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Sinus Bradycardia (brady - slow)
- Sinus bradycardia occurs when the hearts rate is slower than 60 beats per minute. The
sinus bradycardia rhythm is similar to normal sinus rhythm, except that the RR interval is
longer. Each P wave is
followed by a QRS complex in a ratio of 1:1. The PR interval is often slightly prolonged
and occasionally, the P-waves might be abnormally wide.
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The EKG on the top
shows normal sinus rhythm. The EKG at the bottom shows sinus bradycardia |
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- The symptoms of sinus bradycardia include dyspnea, dizziness, and extreme fatigue.
Bradycardia may be accompanied by an increase in stroke volume due to greater end
diastolic pressure (preload). The pulse volume may be greater due to a greater stroke
volume and an increased diastolic run-off time (longer time for blood to
flow away from the
heart).
Sinus bradycardia may occur due to any of the following:
| a. |
Increase in parasympathetic (vagal) tone, for instance, due to training in athletes. This is
a normal response. The heart rate increases with exercise or atropine. |
| b. |
Parasympathetic (vagal) stimulation, for instance, with carotid sinus stimulation.
Stimulation of carotid sinus baroreceptors results in increased parasympathetic
stimulation that decreases the heart rate. |
| c. |
Sick sinus syndrome or sinoatrial (SA) node disease. These are rhythm disorders that
occur if the SA node loses its ability to initiate or increase the heart rate. If the SA
node is unable to properly function due to sick sinus syndrome, the AV node (or
ventricular tissue if the AV node is also not functioning) take over the
initiation of the heart beat, but at a rate that is slower than the sinus rhythm. |
| d. |
Heart block which occurs when the signal from the SA node is slowed or stopped at the
AV node or in the ventricular conducting system. Heart block is described as first, second, or third degree.
The decrease in the
heart rate depends on the degree of heart block. |
| e. |
Acute myocardial infarctions. |
| f. |
Drugs like digitalis and beta-blockers. |
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