Diuretics

Heart failure. Heart failure leads to activation of the renin-angiotensin-aldosterone system, which causes increased sodium and water retention by the kidneys. This in turn increases blood volume and contributes to the elevated venous pressures associated with heart failure, which can lead to pulmonary and systemic edema. The primary use for diuretics in heart failure is to reduce pulmonary and/or systemic congestion and edema, and associated clinical symptoms (e.g., shortness of breath - dyspnea). Long-term treatment with diuretics may also reduce the afterload on the heart by promoting systemic vasodilation, which can lead to improved ventricular ejection.

When treating heart failure with diuretics, care must be taken to not unload too much volume because this can depress cardiac output. For example, if pulmonary capillary wedge pressure is 25 mmHg (point A in figure) and pulmonary congestion is present, a diuretic can safely reduce that elevated pressure to a level (e.g., 14 mmHg; point B in figure) that will reduce pulmonary pressures without compromising ventricular stroke volume. The reason for this is that heart failure caused by systolic dysfunction is associated with a depressed, flattened Frank-Starling curve. However, if the volume is reduced too much, stroke volume will fall because the heart will now be operating on the ascending limb of the Frank-Starling relationship. If the heart failure is caused by diastolic dysfunction, diuretics must be used very carefully so as to not impair ventricular filling. In diastolic dysfunction, ventricular filling requires elevated filling pressures because of the reduced ventricular compliance.

Most patients in heart failure are prescribed a loop diuretic because they are more effective in unloading sodium and water than thiazide diuretics. In mild heart failure, a thiazide diuretic may be used. Potassium-sparing, aldosterone-blocking diuretics (e.g., spironolactone) are being used increasingly in heart failure.

Pulmonary and systemic edema. Capillary hydrostatic pressure and therefore capillary fluid filtration is strongly influenced by venous pressure Therefore, diuretics, by reducing blood volume and venous pressure, lower capillary hydrostatic pressure, which reduces net capillary fluid filtration and tissue edema.

Specific Drugs

Specific drugs comprising the five class of diuretics are listed in the following table.

Adverse Side Effects and Contraindications

The most important and frequent problem with thiazide and loop diuretics is hypokalemia. This sometimes requires treatment with potassium supplements or with a potassium-sparing diuretic. A potentially serious side effect of potassium-sparing diuretics is hyperkalemia. Other side effects and drug interactions are list below: