Rate-dependent bundle branch block

Rate-dependent bundle branch block


Usually left or right bundle branch block
pattern is persistent on ECG. But there are cases when bundle branch block
occurs only at higher heart rate. Bundle branch block is characterized by impaired
conduction via the left or right bundle branch. Rate depended bundle branch block is characterized
by appearance only at the certain heart rate, and it disappears at the heart rate below
this value (also known as critical heart rate). Usually, heart rate at which there is transition
between the normal conduction and the bundle branch block is about 100 bpm. In this case, the term of tachycardia-dependent
bundle branch block is used. But, also, such heart rate at which such transition
is observed may be at the normal range, in other words, bundle branch block occurs when
the heart rate exceeds some limit which is at a range from 60 to 100 bpm. The term of the exercise-induced bundle branch
block is close to the term of rate-dependent bundle branch block. It may be explained by the fact that the heart
rate increases upon physical exertions. It’s established that exercise-induced left
bundle branch block is 3 times more frequent than exercise-induced right bundle branch
block. But rate-dependent bundle branch block appears
at the rest in contrast to exercise-induced one. Left bundle branch block may be caused by
a block either in the main left bundle branch (so-called “Pre-divisional”) or by a simultaneous
block in both anterior and posterior fascicles (so-called “Postdivisional”). Mechanism of rate-dependent bundle branch
block is based on prolongation of the refractory period in cells of the conductive system of
the ventricles. Cells in the refractory period cannot conduct
an impulse. So, in the case of rate-dependent left bundle
branch block, impulse is conducted firstly via the right bundle branch. In the case of rate-dependent right bundle
branch block, impulse is conducted firstly via the left bundle branch. Impulse cannot be conducted via the bundle
branch if the heart rate exceeds the limit at which conduction is normal. If heart rate is higher, the interval between
impulses is shorter, and cells with a prolonged refractory period have not enough time to
restore their conductibility. When the heart rate is lower, cells have time
to go out from the refractory period and they conduct the impulse. Rate-dependent bundle branch block is associated
with chronic hypertension, previous myocardial infarction, coronary artery disease, myocarditis. Rate-dependent bundle branch block may be
the first sign of coronary artery disease. At this ECG we can see the left bundle branch
block occurring at a heart rate of 68 bpm and more. This is a critical heart rate. If heart rate is higher than this value, the
left bundle branch block is obligatory. At heart rate less than 68 bpm there is normal
conduction in the ventricles. Often supraventricular extrasystole creating
a compensatory pause, give time for the conductive tissue to go out from the refractory period
and such extrasystole terminates episode of left bundle branch block. If the heart rate is still frequent, extrasystole
can provide the normal conduction of only 1 impulse that follows this extrasystole. If such patient has paroxysmal supraventricular
tachycardia, left bundle branch block pattern during it, is observed. It should be noted that bundle branch block
may deteriorate heart failure because of an asynchronic contraction of all walls of the
ventricle. Lowering of heart rate may improve conduction
via rate-dependent blocked bundle branch, and thus improve a systolic function
of the ventricles.

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