Story Title

   On the Normal Heart page, we discussed sinus bradycardia. This term is used to describe times when the sinus node fires below the defined range of normal, which is from 60-100 beats per minute. Most of us experience sinus bradycardia on a daily basis. When we are resting, sleeping or even just relaxing, our heart rate will usually drop below 60 beats per minute. Our brain detects the decreased levels of activity and sends signals to the heart that decrease the number of times the sinus node discharges at.  If, however, our heart rate drops too low at an inappropriate time, such as when we are exercising, this could be insufficient to meet the demands the increased activity has placed on our body. This type of bradycardia would be considered abnormal and it would be important to determine why this has occurred.

     There are two primary reasons why the heart would beat too slowly. One is that the signals that cause the heart muscle to contract are not being generated fast enough. If the sinus node does not generate enough signals to meet the body's demands, it may cause symptoms that may include fatigue, periods of dizzyness or even episodes where you feel like you are going to pass out (near syncope). This type of bradycardia is called Sick Sinus Syndrome. Symptoms of sick sinus syndrome usually develope slowly over time and at first, may go unnoticed.

 

Back to Top

Tachycardia

Automaticity abnormalities are also 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.

For more detailed information on the various types of arrhythmias, check the sections specific to the arrhytmia in question.

 

The opposite of bradycardia is tachycardia. This refers to periods when the heart rate goes faster than the normal range. As with slower rhythms, a tachycardia may be completely normal, such as when the heart rate increases in response to exercise or excitement. It is when the heart rate runs fast for no reason that it becomes a problem. This type of rhythm is referred to as an arrhythmia.

Back to Top

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....