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Cardiac Arrhythmias - Tachycardias


Automaticity abnormalities are one of the causes of tachyarrhythmias. The affected tissue generates electrical signals at an accelerated rate. Many forms of atrial and ventricular tachycardias are caused by this mechanism. Automatic tachyarrhythmias, however, only represent a small poriton of abnormal tachycardias seen in the lab. The majority of these rhythms are caused by reentry.Reentry arrhythmias can occur when specific requirements are met.

  1. There must be two conductive pathways that are joined by conductive tissue at both the proximal and distal ends.
  2. One of the pathways must conduct slower than the other.
  3. The slower conducting pathway must have a shorter refractory period than the faster pathway. ie, the pathway that conducts slower must recover faster than the pathway that conducts faster.
When these three conditions are met, reentry becomes a possability. For a tachycardia to be initiated at this point requires a unidirectional block. This can occur when a premature beat is blocked by the normal refractory periods of one of the pathways.

Focal or Reentry

There are two primary types of tachycardias, focal and reentry. A focal tachycardia occurs when a single specific location begins emitting signals at a rate that supercedes the sinus node. The cells of the heart will respond to any signal regardless of the source. This is why the heart responds to signals delivered by devices like pacemakers. When the source is an alternate location within the heart that fires faster than the sinus node, the sinus node will be supressed and the faster location will take over. This type of tachycardia is not easily controlled with medications and may require the physician locate the source of the abnormal signal and eliminate it. More on how this is accomplished will be discussed later in this page.

Unlike focal tachycaridas, reentry arrhythmias do not originate from a single location. Instead, the normal signal of the heart gets caught in a loop that causes the heart to contract over and over again for as long as the loop is maintained. For a loop like this to occur, there must be very specific conditions. First there must be a complete circuit for the signal to travel through. This circuit will have two sections that conduct the normal signal of the heart at different speeds, one faster and the other slower. This complete circuit is not sufficient to cause a reentry arrhythmia. One more piece of the puzzle is needed. The portion of the loop that conducts faster, must take longer to recover and be ready to respond to a new signal. It may seem a bit confusing, so let's see if we can clarify this.... 



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