Overview: Diuretics ("water pills") are used to help the kidneys get rid of unneeded water and salt.
Patients with HCM sometimes develop fluid retention. Puffy ankles are one of the classic symptoms. This is a sign of heart failure. Heart failure means that your heart isn't pumping enough blood, or that it is not filling with blood well enough. It does not normally require a trip to the emergency room (where they will very likely tell you to follow up with your doctor and pay a bill for your visit). But it does need to be addressed right away. Most heart failure is quite manageable, but it makes a big difference if you get attention early on!
Another problem is that sometimes fluid backs up into your lungs (pulmonary edema), causing shortness of breath and coughing. This is caused by a different sort of heart failure. Finally, one way to lower blood pressure is to reduce the amount of fluid in your bloodstream.
Diuretics are frequently used as part of the treatment for each of these three problems.