Hearts in space
Getting objects like satellites into orbit is relatively easy these days, but living things are a different matter. For instance, what happens to the hearts and blood vessels of the people who go there? That’s what University of Texas researcher Ben Levine has devoted his career to finding out, as he explained to Chris Smith…
Ben - It turns out there was one of the big surprises in the early space years that when astronauts came back down to earth and tried to stand up - they couldn’t, they'd faint. It’s important because many people faint on earth. In fact, 1 or 2% of all emergency room visits are because of fainting. So understanding what the effect of gravity is on fainting was really important clinically on earth, and very important for the astronauts. As we’ve extended the human presence in space on to the International Space Station, we become more interested in long duration space flight and that’s important as you think about what the future of space medicine, or really space science is. The ultimate goal is to get to Mars - that’s a long trip. So understanding what happens to the human body over long duration space flight has been our focus for the past 10 years or so.
Chris What have you learnt?
Ben - There are many things that cause fainting. The most common is what we call neurally mediated fainting. That’s fainting that happens when someone stands up and gravity sucks blood into your feet and so you have to constrict the blood vessels. Think about it like putting your thumb over the edge of a hose to squirt the blood back up to your head. And then you also have to speed up the heart rate - if you want to pump more you’ve just got to pump it faster. It turns out that there are sensors located just at the base of your brain which tell your body oh no, not enough blood getting to the brain. Let’s speed up the heart, constrict those blood vessels and deal with all that blood. Now sometimes the body’s just overwhelmed, and we see that, for example, in a soldier who’s very hot and tries to stand at attention, or someone who’s become compromised because they’ve lost a lot of blood. Even the best of reflexes can’t handle that.
Chris - But, that doesn‘t explain what’s going on in the astronaut because they ought to have a normal volume of blood, so why should they get this drop when they stand up?
Ben - Well that’s a wonderful question. And, in fact, nobody faints in space. Just like nobody faints when they’re lying down in bed except in unusual circumstances. But when astronauts go into space, all the blood that’s normally in the lower part of the body rushes up into the chest. It’s so dramatic the astronauts call that “the puffy face, bird leg syndrome.” The legs get skinny and the face gets puffy. But, just like you and I when you lay down at night, the body doesn’t like all that fluid in the upper body so it gets rid of it . That’s why the first thing you have to do when you get up in the morning is go to the toilet - you get rid of that fluid.
Chris - So you’ve shunted a lot of fluid centrally. This is detected and the body interprets that as I must have too much fluid so it throws it away. The astronauts don’t have too much fluid but the body thinks they do, they throw the excess away so they’ve actually got too little circulating volume.
Ben - That’s right. It’s enough circulating volume in space. It only becomes too little when the stand up on earth, as if they’ve lost a pint or two pints of blood.
Chris - So you put the volume back into your astronaut back on earth. They’ve got the right level of blood now, but does the symptoms resolve?
Ben - After the volume has been restored they quickly return back to normal function. The most frequent time that an astronaut might faint is when they first come down and by two or three days afterwards most of that’s back to normal. For a longer duration flight, it may be more severe and that’s because the heart actually gets smaller. We call it the “couch potato’s heart.” If you don’t do anything to prevent this from happening, the heart will shrink it’s muscle mass by about 1% per week.
Chris - We often see pictures sent down from space of astronauts exercising religiously. If you put them on an exercise regime, can you prevent those losses?
Ben - Fortunately you can, and the astronauts do a pretty good job. Most of them may exercise up to 2 hours a day in space, primarily to protect their bones and skeletal muscle, but exercise works for all the body and it protects the heart as well. There’s much less fainting now than there was in the early days of the space programme.
Chris - What about things like heart disease? We know that when we expose people to doses of radiation for things like cancer treatment it does have a bad effect on blood vessels. There is more radiation exposure from cosmic rays in space - does that accelerate the risk of heart disease?
Ben - That’s one of our biggest concerns. Once you’re outside the protective effect of the Earth’s atmosphere, the radiation exposure is quite dramatic, particularly with solar flares and solar events. It can be extraordinary and we worry about accelerating atherosclerosis. Right now, the best that we can do is to identify those individuals at highest risk and try to avoid flying them, so we’ve introduced coronary calcium scanning. Coronary calcium is a way of measuring the footprint of atherosclerosis in the blood vessels. You take a CAT scan and you measure the amount of calcium which signifies that the atherosclrotic process has progressed. Most of the astronauts that have it right now have a coronary calcium score of zero. There are some legacy astronauts though who have been in the programme before the selection who have a fair amount of atherosclerosis. So figuring out ways to protect them from radiation as they go outside the Van Allen belts will be critical because they’re middle aged men and women. And what is it that kills middle aged men and women? It’s cardiovascular disease.
Chris - A lot of the prediction algorithms we have - when you go to your GP and they measure your blood pressure, they take your cholesterol level - they’re good for older people, but they always say they’re notoriously noisy and bad for younger people. So how good is your NASA algorithm for looking at my coronary arteries and giving me a risk at my age (early 40s) of having a heart attack?
Ben - We were unhappy with the quality of the algorithms and that they weren't very good at identifying risk in younger people. So we’ve combined data from three major databases, some of which include a focus more on younger people to look and to see how much better can we do if we include coronary calcium? And we do a lot better.
Chris - Really? So could this be extrapolated into wider clinical care then so we can start using a measure made for astronauts down here on Earth?
Ben - I certainly hope so. We need to get it accepted by the wider clinical audience and published in the medical literature, and then I hope we’ll have an algorithm app that people can use in their GP’s office.