Hearts in Space

NASA has sent hundreds of people into space since 1961, but scientists still don't know enough about the fundamental effects of weightlessness on the cardiovascular system, experts say.

Now, a UofL research team operating under the name "Hearts in Space" is working to change that with a series of experiments that examine basic physiological responses to the zero-gravity environment.

One of the team's researchers, Keith Sharp, Sc.D., has developed a complex computer model that helps explain why astronauts suffer from bouts of lightheadedness, dizziness and occasional blackouts after returning to Earth following space flight.

A better understanding of the causes behind this phenomenon could lead to countermeasures that improve safety for future space travelers, says Sharp, a professor of mechanical engineering at UofL.

"This is a real safety issue because of the time it takes returning astronauts to feel comfortable with standing after they return to Earth," Sharp says. "On average, we're talking about 45 minutes, which is a long time if there were to be a fire or any other type of emergency on board the space shuttle. Some of them might not make it out."

Researchers have long known that the lightheadedness is caused partly by the body's natural response to weightlessness -- it reduces the amount of blood available for circulation by up to one liter.

While that doesn't cause any problems in a zero-G environment, where blood is evenly distributed throughout the body, it can cause real issues upon returning to Earth, where the one-G atmosphere makes blood pool in the legs, reducing its flow to the brain.

This circumstance doesn't explain everything, however. Even if an astronaut's blood supply is replenished, he or she will continue to experience dizziness for up to a day after returning to Earth.

Researchers have proposed several hypotheses to explain the continued dizziness, but Sharp favors a novel idea best described as the "leaky lymph system."

Lymph, a colorless fluid containing white blood cells, surrounds cell tissues throughout the body. It is distributed to those tissues by passing through the walls of nearby capillaries, using pores that are optimized to control the flow of lymph in a one-G environment. The fluid is eventually recollected and funneled back into the circulatory system to form part of the body's blood supply.

Sharp theorizes that the pores in the capillaries grow larger in weightlessness to keep an adequate supply of lymph flowing to tissues. When an astronaut returns to Earth, however, the pores remain too big for a one-G setting, causing "leaks" that drain fluid into surrounding tissues too fast. The process by which lymph is put back into the blood stream doesn't change, however, so the volume of liquid in the circulatory system keeps dropping. The resulting drop in blood pressure means that the heart is unable to maintain adequate blood flow to the brain, and dizziness ensues.

Sharp's computer model allows him to test this theory by manipulating numerous parameters in a simulated cardiovascular system and observing the physiological responses. One of these parameters is the rate of fluid leakage from capillaries.

"We have a simulated astronaut that we subject to a stand test after his return to Earth, and he gets dizzy after about seven minutes in one-G, exactly like astronauts on the space shuttle," he says. "When we manipulate the leakage rate, our model behaves in the same way that actual astronauts do, which suggests that the leakage could be the cause of the phenomenon."

Sharp's encouraging results, obtained with the help of UofL mechanical engineering students Justin Broskey and Jeremy Witten, are expected to be published this fall in the Annals of Biomedical Engineering. Meanwhile, graduate research assistant Dengzhi Wang and mechanical engineering student Brandon Coats are helping continue development of the computer model.

UofL scientists also are examining the most effective means of providing Cardio-Pulmonary Resuscitation in a zero-G environment, says George Pantalos, Ph.D., a professor of surgery and bioengineering at UofL. Pantalos' CPR studies are especially significant given America's renewed interest in long-term space operations.

When President Bush announced in 2004 that America was planning to build a lunar base and launch a manned mission to Mars, NASA revised its Space Life Sciences Critical Path Roadmap -- a document that benchmarks what tasks need to be accomplished as future missions approach, Pantalos says. One such task was the development of effective techniques for delivering CPR in the zero-G environment of space flight or the lunar-G environment of a moon base.

"Nobody expects a crewmember to have a cardiac arrest, but there are potential situations where it could happen," Pantalos says. "One is accidental electrocution. Another is rapid decompression of a spacecraft or lunar base. So NASA wants to know if it's possible to give effective chest compressions and resuscitate a crew member who has experienced cardiac arrest."

Working with Sharp and two other researchers -- third-year medical student Jeff Henson and Mead Ferris, who graduated from UofL with a master's degree in physiology and biophysics -- Pantalos has been testing that question by using instrumented mannequins and human test subjects to collect data.

They have examined four CPR methods:

• Side straddle, in which the victim is strapped to a flat surface and the rescuer is tethered at the waist while applying chest compressions from above
• Modified side straddle, in which two additional straps are stretched across the rescuer's shoulders and attached to the opposite side of the victim to provide additional bracing and counter force
• Vertical CPR, in which the victim is strapped to a flat surface and the rescuer performs a weightless handstand, applying chest compressions by pushing off the walls or roof of the spacecraft with his feet
• Modified Heimlich Maneuver (or backward bear hug), in which the rescuer uses his arms to encircle the victim's chest from behind and applies compressions at the level of the heart

To conduct the study, CPR mannequins were fitted with special instruments that measured a variety of factors, including arterial pressure, chest-compression force and blood flow to the brain, Ferris says.

He and the three other researchers then traveled to Johnson Space Center, Texas, in March of 2007 for two days of flights aboard NASA's specially equipped zero-gravity research plane.

The modified DC-9 is capable of creating a weightless environment for 20 seconds at a time by following a flight path consisting of a series of vertical parabolas, much like an airborne roller-coaster.

After multiple iterations using all four CPR methods, researchers found no significant difference in the effectiveness of each, Ferris says. The backward bear hug was, however, more expedient for practical reasons.

The Vertical CPR method only works if the rescuer is tall enough to push off the walls or ceiling of the spacecraft, Pantalos says. And the first two methods (side straddle and modified side straddle) require that the victim first be immobilized on a flat surface, while the rescuer must be positioned using one or two sets of straps. All of this wastes precious time, Pantalos says. On the other hand, the backward bear hug -- a technique currently not used by NASA -- has no height requirements and can be started immediately after an initial assessment.

"If you see someone go into cardiac arrest, you just come up behind them and start chest compressions right away," Henson says. "It saves a lot of time."

That makes it easy to recommend the backward bear hug as the optimal CPR method in weightlessness, Ferris says.

"All the techniques produce similar augmentations in the physiological parameters and they all have the same ability to produce compressions with the same force and depth, so you would want to go with the technique that can be implemented the fastest," he says.

The findings backed up an earlier study, completed at UofL's Outcomes Research Institute, in which Pantalos' team analyzed the same techniques on Earth using instrumented human test subjects instead of mannequins.

The team just submitted its findings to the American Heart Association for possible presentation at a scientific meeting in November. Pantalos is hopeful the paper will be selected, in part because the studies may mark the first use of conscious human test subjects in CPR research.

"CPR research is typically done on cadavers, animals or mannequins," Pantalos says. "So the idea of using a healthy volunteer test subject was looked at as very novel and very innovative. We spent a fair amount of time preparing our test subjects for these measurements.

"We started out with very gentle test compressions and slowly worked our way up to the point where the test subject could tell us if they were still tolerable. We also were very careful to deliver only partial chest compressions at the same time the natural heart was contracting."

Despite its novel nature, the methodology was so effective for comparative analysis that it may become a standard technique in future CPR research, Pantalos says.

At the very least, UofL's Hearts in Space team will likely use it (and other methods) for ongoing space-related research.

"I can't think of a single colleague who thinks we know as much as we should about the life-sciences aspects of establishing a permanent base on the moon or undertaking an expedition to Mars," Pantalos says. "I think there's still plenty of work to be done in understanding the basic science behind space flight, and we hope to play a continuing role in that process. There is clearly more we need to learn."