Chronic bradycardia and its attendant cardiovascular phenomena present innumerable interesting problems. Many causes are ascribed to the slow heart; this results in a cumbersome and confusing classification. It may not be amiss briefly to summarize the causes generally accepted:
Chemical Substances.—The inhibitory action of the following substances is well known and warrants no discussion here: (a) The neutral non-nitrogenous glucosids and resins; the active principles of digitalis, squills, strophanthus and apocynum; (b) alkaloids such as erythrophlein, veratrin and aconitin; (c) biliary constituents, and (d) inorganic substances, such as barium salts and hydrates.1
Stimulation of the Cardiac Vagus.—Stimulation may be peripheral or central.
Myocardial Disease.—Involvement of the myocardium by disease (a) associated with lesions of the auriculoventricular bundle (complete heart block and delayed impulse transmission), and (b) without involvement of the bundle.
Certain physiologic reactions without cardiac damage.
The early and late stages of asphyxia.2
PHYSIOLOGY OF CHRONIC BRADYCARDIA