The kidneys control the amounts of fluid and electrolytes in the blood. They maintain this control by filtering waste products out of the body, producing urine and controlling the amount of water absorbed into the bloodstream. In cases of acute renal failure, the kidneys do not maintain normal fluid levels in the bloodstream. When this occurs, doctors prescribe diuretics to reduce fluid buildup and prevent complications.
Function
In renal failure, the kidneys do not remove excess fluids from the blood. This allows fluid to accumulate in the circulatory system, making it more difficult for the heart to pump blood. When the heart works harder, blood pressure increases, leading to high blood pressure. Diuretics eliminate excess fluid from the bloodstream to reduce blood pressure and prevent swelling of the eyelids, arms, hands, ankles, legs and feet.
Types
Three types of diuretics eliminate excess fluid from the blood. Loop diuretics increase the excretion of water and sodium by the kidneys. Examples of loop diuretics include furosemide, bumetanide and ethacrynic acid. Potassium-sparing diuretics reduce the amount of potassium excreted from the body. The Merck Manual of Health & Aging reports that these diuretics are weaker than other types of diuretics. Examples of potassium-sparing diuretics include triamterene, amiloride and spironolactone. Thiazide diuretics have milder side effects than loop diuretics, reducing the occurrence of complications. Thiazide diuretics include metolazone, hydrochlorothiazide and indapamide.
Mechanism of Action
Loop diuretics eliminate excess fluid from the bloodstream by affecting the transport of sodium. This results in the absorption of less water by the kidneys. Dr. Richard Klabunde, author of "Cardiovascular Physiology Concepts," indicates that potassium-sparing diuretics do not affect sodium transport in the body. Instead, they increase the excretion of water in the urine. Thiazide diuretics inhibit the transport of sodium in the distal tubules of the kidneys. This also reduces the amount of water absorbed into the bloodstream.
Considerations
Doctors must consider each patient's symptoms, laboratory test results and disease stage when prescribing diuretics for renal failure. Patients with mild swelling and no hypertension may require short-term therapy with diuretics, while those with severe swelling and high blood pressure may have to take a daily dose of a diuretic drug. If a patient does not respond to one type of diuretic, a doctor will increase the dosage of the medication or find an alternative drug.
Complications
Several complications occur with regular diuretic use. If too much fluid leaves the body, patients experience dehydration, with symptoms including dry mouth, thirst, tiredness, decreased urine output, headache, muscle weakness and dizziness. Low potassium levels, also known as hypokalemia, occur with the use of loop and thiazide diuretics. Doctors use potassium-sparing diuretics to prevent these low levels of potassium in the blood. Potential drug interactions exist between diuretics and lithium, digitalis, digoxin, cyclosporine, antidepressants and other medications used to treat high blood pressure.
