Functional Bundle Branch Block
Cycle length greatly influences the refractory period of the various cardiac tissues. Within limits, an increase in cycle length (slower heart rate) prolongs, while a decrease in cycle length (faster heart rate), shortens the refractory period. However, the heart is a heterogeneous structure, made up of different types of fibers which have different electrophysiologic properties. Some cells are specialized for impulse formation and some for impulse conduction, while the myocardial cells perform the work of contraction. It is not surprising, therefore, that the response to drug intervention, or to a rate alteration, might not produce the same changes throughout the heart. For example, acetylcholine shortens the refractory period and speeds conduction in atrial muscle, but prolongs the refractory period and slows conduction in the AV nodal fibers. Similarly, rate-dependent changes in the refractory period may not affect all structures uniformly.For some years,1,2 it has