Measuring Blood Pressure Near Heart May Be Key
Arm-based readings may not give the full picture of a medicine's effectiveness, study finds
SUNDAY, Nov. 13, 2005 (HealthDay News) -- Where blood pressure is measured may be critical in figuring out just how well a drug is working to fight hypertension, a new study suggests.
Researchers assessed treatments focused on calcium channel blocker drugs by measuring blood pressure nearer the heart, where these drugs' activity seems to be more critical.
Using this aortic blood pressure measurement, they found that calcium channel blockers may outperform beta blockers in keeping blood pressure under control.
When blood pressure was measured in the arm, however, the two treatment regimens appeared to be equal.
The findings also suggest that the effectiveness of drugs may rely on their activity near important organs such as the heart or brain, rather than in more distant sites such as the arm.
"Measuring blood pressure in the arm is very important because it tells us whether somebody's pressure is high and whether drugs are lowering that," explained Dr. Bryan Williams, lead investigator and a professor of medicine at the University of Leicester in the United Kingdom. "But there's no doubt that for patients in this study, measurement of the pressure in the arm underestimated the efficiency of the drug."
The results, which were presented at a late-breaking clinical trial session Sunday at the annual meeting of the American Heart Association in Dallas, could change the way different drugs are evaluated and prescribed.
However, other experts pointed to potential problems with the data, including how to parse out which drug is having the most effect.
"There are four drug variables in two arms [of the study]," said Dr. Robert J. Myerburg, professor of medicine and physiology at the University of Miami School of Medicine. "How do you ascribe the benefit?"
Measurements of blood pressure in the brachial artery (running from the shoulder to the elbow) are traditionally used to evaluate how well a drug is working. Some experts posit that measuring central aortic blood pressure (near the heart) may be more indicative of how a drug is performing and also a better predictor of whether a person is likely to suffer a stroke or heart attacks.
"Most of the blood pressure studies assume that differences in outcome are explained by some property of the drug," said Williams. "We said an alternative possibility is that what we measure in the arm isn't always giving us information about what's happening in the central arteries."
The current study, called CAFE (Conduit Artery Function Evaluation) is a sub-study of ASCOT (Anglo-Scandinavian Cardiac Outcomes Trial), which involves almost 20,000 participants and is one of the largest studies of high blood pressure ever undertaken in Europe.
The CAFE study looked at almost 2,200 hypertension patients (mostly males over the age of 55) from ASCOT. The participants had been divided into two groups: those receiving the conventional treatment of beta blocker plus a diuretic and those receiving calcium channel blockers and ACE inhibitors. Participants were started with only beta blockers or calcium channel blockers. If those weren't enough, the other drugs were added, Williams explained.
Central aortic pressure was measured with the SphygmoCor(R) system, which interprets the shape of pulse waves measured at the wrist.
"This is a system being validated by the FDA," Williams said. "It's tricky to use and that's why it's used in a clinical trial, but it certainly provides us with information about what the drug is doing."
The calcium-channel-blocker regimen reduced central aortic pressure by 4.3 millimeters of mercury (mm Hg) more than the beta-blocker regimen, the researchers reported.
"That kind of difference is normally translated into a 20-to-30 percent reduction in risk of stroke," Williams said. "Those kinds of differences in pressure are not insignificant."
There was little difference in brachial blood pressure between the two groups.
When the results were extrapolated to the larger ASCOT group, the lower central aortic pressure was associated with reductions in major cardiovascular events such as stroke and heart attacks as well as death.
"Although the two treatments looked as though they were producing identical falls in blood pressure, we found that the central pressures were much lower on the amlodipine- based [calcium channel blocker] treatment. In fact, it was highly significant, and this was a bit of a surprise," Williams said.
"The thing that was predicting the benefit of the drug wasn't so much pressure in the arm but whether the patient had lower pressure in the central aorta," he added.
But Myerburg questioned the measuring technique. The researchers did not measure the central aortic pressure directly but used a surrogate, he said.
"To what extent is that validated, both for the specific device they use and for the principle in general?" asked Myerburg. "I presume that someone somewhere along the line did a study comparing intra-aortic measure with a catheter to this calculated number and how those calculations are affected by drugs."
In any event, Williams feels the study is a call for more research on how different drugs affect blood pressure in the central arterial system.
"Those drugs lowering pressure closest to the brain and heart are likely to be more relevant," he said.
Facts about high blood pressure are provided by the American Heart Association.