page_title: Tachycardia-dependent bundle branch block
section_title: Rate-dependent bundle branch block
hierarchical_section_title: Tachycardia-dependent bundle branch block / Rate-dependent bundle branch block
caption_attribution_description: "Schematic diagram of normal sinus rhythm for a human heart as seen on ECG" (with English labels)
context_page_description: A tachycardia-dependent bundle branch block is a defect in the conduction system of the heart, and is distinct from typical bundle branch blocks due to its reliable, reproducible onset related to an increase in the rate of cardiac contraction. Tachycardia-dependent bundle branch block can prevent both ventricles from contracting efficiently and can limit the cardiac output of the heart.
context_section_description: As a result of underlying heart disease, this cardiac conductive tissue can become damaged from ischemia (a deprivation of oxygenated blood). This damage results in the inability of this neural-like tissue to conduct electrical signals and control the heart as efficiently as before, resulting in the cardiac abnormality known as a bundle branch block (BBB). This can affect either side of the heart, and is described as a right or left BBB. Some BBB are permanent and involve a complete bundle blockage  while others are dependent on the underlying activity of the heart. For example, certain situations of excessive or reduced heart rate (tachycardia or bradycardia, respectively) can cause a BBB known as a rate-dependent bundle branch block (RDBBB). This manifests in a similar fashion to a regular bundle branch block, but occurs only under conditions that affect contractile rate. Tachycardia-dependent bundle branch block (TDBBB) can affect either ventricle in the heart, and occurs when the heart's rate of contraction reaches an elevated level and becomes uncoupled from the heart's refractory period (the time it takes for a cardiac cell to "reset" for future contraction). Thus the cell is unable to contract by the time the next electrical stimuli is present, and a blocking of this signal occurs. This prolongation of the refractory period is related to a decreased sodium channel response in damaged tissue that shows an inactivation of sodium channels responsible for repolarization.
caption: The various components of a normal, healthy ECG tracing.
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