Echocardiographic Assessment of Cardiovascular Adaptation and Countermeasures in Microgravity
James D. Thomas, M.D.
The Cleveland Clinic Foundation
Dr. James Thomas is exploring echocardiography (ultrasound) as a reliable, portable, noninvasive method to detect and quantify changes that the body undergoes in space. This machine will provide imaging of the heart, abdomen and blood vessels when astronauts suffer a medical problem in space, and it will obtain high quality research data to better understand the impact of long-term space flight on the human body. His project will establish an echocardiograph facility for training non-physicians to use ultrasound and for assessing data from flight and ground studies.
There is some evidence that the heart undergoes a loss of muscle during long-term space flight, which might limit an astronaut’s ability to work in space or back on the ground. By studying the changes that occur in patients’ hearts after aortic valve replacement, Dr. Thomas hopes to determine some of the factors responsible for this loss of muscle in space.
NASA Taskbook Entry
The cardiovascular system undergoes significant changes in microgravity, including an early cephalad shift of lower extremity blood volume, loss of plasma volume over 24 to 48 hours, and long-term reduction in ventricular chamber volume and mass. In the weightless environment, these alterations generally are well tolerated, but upon return to Earth, astronauts often suffer from serious orthostatic intolerance and reduced exercise capacity, changes that may limit the long-term presence of man in space. It is essential that the mechanisms for these alterations be understood so that reliable countermeasures can be tested and implemented. Hypovolemia, cardiac atrophy, and autonomic dysfunction have each been hypothesized to contribute to this post-flight debility, but their relative importance is unclear. Furthermore, it is unknown whether actual abnormalities in the myocardium itself develop with long-term space flight. Therefore, reliable portable noninvasive methods will be needed in order to detect and quantify these changes.
Alone among such imaging modalities of radiography, magnetic resonance imaging and computerized tomography, echocardiography has the unique ability to characterize cardiovascular anatomy and physiology in ground-based models, pre- and post-flight, and most importantly during flight. Indeed, the Science Working Group (SWG) for the International Space Station (ISS) Human Research Facility (HRF) has recognized the primacy of ultrasound for medical diagnosis and physiology research, with plans to launch a specially modified commercial ultrasound instrument to the ISS in 2001. Echocardiography is similarly being used before and after shuttle flights and in a variety of bed-rest studies sponsored by NSBRI and NASA. Unfortunately, while ultrasound has the potential for high spatial and temporal resolution imaging of the heart, in the past it has been severely limited by operator inexperience and inconsistency in its subjective interpretation. Needed are new methodologies for assessing the load-independent function of the heart and consistent, objective quantification of a wide range of NASA echo studies, whether obtained on the ground, in flight or in experimental models. We propose to provide such a facility while validating novel methods for the load independent assessment of myocardial function. Our central hypothesis is that:
Microgravity affects cardiovascular function not only through changes in chamber volume and mass but also through changes in myocardial properties.
A definitive test of this hypothesis is at least several years away when dedicated life science missions are possible aboard the ISS. However, within the scope of this grant, we propose several specific aims that will be critical to the ultimate comprehensive study of the cardiovascular system in space. Key issues: Validation of non-invasive Doppler echocardiographic indices for the assessment of left ventricular contractility and relaxation including color M-mode Doppler derived diastolic intraventricular pressure gradients (IVPG) and tissue Doppler derived myocardial systolic and diastolic strain rates (e's, e'd); Validation of Doppler derived exercise cardiac output and contractile reserve and their potential utility for the early detection of myocardial dysfunction during prolonged space flight. Additional deliverables to NSBRI: Development and distribution of stand-alone software and algorithms for implementing the quantitative analysis of Doppler echocardiographic data, as described above, so they may be applied to ultrasound data obtained from remote sources; 4) Establishment of an Echocardiographic Core Facility to the NASA research and clinical community, capable of applying standard and novel analysis techniques in a rigorous fashion to echocardiographic data obtained from selected ground-based experimental models, pre- and post-flight examinations, and eventually from in-flight acquisitions.
If successfully implemented, these aims will allow the cardiovascular sequellae of space flight to be studied much more rigorously, while providing consistent, objective echocardiographic interpretation to the entire NASA community.