The document discusses electrophysiologic testing for bradycardia. It defines bradycardia as a heart rate below 60 bpm in adults and sinus node dysfunction (SND) as a sinus rate below 50 bpm. It describes how autonomic influences can cause sinus bradyarrhythmias and AV block due to the rich innervation of the sinus and AV nodes by the autonomic nervous system. Neurally-mediated bradyarrhythmias are characterized by ventricular asystole from sinus arrest or exit block, with AV block seen less frequently due to the lack of atrial activity during sinus arrest.