Slow heart rate, also known as bradyarrhythmias, are generally
caused by two fundamental problems. One is the inability for
the normal pacemaker mechanism of the heart to generate an
impulse at the normal rate (50-100 bpm). Thus, this results
in heart rates less than 50 bpm. Common causes include medi-
cations or an age related problem known as sinus node dysfunction.
Although syncope can occur in association with sinus bradycardia
or sinus node dysfunction, a more common complaint is exercise
intolerance or fatigue. This is usually termed chronotropic incom-
petence, because it refers to the inability for the normal pacemaker
mechanism to increase rate in accordance with body needs.
The other abnormality responsible for bradyarrhythmias is
impairment in impulse conduction through the small connection
between the atria and ventricles, the AV node. As noted above,
this structure is responsible for transmitting the impulse from
the upper chambers into the lower chambers. Such impairment
in impulse conduction is referred to as AV block and is usually
associated with the development of an escape rhythm. Usually
either the junction or the ventricles take over, hence junctional
or ventricular escape rhythms are responsible for the heart rate.
The treatment of choice is a permanent pacemaker, usually a dual
chamber device.
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