A regular but unusually slow heart beat (60 beats/minute or less at rest). Sinus bradycardia can be the result of many things including good physical fitness, medications, and some forms of heart block. Low blood calcium makes the nervous system highly irritable with tetany (spasms of the hands and feet, muscle cramps, abdominal cramps, and overly active reflexes). Chronic hypocalcemia contributes to poor mineralization of bones, soft bones (osteomalacia) and osteoporosis.. What are the causes of slow heart rates? . . Abnormally slow heart rates (bradycardias) can result from diseases affecting the SA node, the conduction tissues, and the AV node. Sick sinus syndrome is a disease wherein the SA node cannot generate signals frequently enough to maintain adequate heart rate. Heart blocks are conditions where diseases (such as heart attacks) or degeneration (due to processes such as aging) of the AV node and/or the conduction tissues impair the transmission of signals from the SA node to the heart muscles.. . Some medications can cause bradycardia. Examples of these medications include calcium channel blockers such as verapamil (Calan) and beta-blockers such as propanolol (Inderal) and digoxin (Lanoxin). These medications can also aggravate bradycardias in patients with existing diseases of the SA node, AV node, and other parts of the heart's conduction system.