in the HR

• Modulated by metabolic and electrolyte abnormalities and the effects of drugs

• Relatively long cycle was followed by a relatively short cycle, the beat with a short cycle often has RBBB morphology

• Most commonly observed during atrial fibrillation when there are frequent episodes of long-short RR cycle lengths

Summary• Rate related aberrancy

• Tachycardia induced - Phase 3 block

• Bradycardia induced - Phase 4 block

• Most common rate related aberrancy - RBBB

• Usually benign

• Exercise induced BBB

• RBBB - benign

• LBBB - higher possibility of obstructive CAD, esp < 140/min