Dromotropic Action

DROMOTROPIC ACTION (from Greek dromos-running), the influence of various factors on the heart's ability to conduct impulses. The term D. d. was introduced in 1900 by Engelmann. Irritation of the cardiac branches of the vagus nerve causes slowing in impulse conduction (negative D. d.), while irritation of the cardiac branches of the sympathetic nerve leads to acceleration of impulse conduction (positive D. d.). For the most part, D. d. changes in the same direction as inotropic action, but there are exceptions to this rule (greatly slowed conduction with unchanged volume of contractions during incomplete block, etc.). Positive D. d. is produced by Ca, Sr, Ba (in small amounts), adrenaline (by irritating the endings of the sympathetic nerve), and atropine (by paralyzing the endings of the vagus nerve). The positive D. d. caused by the latter two substances is sometimes paralyzed due to exhaustion of the conducting ability as a result of an increase in the number of impulses. Negative D. d. is produced by K, Ba (in large amounts), high concentration of H-ions, digitalis, chloroform, chloral hydrate, atropine (in small doses), quinine (a protoplasmic poison), nicotine, sparteine, and cocaine (acting directly on the heart muscle), morphine, physostigmine, and muscarine (acting irritably on the endings of the vagus nerve). Negative dromotropic action can lead to complete dissociation of the activity of the atria and ventricles.