A tachycardia-dependent

bundle branch block A bundle branch block is a partial or complete interruption in the flow of electrical impulses in either of the bundle branches of the heart's electrical system. Anatomy and physiology The heart's electrical activity begins in the sinoatri ...

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 In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols Q, \dot Q, or \dot Q_ , edited by Catherine E. Williamson, Phillip Bennett is the volumetric flow rate of the heart's pumping output: tha ...

of the heart.

Anatomy

The human heart is a four-chambered organ responsible for the distribution of blood throughout the body. While every physiological effort is made to ensure that such a vital organ can operate continuously without error, sometimes a pathological situation arises and the function of the heart is compromised. One such pathology arises when the electrical signal propagated throughout the heart (responsible for the heart's highly organized contractions) is hindered, resulting in a degradation of said conduction. This is referred to as a bundle branch block and is seen clinically as rate-dependent bundle branch block,

right bundle branch block A right bundle branch block (RBBB) is a heart block in the Bundle branches#Structure, right bundle branch of the Electrical conduction system of the heart, electrical conduction system. During a right bundle branch block, the right ventricle (h ...

left bundle branch block Left bundle branch block (LBBB) is a conduction abnormality in the heart that can be seen on an electrocardiogram (ECG). In this condition, activation of the left ventricle of the heart is delayed, which causes the left ventricle to contract late ...

Normal functioning

In a healthy heart, specialized cells in the

sinoatrial node The sinoatrial node (also known as the sinuatrial node, SA node, sinus node or Keith–Flack node) is an ellipse, oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of Cell (biology), cells known as pa ...

are responsible for the generation of contractile stimuli that are directed throughout the heart. A direct neural-like pathway propagates this electrical signal from the SA node through the atrial tissue to a central nodal point in the center of the heart, the

atrioventricular node The atrioventricular node (AV node, or Aschoff-Tawara node) electrically connects the heart's atria and ventricles to coordinate beating in the top of the heart; it is part of the electrical conduction system of the heart. The AV node lies at the ...

. From here, the signal travels down a signal conducting channel (the

bundle of His The bundle of His (BH) or His bundle (HB) ( "hiss"Medical Terminology for Health Professions, Spiral bound Version'. Cengage Learning; 2016. . pp. 129–.) is a collection of heart muscle cells specialized for electrical conduction. As part of ...

) where it will diverge into a left and right branch that will control the left and right ventricle, respectively.

Rate-dependent bundle branch block

As a result of underlying heart disease, this cardiac conductive tissue can become damaged from

ischemia Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems ...

(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

(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 Tachycardia, also called tachyarrhythmia, is a heart rate that exceeds the normal resting rate. In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults. Heart rates above the resting rate may be normal ...

bradycardia Bradycardia, also called bradyarrhythmia, is a resting heart rate under 60 beats per minute (BPM). While bradycardia can result from various pathological processes, it is commonly a physiological response to cardiovascular conditioning or due ...

, 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 In neuroscience, repolarization refers to the change in membrane potential that returns it to a negative value just after the depolarization phase of an action potential which has changed the membrane potential to a positive value. The repolarizat ...

Diagnosis

TDBBB can be diagnosed with use of an electrocardiogram (ECG) which will "trace" the electrical activity of the heart, providing an overall view of the hearts electrical system. Typically, TDBBB will be evident on an ECG and manifest as a prolongation of the

QRS complex The QRS complex is the combination of three of the graphical deflections seen on a typical electrocardiogram (ECG or EKG). It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the ri ...

(a QRS complex completion time that exceeds 120ms),

notching Notching is a metal-cutting process used on sheet-metal or thin bar-stock, sometimes on angle sections or tube. A shearing or punching process is used in a press, so as to cut vertically down and perpendicular to the surface, working from the e ...

or slurring of the R wave, or the absence of Q waves should the TDBBB affect the left ventricle.Kumar V, Venkataraman R, Aljaroudi W, Osorio J, Heo J, Iskandrian AE, Hage FG. "Implications of Left Bundle Branch Block in Patient Treatment", ''Am J Cardiol'' 2013;111:291-300.

Treatment

If an affected individual begins to experience severe TDBBB, then medical intervention is often advised. Suggested therapy for the treatment of TDBBB can include the prescription of certain medications or the implantation of a

pacemaker A pacemaker, also known as an artificial cardiac pacemaker, is an implanted medical device that generates electrical pulses delivered by electrodes to one or more of the chambers of the heart. Each pulse causes the targeted chamber(s) to co ...

device.Shlipak M. G. et al. "Treatment and outcomes of left bundle-branch block patients with myocardial infarction who present without chest pain". ''Journal of the American College of Cardiology''. 36 (2000) 706-712 Advised medications would possess anti-coagulant mechanisms to reduce the risk of blood clot formation ensuring that no further restriction of arteries would deprive the heart of oxygen and further damage the bundle branches. The use of a pacemaker would ensure that the heart receives a constant rhythmic electrical input that never changes in frequency. While this would effectively eliminate the occurrence of TDBBB, the pacemaker would restrict the patient's heart to a permanent rhythm, eliminating the ability of patients to perform physical activity.Hayes D. L. "Advances in pacing therapy for bradycardia". ''International Journal of Cardiology''. 32 (1991) 183-196 Future pacemakers that adaptively respond to physiological requirements are being developed in order to negate the limitations observed with their current use.

References

{{reflist Cardiac arrhythmia