Cardiac arrhythmias (heart rhythm problems) can be divided into two general types: those that make the heart rhythm too slow -- the bradycardias -- and those that make the heart rhythm too rapid -- the tachycardias. This article briefly reviews bradycardias.
Bradycardias also can be sub-divided into two general types: Bradycardia due to sinus node disease, or bradycardia due to conduction system disease.
Whatever the cause, when the heart beats more slowly than normal, symptoms can vary from none at all (for mild bradycardia), to lightheadedness, dizziness or even loss of consciousness (when more severe slowing occurs).
Bradycardia from Sinus Node Disease
The sinus node is a tiny structure located in the upper part of the right atrium of the heart that produces the heart's electrical signal. ( See this article on the cardiac electrical system.) The electrical signals rhythmically produced by the sinus node spread across the heart, causing the heart to beat. One heart beat occurs for each electrical signal produced. Normally, the sinus node "drives" the heart to beat 50 to 90 times per minute while a person is inactive, and up to 200 times per minute (or even more, in young people) during maximum exercise.
When the sinus node becomes diseased, it produces electrical signals more slowly than normal, so the heart rhythm becomes slower than normal. Sinus node disease is caused by a degenerative process, most often related to aging, that causes fibrosis (scar tissue) in the area of the sinus node. It is rarely seen in patients under 60 years of age. If the bradycardia produced by sinus node disease is severe enough to cause symptoms (a condition sometimes referred to as "sick sinus syndrome"), a pacemaker may be required.
Conducting System Disease and Heart Block
The cardiac conducting system consists of three oddly named structures: The AV node, the bundle of His and the bundle branches. The AV node and the bundle of His allow your heart's electrical signal to pass from the atria to the ventricles. Once the electrical signal has reached the ventricles, it is distributed through the cardiac muscle by the bundle branches. ( For more details, see this article on the cardiac electrical system.)
Disease in the AV node or the bundle of His can prevent some or all of the electrical signals from reaching the ventricles, thus slowing (or even, potentially, stopping) the heart rate. Slowing of the heart rate due to conducting system disease is referred to as heart block.
As in sinus node disease, the symptoms of heart block (lightheadedness, dizziness, or loss of consciousness) depend on the degree of slowing. Significant heart block is treated by inserting a pacemaker.
