DALLAS (Oct. 11, 2005) -- Reduced blood flow to the skin's surface maybe a key cause of heat-related illnesses in patients with congestiveheart failure, UT Southwestern Medical Center researchers have found.
The first study to investigate how heat affects people with heartfailure shows that one of two ways the body can cool itself is not aseffective in those with congestive heart failure relative to healthyindividuals. The results, published in today's issue of the journalCirculation, emphasize the need for people with heart failure to takespecial care when the weather is hot, said Dr. Benjamin Levine,professor of internal medicine at UT Southwestern Medical Center andone of the study's two senior authors.
"We wondered whether either sweating or skin/blood responses would beimpaired in heart failure patients," said Dr. Craig Crandall, associateprofessor of internal medicine at UT Southwestern and the other seniorauthor of the study. "We found that for the same level of internaltemperature, the heart failure patient does not dilate blood vessels ofthe skin as much."
He and his colleagues discovered no difference in sweating responsesamong study participants with heart failure or healthy subjects.However, the skin/blood flow response in those with heart failure wassignificantly impaired, by as much as 50 percent when compared to thecontrol group.
"The purpose of this study was to find out why patients with heartdisease and heart failure are at great risk for having complicationswhen the weather gets hot," said Dr. Levine, director of the Institutefor Exercise and Environmental Medicine, a collaboration between UTSouthwestern and Presbyterian Hospital of Dallas. "We saw this inparticular in the heat wave that hit Chicago in 1995. Of the reporteddeaths, a large number had a prior heart condition. We wondered why."
The study included 28 participants between the ages of 47 and 55 --half with congestive heart failure and half with healthy hearts. Fortesting, all were put into tube-lines suits and the temperature of thewater perfusing the suits was elevated, resulting in increases in skinand internal temperatures. Researchers then studied skin-blood flow andsweating, the two main mechanisms for the body to maintain temperaturecontrol.
Increased blood flow to skin works as a kind of radiator for the body.When your heart pumps more blood to the skin's surface, it is drawingout the heat. A healthy person may have to pump three times as muchblood as normal if the outside temperature is hot.
Study participants' heat-stress responses were obtained during restingconditions only, not body heat generated through exercise. Bloodpressure, heart rate, forearm skin blood flow and sweat rate werecollected while 93.2-degree Fahrenheit water perfused through the suit.After six minutes, whole-body heating began by elevating the skintemperature to 100.4 F, a temperature high enough to cause sweating andelevated blood flow to the skin.
"If a person doesn't have enough pump function to get the bloodflowing, they're going to have difficulty controlling their bodytemperature," Dr. Levine said. "Heart failure itself is the inabilityto pump enough blood to meet the demands of the body -- it usuallyhappens in patients who have had a heart attack or other diseases thathave compromised the heart muscle."
Dr. Crandall added that physicians have long known anecdotally thatpeople with heart failure are more susceptible to heat-relatedillnesses. The mechanism for this, however, was not known.
"Since so many variables could impact the level of heat stress such aswind and humidity, we don't have a set an environmental temperaturethreshold to give to patients with heart failure," Dr. Crandall said."What we can say is that they should be more aware of the heat and, ifthey begin to feel overheated, they should get into an air-conditionedenvironment."
Other UT Southwestern researchers participating in the study were Drs.Armin Arbab-Zadeh and Anand Prasad, postdoctoral trainee clinicians.
The study was supported by the Doris Duke Charitable Foundation, theNational Heart, Lung and Blood Institute, and the American HeartAssociation.
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