MONDAY, Nov. 24 (HealthDay News) -- Round-the-clock blood pressure measurements, especially those during the night, are better predictors of major cardiovascular problems than readings taken in a doctor's office for people whose high blood pressure is hardest to treat, a Brazilian study shows.

"Ambulatory blood pressure monitoring should be performed during the whole 24 hours, with separate analyses of the daytime and nighttime periods, because it seems that nighttime blood pressures are better cardiovascular risk factors than are daytime blood pressure," said the report in the Nov. 24 issue of Archives of Internal Medicine by physicians at the Federal University of Rio de Janeiro.

The researchers followed 556 people with what doctors call "resistant hypertension" -- blood pressure that remains dangerously high despite standard drug therapy. More than 10 percent of people with high blood pressure are in that category, the report said.

Some of the participants in the Brazilian study underwent usual regular blood pressure measurements in a doctor's office, while others had continuous monitoring at home (ambulatory monitoring) -- every 15 minutes during the day and every 30 minutes at night.

The clear and present danger of high blood pressure was evidenced by the high incidence of strokes, heart attacks, heart failure and deaths, which occurred in 19.6 percent of the study group, with an average follow-up period of 4.8 years. Office blood pressure measurements did not predict any of these events, whereas higher round-the-clock readings did, the report said.

The report is yet another instance of a common heart test failing to predict trouble ahead. A study reported earlier this month by physicians at the London Chest Hospital in England found that performing an electrocardiogram, the standard test for measuring the activity of the heart, is of little use in forecasting future coronary problems for people with chest pain.

On the hypertension front, previous studies have indicated that "ambulatory blood pressure measurements do predict things better, in part because they are so frequent," said Dr. William C. Cushman, professor of preventive medicine and medicine at the University of Tennessee.

"There is a growing body of evidence that ambulatory blood pressure measurements may be more accurate than taking office blood pressures," Cushman said.

And yet, he added, "there clearly have been dramatic benefits from treatment based on office blood pressures."

An unanswered question is whether treatment based on nighttime blood pressure measurements can reduce the risks of hypertension, Cushman said. "The real problem is that virtually all the studies showing that treatment is effective have been based on office readings," he said.

That echoed the findings of the Brazilian researchers, who wrote that their study "raises the question of whether therapeutic interventions directed specifically at controlling nighttime hypertension will be able to improve cardiovascular prognosis compared with the traditional approach of controlling daytime blood pressure levels. This important clinical question should be addressed in future prospective interventional studies."

What is needed is a study in which people with resistant high blood pressure would be randomly selected to receive treatment based either on office readings or round-the-clock measurements, Cushman said. "We don't have treatment studies that have used that approach," he said.

Resistant hypertension can be treated successfully with combinations of drugs such as diuretics, ACE inhibitors and calcium channel blockers, he said.

Not all doctors are convinced that round-the-clock monitoring is the total solution.

It's clearly of value in treating people with resistant hypertension, but often is "not practical in terms of convenience to the patient," said Dr. David A. Calhoun, professor of medicine at the University of Alabama at Birmingham.

"We tell all our patients to invest in a home monitor," Calhoun said. "Once we have done that and have confidence in their measurements, we have them keep a blood pressure diary and bring it in each week as a guide to decision-making."

Dr. John Bisognano, associate professor of medicine and cardiology at the University of Rochester, in New York, said continuous monitoring doesn't solve the basic problem with resistant hypertension, which is that "these are people who need better blood pressure control."

"It's like putting a better thermometer in a burning building," he said. "It doesn't really matter whether you get more accurate temperatures or not."

Continuous monitoring is of more value for people with borderline hypertension, Bisognano said. "It can tell you whether to initiate treatment and how intensely you need to treat them," he said.

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