By James Somerville, MD, nephrology, InterMed Consultants, Ltd., a Fairview Physician Associates clinic

High blood pressure, the most common reason for non-pregnant adults' health care visits and prescribed medications, afflicts 30 percent of people older than 18 and 50 percent of those older than 65 nationwide. Rates likely will increase as adults become proportionally older and more overweight.

Hypertension is "resistant" when blood pressure-usually systolic-remains higher than goal (usually <140/90 mm/Hg) despite using at least three medications of different classes. More prevalent in older, overweight, diabetic, female and African American adults, at least 20 to 30 percent of clinical trial participants exhibit resistant hypertension. Some studies show only half the subjects achieve goal blood pressure.

Watch for 'white coat' effect

Hypertension inaccurately labeled "resistant" may stem from poor measurement, such as using an inadequately sized cuff in individuals with large arm circumference, or poor adherence to prescribed treatment. Monitoring ambulatory blood pressure can identify the "white-coat" effect, possibly responsible for 25 percent of seeming resistant hypertension.

Obesity, excessive dietary salt, natural licorice or alcohol intake can contribute to resistant hypertension. Medications also can interfere;

the most common classes include NSAIDS, sympathomimetics, stimulants, oral contraceptives, corticosteroids and herbal supplements containing ephedra.

Secondary causes include primary aldosteronism

More common than previously suspected, primary aldosteronism, chronic kidney disease and renal artery stenosis represent the most frequent "classic" secondary causes of resistant hypertension. Primary aldosteronism, identified by measuring renin and aldosterone levels, is present in up to 20 percent of resistant hypertensives. Sleep apnea also is common in resistant hypertensives, although it is not usually listed as a "classic" secondary cause.

Suboptimal therapy, in particular failure to counteract extracellular volume expansion through underuse or misuse of diuretics, may contribute to resistant hypertension. The long-acting thiazide chlorthalidone may be more effective than hydrochlorothiazide. Using loop diuretics in patients with chronic kidney disease will control volume expansion more effectively than thiazides. Using aldosterone antagonists, such as spironolactone, also may control resistant hypertension.

For a scientific statement on hypertension, visit the American Heart Association's journal Hypertension, June 2008 at hyper.ahajournals.org. Also contact Somerville, 952-920-2070, somer5mn@netscape.net.