1.) An unopposed vagal tone secondary to sympathectomy. This sympathectomy occurs 2-6 dermatomes higher than sensory block, so that a sensory block of T6 can conceivably inhibit all of the sympathetic innervation to the heart.
2.) The Bezold-Jarisch reflex – which may be widely under appreciated phenomenon. You’ll be surprised how many clinicians are not well versed in this essential physiology.
Sympathectomy → reductions in venous tone → profound decreased venous return. This activates mechanoreceptors embedded within the walls of the myocardium → stimulating vagal afferents → vasomotor center of medulla → increased vagal tone to the heart, thus slowing or stopping the heart entirely to allow enough filling time to generate an effective stroke volume.
http://allnurses.com/certified-registered-nurse/sympathectomies-and-atropine-124910.html