Cardiac glycosides are drugs used to treat CHF and arrhythmias.  They help increase the force of contraction and slow the heart rate.  The most common cardiac glycosidic drugs are Digitalis (aka Digoxin).

These drugs work by prolonging the plateau phase (Phase 2) of cardiac depolarization.  This slows the rate of contraction and allows for more time for ventricular filling, effectively increasing the force of contraction and thus ultimately increasing the CO.

Mechanism: Normally, sodium/potassium pumps pump Na out and K in.  Cardiac glycosides inhibit this pump.  Inhibition of this pump increases the intracellular concentration of Na.  A second pump, the Na/Ca exchanger (NCX) pumps 3 Na ions in and 1 Ca ion out.  Elevated intracellular Na concentration inhibits this pump, which results in an increase in intracellular Ca.  This increase in Ca results in a higher uptake into the sarcoplasmic reticulum and higher Ca release upon stimulation, effectively releasing more Ca per contraction which increases the force of contraction per each crossbridge cycle.