A Duke University Medical Center study of dialysis patients indicates that a new way of looking at traditional blood pressure numbers may be important in determining which patients are at highest risk of dying.
Almost 20 percent of dialysis patients die each year, usually from heart disease. By better understanding pulse pressure, doctors may be able to identify high-risk patients and take steps to keep those patients alive. The study is featured in the March 27, 2002 issue of the Journal of the American Medical Association.
The study looked at pulse pressure, which is the difference between blood pressure's two key numbers: systolic blood pressure (top number) and diastolic blood pressure (bottom number). For example, a blood pressure 120/80 has a pulse pressure of 40. Pulse pressure has previously been shown to be a powerful predictors of heart attacks, heart failure and death in patients who are not on dialysis.
The Duke University researchers examined pulse pressure in dialysis patients because this population shows an unusual relationship between traditional blood pressure measurements and death.
"In the general population, people who have higher blood pressure have a greater risk of dying," said Dr. Preston Klassen, lead author of the study and nephrologist at Duke University Medical Center. "In dialysis patients, that relationship at first glance appears to be reversed. Dialysis patients with higher blood pressures seem to live longer than dialysis patients with lower blood pressures. We decided to examine pulse pressures in dialysis patients to see what the relationship was between pulse pressure and death."
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The Duke researchers examined existing data from more than 37,000 patients undergoing hemodialysis across the United States. They analyzed patients' pulse pressures and then correlated that information with which patients died during the course of one year. The analysis showed that dialysis patients, like patients in the general population, had a greater risk of dying if they had higher pulse pressures.
"In particular, these results suggest that for any given level of systolic blood pressure, greater pulse pressure correlates with increased risk of death. Let's say you have one person with a blood pressure of 140/60 and another person with a blood pressure of 140/90," said Klassen. "Conventional wisdom would say that the second person is at greater risk than the first person because their bottom number is higher. But in fact, it's the first person that is at greater risk. The first person has a wider distance between the two numbers, a pulse pressure of 80, compared to the second person with a pulse pressure of 50."
While the study shows that a high pulse pressure is an indicator for risk of death in dialysis patients, researchers don't know how to lower pulse pressures specifically. "Even if we did know how to lower pulse pressures, we wouldn't know if this would reduce death," Klassen said.
"We need more research to understand if diet, exercise or drug therapy may be useful in lowering pulse pressures." At that point, researchers would need to see if lowering pulse pressure does, in fact, help patients.
Klassen warns that the wrong message to take away from this study is that dialysis patients should have high blood pressure. He suggests that physicians need to be more aggressive in identifying and treating high blood pressure in all patients, particularly in those with kidney disease before they start dialysis.
"The reason that dialysis patients with higher systolic blood pressures appear to live longer than those with normal or lower pressures is probably because of years of hypertension," he said.
"Hearts subjected to longstanding hypertension may ultimately fail and be unable to generate high blood pressure. Their blood pressures are low because they have weaker hearts, which also puts them at greater risk of dying from heart attacks, heart failure and other causes." Klassen said most dialysis patients have high pulse pressures. A pulse pressure above 50 is considered elevated and thought to represent increased risk for heart disease and death in the general population.
"We found that over 90 percent of dialysis patients had a pulse pressure greater than 50. This is probably related to stiffening of blood vessels, which happens more frequently in dialysis patients."
During hemodialysis a patient's blood is passed through a machine that filters water and toxic compounds created by the body. Approximately 63 percent of patients with kidney failure receive hemodialysis to replace kidney function, while 30 percent have a kidney transplant and 7 percent receive another form of dialysis called peritoneal dialysis. The most common causes of kidney failure are diabetes and hypertension.
Klassen said this research might also have implications for the general population as well. "This study will hopefully help us learn more about the best way to look at blood pressure in patients," he said. "Researchers are starting to get the idea that it is important to look at both the top and bottom numbers and how they relate to one another when studying blood pressure.
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It's not just the top number by itself or the bottom number by itself that matters. It may be the relationship between the numbers -- the distance between them -- that gives important information."
The study was partly funded by an American Kidney Fund Clinical Scientist in Nephrology grant. Co-authors include Donal N. Reddan, Elizabeth R. DeLong, Ph.D., Joseph Coladonato, M.D., and Lynda Szczech, M.D., all of Duke; William Owen, M.D., previously of Duke, now with Baxter Healthcare International, Waukegan, Ill.; and Edmund G. Lowrie, M.D. and J. Michael Lazarus, both with Fresenius Medical Care North America, Lexington, Mass. Lowrie is also affiliated with Duke.
Source: Duke University Medical Center