The normal heartbeat originates in the specific part of the upper chamber of the heart in a region called the called the sino atrial (SA) node, the heart's "natural" pacemaker. The electrical signal then travels through the top chambers of the heart (atria) to the portion of the heart called the atrioventricular (AV) node, which connects connects the top and bottom chambers of the heart. The electrical impulse goes through the AV node andthen travels to both bottom chambers of the heart (ventricles), the chambers that create the pulse or heartbeat that we feel.
Slow heart rhythms, or bradycardias, occur for two basic reasons: the sinus node does not produce beats at an adequate rate or not all beats get through from the top chamber to the bottom chamber, a condition called AV block. When there is a slowing or absence of the electrical signal from the SA node, a condition called sinus node dysfunction is present. As a result, the pulse rate can become slower, or the heart beat may not increase with activity or the need for a faster pulse (fear/exercise). The inability to increase the heart rate to meet the bodys needs is called "chronotropic incompetence."
There are different severities of the problems with the heart beat traveling from the top to the bottom heart chambers. The most common description of the severity is the degree of AV block. In first degree AV block there are no blocked heart beats. The beats just take a longer time to get from the upper chambers to the lower chambers. On the ECG the measurement of the time if reflected in an interval called the PR interval. In first degree AV block the PR interval is greater than 0.2 seconds. In second degree AV block there are occasional beats that do not make it from the upper to the lower heart chambers. Very commonly there is only one blocked beat. The level of the block may be within the AV node itself or a lower site called the His bundle. The lower the level of the block the more serious the problem in general. Some patients may have complete or third degree AV block. The patients do not have signals that travel from the upper chambers to the lower heart chambers. The patient relies on another source of the heart rhythm called an escape rhythm. This rhythm maintains the patients rhythm at a slow rate but one that can maintain the patients blood pressure and meets much of the patients bodily demands.