Apollo Lunar Astronauts Dying of Heart Disease Pose CV Safety Questions for Deep Space

Astronauts, by occupation, are healthier than the general population and have continued access to excellent medical care, but new research suggests they might be particularly vulnerable to dying from cardiovascular disease.

Those participating in Apollo lunar space missions—the only astronauts who traveled outside the Earth’s protective magnetic shield—were five times more likely to die from cardiovascular causes when compared with astronauts who never flew in space and four times more likely to die of cardiovascular disease than astronauts who never flew beyond the Earth’s lower orbit.

“These data suggest that human travel into deep space may well be more hazardous to cardiovascular health than previously estimated,” conclude Michael Delp, PhD (Florida State University, Tallahassee, FL), and his NASA-affiliated colleagues in a paper published July 28, 2016, in Nature’s Scientific Reports.

The early reasoning for the increased cardiovascular mortality risk among Apollo astronauts is exposure to deep space radiation. In their paper, the researchers explain that when traveling beyond the earth’s lower orbit to the moon, the Apollo lunar astronauts navigated their way across “regions of geomagnetically trapped electrons and protons known as the Van Allen belts,” and depending on the duration of their mission and specific activities, “were continuously subjected to varying levels of high-energy cosmic rays.” There are epidemiological data linking ionizing radiation exposure to cardiovascular disease risk, they add.

Of the astronauts who didn’t fly on any orbital missions and astronauts who flew only in the earth’s lower orbit, where the Earth’s magnetosphere protected them from ionizing radiation, 9% and 11% died from cardiovascular causes, respectively. In contrast, 43% of Apollo lunar astronauts died from cardiovascular disease.

The study is not without limitations, not the least of which is the small sample size. The Apollo program ran from 1961 to 1972 and to date just 24 astronauts have flown into deep space. Of those, eight Apollo astronauts have died. Three of them—Neil Armstrong (Apollo 11), James Irwin (Apollo 15), and Ronald Evans (Apollo 17)—died from cardiovascular causes.

For Benjamin Levine, MD (University of Texas Southwestern Medical Center, Dallas), the team leader of the cardiovascular section at the National Space Biomedical Research Institute (NSBRI), the data are simply not strong enough to definitively state that deep space travel leads to an increase in cardiovascular mortality among astronauts. Attributing the small number of deaths to radiation exposure decades prior—without controlling for any other variables—is not good science, he said. The association, he added, is “simply untenable.”

“The Apollo astronauts were a decade older than the others,” said Levine, “so even the comparison group is not reasonable. There were only seven deaths, with three astronauts dying from cardiovascular causes. Maybe those were the three who were big-time smokers? Maybe they had high cholesterol before the use of statins?”  

To TCTMD, Levine said the relationship between radiation exposure and accelerated coronary disease has been documented for many decades, going back to the launch of the atomic bomb. At NASA, clinicians and others involved in the selection process for choosing astronauts spend a lot of effort to not fly individuals with premature cardiovascular disease, so much so that those with coronary artery calcium score of 10 are unable to enter the NASA Astronaut Corps (the unit of NASA that selects and trains astronauts), said Levine.

Animal Study Is Interesting, an Advance

To address the potential role of exposure to ionizing radiation and weightlessness on cardiovascular disease, Delp and colleagues also conducted a follow-up study in a mouse model. In total, 44 male mice were divided into four groups: controls, hindlimb unloading alone (to stimulate weightlessness), total body irradiation alone, and hindlimb unloading and total body irradiation.

In the mice exposed to weightlessness and radiation, the endothelial cells producing nitric oxide became disabled, leading to vessel narrowing and atherosclerosis. Seven months later, the mice exposed to the space-relevant radiation had sustained vascular endothelial cell dysfunction, while those exposed to weightlessness did not, report the researchers.

Despite being critical of the epidemiologic association between radiation exposure and cardiovascular mortality in the lunar astronauts, Levine praised the strength of the basic animal science, something he hopes isn’t obscured by the flashy but weaker mortality findings. The basic animal science supports previous research, including work done by Dan Berkowitz, MD (Johns Hopkins Medicine, Baltimore, MD), showing that oxidative stress contributes to vascular endothelial dysfunction, among other findings.

“You have to ask yourself if there is anything about the space-flight environment that might change the equation, that might accelerate cardiovascular disease,” said Levine. “I have been very worried and led many studies to get the best people to work on what happens to the blood vessels in space. I think it’s a very important problem, and I’m pleased Dr. Delp has published the animal work, but it is still a long way from impaired endothelial function to a heart attack.”

Despite the exposure to radiation, the Apollo lunar astronauts did not have an increased rate of cancer-related mortality compared with those who stayed on earth and those who only stayed within the earth’s lower orbit.

Source: 
Delp MD, Charvat JM, Limoli CL, et al. Apollo lunar astronauts show higher cardiovascular disease mortality: possible deep space radiation effects on the vascular endothelium. Sci Rep. 2016;6:29901.