Quinidine sulfate is a salt form of the drug quinidine, and it is used to treat arrythmias, medical conditions that cause the heart to beat abnormally. It is known as a class IA antiarrhymiatic agent, based on how the drug functions. Sodium channels that cause the cells of the heart to contract are blocked by this medication.
Commonly, several types of arrythmias can be treated with quinidine sulfate, although the presence of congestive heart failure can prevent it from being effective. Its ability to treat several conditions is due to its use dependent blocking abilities. When the heart rate increases, quinidine's ability to block sodium channels increases, but as the heart rate slows down, the medication does not block sodium channels to the same extent. Other ion channels are blocked by quinidine, which also contributes to its usefulness.
Humans are not the only species that quinidine sulfate is used to treat. This drug is effective for treating a condition known as sustained atrial fibrillation in horses, which causes the upper chambers of the heart to shake instead of contract normally. When treating horses, this medication is administered at doses of 22 milligrams (mg) per kilogram (kg) of the horse's weight. Every two hours, this dose is repeated until the heart contractions stabilize. Often, this medication is given to horses through a tube that runs from the nose to the stomach.
Another use for quinidine sulfate is for the treatment of malaria. Parasites that cause malaria are quickly killed when this medication is administered. Usually, it is given in a tablet form for this purpose, instead of through tubes. Originally, this drug was used to treat malaria, but merchants and others taking this medication quickly found that it could help regulate the heartbeat as well, leading to its more common use today.
Although quinidine sulfate is highly effective in treating atrial fibrillation in horses, it has side effects, like any other medication. When this medication was given to 77 horses with heart conditions in one study, almost half developed side effects of some type. Nasal mucosal edema, or a swelling of tissue inside of the nose, anorexia, diarrhea, and colic were the most common side effects that were seen. These side effects did not inhibit the ability of quinidine to help treat atrial fibrillation, however, and it was found that more rapid therapy was associated with better outcomes, without a higher risk of side effects.