<prompt> Digitalis does not increase which parameter of heart-Force of contractionMyocordial contractilityMyocardial oxygen demandCardiac output  <response>: Ans. is c i.e., Myocardial oxygen demand Effect of digitalis on hearto Main mechanism of action of digitalis is to increase in force of contraction of heart - a positive inotropic action.o Amongst given options, option d is the best choice - heart rate is decreased by digitalis and there is increase myocardial contractility without a propartionate increase in O2 consumption.How does digitalis decrease heart rate? Heart rate is decreased by digitalis mainly in patients with CHF. In CHF there is sympathetic overactivity and i vagal tone because of CO2 Digitalis increases myocardial contractility major action - CO and better emptying of ventricle - improvement in circulation - restoration of diminished vagal tone and abolish sympathetic overactivity - Heart rate. Beside above main mechanism digitalis slows the heart rate by -A Increased vagal tonei 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 