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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.
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
.
f
.
Acute myocardial infarctions.
Drugs like digitalis and beta-blockers.