Tachycardia pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor-In-Chief: M.Umer Tariq [2]

Pathophysiology

Hemodynamic Responses

The body contains several feedback mechanisms to maintain adequate blood flow and blood pressure. If blood pressure decreases, the heart beats faster in an attempt to raise it. This is called reflex tachycardia. This can happen in response to a decrease in blood volume (through dehydration or bleeding), or an unexpected change in blood flow.

Sinus Tachycardia

The most common type of tachycardia is sinus tachycardia, which is the body's normal reaction to stress, which includes scenarios involving fever, dehydration, or blood loss (shock). It is a narrow complex tachycardia. In the absence of heart disease, sinus tachycardia tends to have a narrow QRS complex on ECG. Treatment is generally directed at identifying the underlying cause.

Ventricular Tachycardia

Ventricular tachycardia (VT or V-tach) is a potentially life-threatening cardiac arrhythmia that originates in the ventricles. It is usually a regular, wide complex tachycardia with a rate between 120 and 250 beats per minute. Ventricular tachycardia has the potential of degrading into the more serious ventricular fibrillation. Ventricular tachycardia is a common, and often lethal, complication of a myocardial infarction (heart attack).

Exercise-induced Ventricular Tachycardia

Exercise-Induced Ventricular Tachycardia a phenomenon related to sudden death, seen in patients with severe heart disease (ischemia, acquired valvular heart and congenital heart disease) accompanied by left ventricular dysfunction. A case of a death from exercise-induced VT occurred in Loyola Marymount basketball player Hank Gathers,in March of 1990. Both of these rhythms usually last for only a few seconds to minutes (paroxysmal tachycardia), but if VT persists it is extremely dangerous, often leading to ventricular fibrillation.

Supraventricular Tachycardia

A supraventricular tachycardia (SVT) is a tachycardia or rapid rhythm of the heart in which the origin of the electrical signal is either the atria or the AV node. These rhythms, by definition, are either initiated or maintained by the atria or the AV node. This is in contrast to ventricular tachycardias, which are rapid rhythms that originate from the ventricles of the heart, below the atria or AV node. The term SVT encompasses a large number of arrhythmias arising from the atria and AV node, and the term SVT is often incorrectly applied only to the subgroup of AV nodal re-entrant tachycardias.

Atrial Fibrillation

Atrial fibrillation is one of the most common cardiac arrhythmias. It is generally an irregular, narrow complex rhythm. However, it may show wide QRS complexes on the ECG if a bundle branch block is present. At high rates, the QRS complex may also become wide due to the Ashman phenomenon. It may be difficult to determine the rhythm's regularity when the rate exceeds 150 beats per minute. Depending on the patient's health and other variables such as medications taken for rate control, atrial fibrillation may cause heart rates that span from 50 to 250 beats per minute (or even higher if an accessory pathway is present). However, new onset atrial fibrillation tends to present with rates between 100 and 150 beats per minute.

AV Nodal Reentrant Tachycardia (AVNRT)

AV nodal reentrant tachycardia is the most common reentrant tachycardia. It is a regular narrow complex tachycardia that usually responds well to vagal maneuvers or the drug adenosine. However, unstable patients sometimes require synchronized cardioversion. Definitive care may include catheter ablation.

AV Reentrant Tachycardia

AV reentrant tachycardia (AVRT) requires an accessory pathway for its maintenance. AVRT may involve orthodromic conduction (where the impulse travels down the AV node to the ventricles and back up to the atria through the accessory pathway) or antidromic conduction (which the impulse travels down the accessory pathway and back up to the atria through the AV node). Orthodromic conduction usually results in a narrow complex tachycardia, and antidromic conduction usually results in a wide complex tachycardia that often mimics ventricular tachycardia. Most antiarrhythmics are contraindicated in the emergency treatment of AVRT, because they may paradoxically increase conduction across the accessory pathway.

Junctional Tachycardia

Junctional tachycardia is an automatic tachycardia originating in the AV junction. It tends to be a regular, narrow complex tachycardia and may be a sign of digitalis toxicity.