The autonomic nervous system, the same part of the nervous system as controls the blood pressure, controls the firing of the sinus node to trigger the start of the cardiac cycle. (medterms.com)

If the sinus node is not functioning normally -- due to damage from surgery, drugs, congenital heart defects or other causes -- the heartbeat may become very slow with a decrease in blood pressure. (University of California)

Extrinsic sinus mechanisms of sinus node dysfunction could be drug effects or due to autonomic influences. The important drugs which could suppress sinus node function are beta blockers, non-dihydropyridine calcium channel blockers (e.g. verapamil and diltiazem), digoxin, clonidine, methyl dopa, class IA, IC and III antiarrhythmic agents and lithium. (Cardiophile for Medical Professionals)