quinidine, heart muscle relaxant used to maintain regular heart rhythm patterns. It is an alkaloid chemically similar to quinine and, like quinine, occurs naturally in some species of cinchona trees. Quinidine slows the rate of blood flow in heart chambers and lowers the excitability of the muscle. Quinidine is a general relaxant of smooth muscle and acts as a dilator of larger blood vessels. It has also been used to reduce fever and to treat malaria. Synchronized electric shock has largely replaced quinidine as a method of establishing more normal heart rhythms.