STS-95 PAYLOADS - Human Research
Getting Pumped Up in Space: Cardiovascular Investigations

In the microgravity of spaceflight, bodily fluids are shifted to the upper body. As the body seeks to regulate this fluid movement, the cardiovascular system is affected during and after spaceflight. During spaceflight, pressure sensors in the head, neck and chest are stimulated by the fluid shift, which the central nervous system interprets as excess fluid volume in the heart and lungs. To reduce this perceived excess, body fluids are either directly eliminated through urination and decreased thirst or redistributed throughout the blood vessels. After a few days, the body adapts to the microgravity environment and the process of fluid redistribution is complete. A new difficulty arises when the astronaut returns to Earth and again feels the effects of gravity. The reduced blood volume, combined with less responsive reflexes, may not provide adequate blood pressure to the head during entry or landing. Astronauts often experience lightheadedness, and sometimes even fainting, when they stand for the first time upon returning to Earth. The natural loss of body fluids in space may also upset the chemical balance of the blood and thereby interfere with the normal functioning of the heart, possibly resulting in irregular heartbeats.

Three studies planned for STS-95 will address the cardiovascular effects of spaceflight, with a focus on the older individual. Each cardiovascular investigation for STS-95 requires multiple repetitions of the experiment procedure before and after the flight.

Cardiovascular Responses to Standing Before and After Spaceflight will build upon an existing database of 43 shuttle astronauts, including three who flew long-duration missions on the Russian space station Mir. This investigation will assess the many physiological responses involved in standing up in the normal gravity environment after the body has adapted to 9 days in microgravity. Immediately after landing, astronauts are transported to the clinical test facility, where the cardiovascular response to Earth's gravity can be examined with the enhanced tilt test procedures. These procedures allow researchers to collect detailed cardiovascular measurements as the astronaut is telted upright after lying down. Cardiovascular response mechanisms will be measured with a blood pressure monitor, an electrocardiograph for collecting heart rate and rhythm information, and an ultrasound echocardiograph for measuring the volume of blood pumped by each heart beat. With this information, the degree of arterial constriction can be calculated both at rest and under the gravitational stress of standing. A second, minimally invasive technique involves the re-breathing of a small amount of carbon monoxide to allow the calculation of blood volume.

An astronaut dons his launch and entry suit, which contains a compact blood pressure monitor and tape recorder for measuring cardiovascular functioning.

Orthostatic Function During Entry, Landing and Egress (with an existing database of 66 shuttle astronauts, including five from Mir) will focus on two parameters, blood pressure and heart rate, during the entry and landing phases of the flight. A compact blood pressure monitor and a small tape recorder will record cardiovascular data for the intervals before, during and after entry and landing. These deveces are integrated into the landing and egress suit. Data collected during these phases will document the real-time response of the cardiovascular system to Earth's gravity after more than 9 days of exposure to microgravity. Comparison with data from the Cardiovascular Response experiment will indicate how much readaptation takes place in the first hours after landing. The third investigation, In-flight Holter Monitoring (with an existing database of 24 shuttle astronauts, including four who flew on Mir), will use a standard Holter recorder to collect 24 hours of electrocardiogram data. The Holter recorder is a vest-like garment with embedded sensors. Data collected before, during and after the flight will reveal what difference, if any, exists between the occurrence of irregular heartbeats in spaceflight and on Earth.

The Holter monitor consists of an electrode set and data recorder that continuously measure electrocardiograph data.

The consequences of an inadequate cardiovascular response to entry and landing and to the gravitational loading on Earth could compromise the health and safety of crewmembers after spaceflight. Development of improved countermeasures and treatments for astronauts requires a complete understanding of how the body adapts to the microgravity of spaceflight. Similar mechanisms may be involved in the development of many cardiovascular diseases on Earth, such as abnormal blood pressure, blood loss and heart failure. These diseases are particularly prevalent in older people. The same knowledge that benefits returning astronauts may be applied to protecting patients here on Earth, young and old alike.