OBJECTIVES:
As astronauts venture farther into space, the impact of long-term microgravity on cardiovascular function may become a critical limitation to mission safety and success. The first task in this experiment was to develop and validate methodology to extract strain and torsion from the International Space Station (ISS) echocardiograms and combined it with the numerous pre- and post-flight studies that were conducted over the years. This gave investigators a comprehensive view of the heart in space, information which was integrated into evolving mathematical models of the heart.
Exposure to microgravity induces short and long-term changes in the cardiovascular system, with cardiac atrophy, orthostatic hypotension and impaired thermoregulation being the most recognizable. The most obvious issue, noted in the majority of astronauts after long-term space flight, is orthostatic hypotension. While its importance is clear, the etiology remains uncertain, with proposed mechanisms including hypovolemia, impaired baroreflexes, and left ventricular atrophy leading to systolic and/or diastolic dysfunction. In order to better define these issues, NASA conducted a flight study titled Cardiac Atrophy and Diastolic Dysfunction During and After Long Duration Spaceflight: Functional Consequences for Orthostatic Intolerance, Exercise Capacity, and Risk of Cardiac Arrhythmias. This study is also referred to as the Integrated Cardiovascular Study, or ICV.
The investigator’s team were extensively involved in the development of the next generation of echo machines and had the unique opportunity to develop and validate advanced applications for space use. They focused on massively parallelized echo machines capable of real-time 3D imaging with automated volume measurements and comprehensive 3D strain and torsion analysis. This study had the following specific aims:
- Validate strain and torsion data from HDI-5000 data.
- Quantitative analysis of echo data.
- Integration of the pre-, in-, and post-flight data into evolving numerical models of the cardiovascular system.
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APPROACH:
As part of this investigation, detailed imaging studies were conducted on astronauts before, during, and after space flight, including an extensive series of in-flight resting and exercise echocardiograms. The PI monitored all in-flight echoes remotely in real-time, and he and his colleagues at the Cleveland Clinic served as the echocardiographic core lab for this study. Investigators were in a unique position to guide on-flight acquisition as well as perform detailed examination of the ultrasound studies received. However, ICV was initially proposed in 1999 with echocardiographic techniques that are now over ten years old, focusing mainly on ventricular size, mass, and simple measures of systolic function, such as ejection fraction and stroke volume. Echocardiography has advanced considerably since then in the sophisticated ventricular mechanical data that can be extracted from ultrasound data. In the current study, investigators aimed to validate extraction of these novel echocardiographic indices of ventricular mechanics (two-dimensional strain and torsion, among others) from the in-flight data acquired on the 10-year-old HDI-5000 ultrasound system aboard the ISS, which was never designed to provide such data. Once validated, investigators were able to derive detailed regional ventricular mechanics from all of the in-flight studies, allowing direct comparison with the pre- and post-flight examinations to gain a much better understanding of the magnitude and time course of structural and functional changes in the cardiovascular system in microgravity.
RESULTS:
Task 1: Analysis of DICOM images yields lower absolute global (GLS) and regional (RLS) longitudinal strain values compared to source polar images. GLS from DICOM images has comparable reliability and reproducibility as from the source Polar images. Both methods of strain analysis are independently reliable; however, strain values from Polar and DICOM analyses are not interchangeable.
Task 2: An interim analysis from the effect of microgravity on myocardial strain was presented at ASE 2012 using pre-, in-, and post-flight data from 6 astronauts with a median space stay of 166 days. Pre- and post-flight images by experienced sonographers on earth and images in space acquired by the astronauts who had echo training before space flight (with real-time guidance of ground-based investigators). Strain data was assessed using a generalized mixed model in three different conditions (pre-, in- and post-flight). In this analysis, there appears to be reduction in absolute GLS during flight, with normalization post-flight.
Task 3: Integrated cardiac function model: A semi-automated system allows ultrasound images to be segmented and fitted to a 3D finite-element mesh of the left ventricle. The segment boundaries are tracked using speckle tracking and the motion is encoded onto a geometrically accurate left ventricular mesh. Strain results are based on the mesh deformation and mesh and analysis data are integrated into the standard DICOM format and consequently into the PACS workflow.
Task 4: To begin assessment of 3D strain acquisitions, investigators have implemented a protocol using the GE Vivid E9 to acquire both 2D and 3D strain in a variety of clinical patients. Analysis suggests slight underestimation of longitudinal strain by 3D, likely related to lower frame rate.
Hussan JR, Hunter PJ, Gladding PA, Greenberg N, Christie R, Wu A, Sorby H, and Thomas JD. ICMA: an integrated cardiac modeling and analysis platform.
Bioinformatics. 2015. April 15; 31(8):1331-3.
Iskovitz I, Kassemi M, and Thomas JD. Impact of weightlessness on cardiac shape and left ventricular stress/strain distributions.
Journal of Biomechanical Engineering. 2013. December; 135(12):121008.
Crew health and performance is critical to successful human exploration beyond low Earth orbit.
The Human Research Program (HRP) investigates and mitigates the highest risks to human health
and performance, providing essential countermeasures and technologies for human space exploration.
Risks include physiological and performance effects from hazards such as radiation, altered gravity,
and hostile environments, as well as unique challenges in medical support, human factors,
and behavioral health support. The HRP utilizes an Integrated Research Plan (IRP) to identify
the approach and research activities planned to address these risks, which are assigned to specific
Elements within the program. The Human Research Roadmap is the web-based tool for communicating the IRP content.
The Human Research Roadmap is located at:
https://humanresearchroadmap.nasa.gov/
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