Abstract
Unstable bundle branch blocks may be tachycardia dependent, bradycardia dependent, or rate independent. When appearing at the "critical" rates or "critical" cycle lengths they may seem to be rate independent. Conversely, "true" rate-independent blocks may be reversible or irreversible. Determining if a rate-unrelated block may disappear can be difficult because irreversile bundle branch block seems to develop through a slow process in time during which rate-dependent, rate-independent, and even normal conduction alternate in successive electrocardiograms.