The mechanism of action of digitalis is -

Ans. is 'd' i.e., Decrease in hea rate with decrease in oxygen consumption Effect of digitalis on hea o Main mechanism of action of digitalis is to increase in force of contraction of hea - a positive inotropic action. o Amongst given options, option d is the best choice --> hea rate is decreased by digitalis and there is increase myocardial contractility without a propaionate increase in 02 consumption. How does digitalis decrease hea rate? o Hea rate is decreased by digitalis mainly in patients with CHF. In CHF there is sympathetic overactivity and vagal tone because of CO. o Digitalis increases myocardial contractility (major action) T CO and better emptying of ventricle --> improvement in circulation --> restoration of diminished vagal tone and abolish sympathetic overactivity Hea rate. Beside above main mechanism digitalis slows the hea rate by ? A) Increased vagal tone i) Reflexly through nodosa ganglion. ii) Direct stimulation of vagal centre. iii) Sensitization of SA nodes to Ach. B) Extravagal - Direct depressant action on SA and AV nodes. "The vagal action manifests early and can be blocked by atropine, whereas extravagal action becomes prominent later and cannot be reversed by atropine". Remember following impoant points about pharmacological actions of digitalis. o Main mechanism of action of digitalis is to increase in force of contraction of hea a positive ionotropic effect. o Cardiac output is increased. o Hea rate is decreased.