Students' Research Provide Clues to Heart Function
Every year, scientists are coming to more fully understand the mysteries of the human heart. New work on a poorly-understood phenomenon called cardiovascular drift by two Willamette undergraduates at is adding to the knowledge that one day may help enhance the performance of top athletes.
Heather Clifton, of Grass Valley, Calif, and Garrit Southard, of Salem, Ore., both seniors majoring in exercise science, are interested in how the body functions during exercise. In 2003, they applied for and received a Carson Undergraduate Research Grant, a summer stipend that's designed to encourage independent student research. They studied cardiovascular drift, an intriguing physiologic anomaly in which the heart rate slowly and steadily increases during long-term exercise, and they recently presented their findings before fellow students and faculty members.
Cardiovascular drift is a hot research topic right now because it goes against conventional wisdom about how the heart functions during exercise. "We used to think that when you're exercising at a steady level, the body reaches a steady state where the heart rate remains the same," explained Clifton in a recent interview. "When you look with more sophisticated equipment, you find that the heart rate slowly climbs. That's cardiovascular drift."
Most of the research in the past few years on has focused on why cardiovascular drift occurs. Scientists know it occurs when people exercise for long periods of time without drinking enough water. Exercise causes the body to sweat, which decreases the blood's plasma volume and makes less blood available for the heart to pump. To provide the exercising muscles the needed oxygen and nutrients, the body tries to compensate by producing more blood faster, which increases heart rate. The heart also tries to pump more blood with each beat, but when that accommodation can't keep up with demand, the heart beats faster to compensate.
Clifton and Southard wanted to know what effect, if any, cardiovascular drift has on the body. "There isn't anything in the literature on the energetics of cardiovascular drift," said Southard. "What are the energy demands of cardiovascular drift? Does it cause fatigue? Does it impact performance?"
"I wanted to find out what kind and how much fuel is burned with cardiovascular drift," Clifton explained. "Our hypothesis was that if the heart has to work harder during cardiovascular drift and changes the energy demands, then it would disrupt the steady state the body has achieved. In a steady state, you have a dynamic equilibrium between energy demands and the body's ability to meet those demands. If your heart rate increases the energy demands, you're no longer in this steady state."
Clifton and Southard selected four male elite athletes: three were members of Willamette's cross country team. A metabolic analyzer captured each subject's breath for analysis. The machine analyzed, among other things, oxygen consumption (called VO2), carbon dioxide exhalation and how much and what kind of fuel the body is burning.
"The first day we established the subjects' lactate threshold and their maximum oxygen consumption (VO2 max) to determine their capacity for aerobic performance," explained Southard. This enabled the students to determine how hard the athletes had to work to make cardiovascular drift occur.
Three days later, the athletes, who weren't given water before or during the test, rode exercise bikes at the pre-determined levels for 70 minutes while the analyzer took measurements. The students also took tiny blood samples for blood analysis.
"The analyzer measures roughly 30 different measurements per breath so we collected an amazing amount of data," said Clifton. "At the end of 70 minutes, we literally had 15 to 20 pages of data on each subject."
What did Clifton's and Southard's research find? Despite the fact that the heart rate increases, amazingly the body is able to maintain its steady state. "We found no lactic changes and no changes in oxygen consumption," said Southard.
"There were no detectable changes," said Clifton. "That doesn't mean there aren't changes. It could be that the changes are so miniscule that our state-of-the art metabolic analyzer simply couldn't detect them." Another possibility is that the body is meeting the increased demands in creative ways. "Oxygen, for example, may be redirected to the heart so that even though the body is using the same amount of oxygen, it's being used differently by different tissues. The only way for us to know is to have more finely-tuned equipment that can measure those changes, but that equipment doesn't exist yet."
Southard offered yet another explanation. "These are highly trained athletes whose bodies have seen cardiovascular drift before," he said. "It may be that their bodies are able to make really small changes that can accommodate the increase in heart rate."
Southard's next line of research will explore how untrained individuals respond to cardiovascular drift. The results of that research will become part of his senior thesis this year at Willamette.
The students plan to present their research findings at the regional meeting of the American College of Sports Medicine (ACSM) in March in Seattle where they are one of only two undergraduate projects that were awarded a podium presentation. They plan to go onto the ACSM national meeting in Indianapolis in June.
So what can we conclude from Clifton's and Southard's work? "We know we can cause cardiovascular drift to occur in the laboratory," said Southard. "For highly trained athletes at least, cardiovascular drift isn't something they have to worry about."
Perhaps most importantly, the students' work is increasing the knowledge about how the heart works. "No one has asked how cardiovascular drift damages the heart or strengthens the heart, what happens over time and how long it takes to recover from it. Garritt and I have started the process of looking at some of those questions. We're just at a starting point, but the research definitely adds to our knowledge about how the heart works and what it does during exercise."