By: Thomas Pickering, MD, DPhil, FRCP, Director of Integrative and Behavioral Cardiology Program
of the Cardiovascular Institute at Mount Sinai School of Medicine, New York.

Diuretics remain the mainstay of the drug treatment of hypertension, and have been proven to be very effective at lowering the rates of cardiovascular events (strokes and heart attacks) in hypertensive individuals. One of the commonest side effects, which usually goes unnoticed by the patient, is to produce a low-blood potassium (technically called hypokalemia). This occurs because the diuretics cause potassium to be washed out in the urine as well as sodium.

The Systolic Hypertension of the Elderly Program (SHEP) was one of the largest trials of the effects of hypertension treatment on reducing cardiovascular events, in which elderly patients were treated with either a diuretic (chlorthalidone- Hygroton) or an inert placebo. If the blood potassium went low (below 3.5 mmol/L), potassium supplements were supposed to be prescribed. An analysis of 4,126 patients in SHEP found that 7% of the patients being treated with the diuretic had a low-blood potassium one year into the trial. After five years when the study ended, it was found that although the rate of cardiovascular events was significantly reduced in the diuretic treated group as a whole (by about 30%), this reduction was not seen in the subset of patients who developed a low-blood potassium. The main implication of the study was that it is important to keep the blood potassium level normal in patients on diuretic treatment.

Doctor’s comments

This is an important finding, with practical implications. One reason why a low-blood potassium may be bad is that it predisposes the heart to rhythm abnormalities (arrhythmias), and sudden death, but this did not appear to be the reason for the worse prognosis in the patients with low potassium in SHEP. Another reason may be that potassium actually slows the progression of vascular disease, and we have posted articles showing that people who eat a diet that is high in potassium are at reduced risk of having a stroke. There are two ways of preventing low-blood potassium in patients taking a diuretic. One is to take potassium supplements (usually in the form of pills), and the other is to take a second diuretic that retains potassium.

Where it was published