2. NIH defines bradycardia as heart rate <60 bpm in adults other than well
trained athletes.
AHA defines SND as sinus rate <50 bpm.
3. Autonomic/Neurally Mediated
Bradycardia
Since the sinus node and AV node are richly innervated by the autonomic
nervous system, both sinus bradyarrhythmias (sinus pauses, sinus arrest,
and sinus arrhythmia) and AV block (typically type I AV block, or
intermittent complete block) can be caused by autonomic influences
without any underlying conduction system disease.
Increases in parasympathetic (vagal) tone can be triggered by a variety of
events that can result in bradyarrhythmias and include hypersensitive
carotid sinus syndrome, vasovagal syncope, cough syncope, stimulation of
the Bezold- Jarisch receptors during an inferior MI, autonomic dysfunction,
and others
4. ecg
Neurally- mediated bradyarrhythmias are characterized most frequently by
ventricular asystole caused by cessation of atrial activity as a result of sinus arrest
or SA exit block.
AV block is observed less frequently, probably in part because the absence
of atrial activity from sinus arrest precludes the manifestations of AV block.
However, if an atrial pacemaker maintained an atrial rhythm during the
episodes, a higher prevalence of AV block would probably be noted.
