Rate dependent bundle branch block can occur when the heart rate increases or decreases past a critical threshold. Tachycardia induced bundle branch block, also called phase 3 block, occurs when the heart rate increases and the conduction system cannot repolarize fast enough between beats. Bradycardia induced bundle branch block, or phase 4 block, occurs when the heart rate decreases too much and the conduction system loses its resting potential. Right bundle branch block is the most common morphology seen. Exercise induced left bundle branch block may indicate underlying coronary artery disease, especially at heart rates under 140 beats per minute.

1. Rate dependent
Bundle Branch Block

2. • 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

3. • 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

4. • 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

5. • 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

6. • 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.

10. • 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

12. 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

14. • 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

16. • 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

17. • 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

18. 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

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

22. 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