Up in Space

"Up in Space: Medicine off the Earth” was the topic of a lecture by Jonathan B. Clark at Kidney Week 2011. Clark is assistant professor of neurology and space medicine at Baylor College of Medicine and Center for Space Medicine. He is also clinical assistant professor at the University of Texas Medical Branch. He worked at NASA as a space shuttle crew surgeon from 1997 to 2005.

Here ASN Past President Joseph V. Bonventre interviews Dr. Clark about the health effects of space and the space program’s potential long-term benefits for humans.

For the full interview, see the Kidney News online app.

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Dr. Bonventre: In your talk you mentioned issues with bone loss and kidney stones. Could you talk a little about bone loss, the release of calcium, and the formation of stones?

Dr. Clark: Astronauts undergo rigorous physical screening. They do not have a history of kidney stones when they come into the astronaut program. In the astronaut corps of approximately 300 astronauts, 12 have had kidney stones, and two have had repeated kidney stones. Many of those stones developed in the postflight period. Bodies adapt to the absence of gravity by releasing calcium, specifically from the weight-bearing bones. That release of calcium causes hypercalciuria, one of the major risk factors for kidney stones. In the 1980s, one of the Russian cosmonauts developed a kidney stone in space.

At that time, the United States was focusing on shorter missions, but the Russians were supporting longer missions and were at greater risk for kidney stone formation. We know that there is a reduction in bone density in space crews at a rate about 10 times greater than the calcium mobilization experienced on earth. We see an increase in calcium in the urine and a decrease in bone density, similar to what is seen in postmenopausal osteoporosis but at a 10 times greater rate. Whereas a postmenopausal woman might lose bone density at a rate of about 1–2 percent per year, those in space lose bone density at a rate of about 1–2 percent per month. Bone loss in patients with spinal cord conditions seems to plateau after about 2.5 years. So far, no one has flown in space for longer than 14 months, and in those missions we have not seen the plateau in bone density loss.

Dr. Bonventre: How much of the bone loss is reversible?

Dr. Clark: Well, bone density goes down as we age. This isn’t my area of expertise, but for accelerated bone loss in space, the recovery period is much longer than the loss period. Astronauts will recover bone density, but it may take three times longer to recover the bone density as it took to lose it. We have seen some instances of male crew members who have experienced hip and femoral fractures not due to trauma.

Dr. Bonventre: One of the goals of the space program is to go to Mars. A Mars trip would take about 6 months travel time each way and a year on Mars; is that right?

Dr. Clark: A Mars mission might range from 13 months to 30 months: a 6-month transit period each way and a stay of either 1 month or 1 year. The challenges include minimizing travel time in microgravity and addressing the exposure to radiation in deep space, which is very problematic. Interestingly, some recent studies indicate that radiation also contributes to bone loss, perhaps by an effect on the bone-forming cells in our long bones.

Dr. Bonventre: So bone loss is one problem with that kind of mission. Another would be a medical emergency in deep space. How is NASA preparing for that?

Dr. Clark: Dealing with a medical emergency in space is challenging, even in low earth orbit. We just returned to a six-person crew on the space station; imagine how difficult it would be to take care of a medical emergency with just a three-person crew. Right now the best plan for a Mars mission would be to have a six- or seven-person crew. NASA is evaluating training, equipment, and procedures that would work best in a deep space mission. In addition, the length of time it takes for a signal to be sent back and forth poses challenges. Going to Mars, currently you would experience a time delay of 14–40 minutes for two-way communication, with no ability to get real-time feedback on a medical problem.

Right now we’re developing some advanced diagnostic imaging, primarily with the ultrasound machine. Ultrasound has drastically improved; the original ultrasound machine on the space station was like the old-time cart in the cardiology suite—a fairly large machine. Now you can get ultrasound machines the size of midsized laptops that provide real-time feedback. New technologies have been developed that support expeditionary medicine and medicine in austere environments, such as natural disasters in remote locations. It’s interesting that so many NASA technologies support life on earth, not just the few people who travel up in space.

Dr. Bonventre: One other implication of space travel is muscle wasting, both cardiac and skeletal. What are the countermeasures, and can the loss be reversed?

Dr. Clark: The human body is an amazing system. It’s very adaptive. In microgravity the body senses that it doesn’t need muscles, bones, or a cardiac system as strong as what it requires on earth. The body adapts to the absence of gravity, but this is of course maladaptive for the return to gravity. Adjusting to the absence of gravity, the body dumps excess bone and calcium and reduces skeletal muscle mass. Cardiac muscle mass is lost because the body doesn’t need to maintain blood pressure the way it does in a gravity environment. Cardiac echo ultrasounds have shown a reduction in cardiac mass and a reduction in aerobic fitness as measured by cardiac output and heart rate in response to exercise challenge. There has been a huge effort to counteract those degrading systemic effects on the body. Most of the countermeasures involve some form of exercise: to enhance cardiac fitness and to enhance musculoskeletal strength with resistance exercise.

In space station crews, with missions lasting 6–7 months, muscle mass and aerobic capacity in space decline very rapidly: down 20–30 percent compared with normal. This slowly recovers toward the end of the flight, but it is still 10–20 percent from normal. In the first week after a flight, astronauts have the same deficit experienced in space but recover after a month. What we don’t know is what the recovery might be for someone who is already in a cardiac-compromised state. This ties into concerns regarding commercial space flight and the likelihood that we’ll see people in less than pristine medical shape. Commercial space travelers may have underlying pathologic conditions of any organ system. The majority of early commercial space flights will be suborbital and might last for 5–10 minutes. Space tourists have stayed on the space station for 10–14 days but no longer. We may see commercial flight participants flying for longer periods of time—up to 6 months or longer. The medical communities will be challenged. Making sure a healthy person doesn’t decline in capabilities is already a challenge, but what about someone not in the best physical shape, or who has underlying medical conditions?

Dr. Bonventre: You mentioned increased intracranial pressure, exemplified by eye changes in the astronauts. Is that also associated with elevations in systemic blood pressure or other complications? Are there countermeasures in place, especially for longer-term space travel?

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Dr. Clark: The problem with increased intracranial pressure has only recently been recognized by NASA. The first case was recognized in 2005, and since then we’ve had eight cases in 34 crew members. This may represent the gravest concern for human space flight. Some of these people exhibit elevations, even years later, as if their bodies had adapted to this state and never fully returned to normal. NASA has initiated a massive effort to understand more about this problem, and it encompasses all specialties, including nephrology. For instance, carbonic anhydrase inhibitors, also used to treat mountain sickness and high-altitude cerebral edema, have been used to treat intracranial pressure. Unfortunately, the side effect of that treatment is increased risk of kidney stones—a risk already associated with space flight. We don’t understand all aspects of this problem; there is speculation that the choroid plexus is involved, and some of the secretion and absorption phenomena of the brain to wash out contaminants and bring in nutrients may have some similarities to kidney function. I’m interested in connecting with nephrologists, who understand these cellular-level activities and might help us understand more about increased intracranial pressure in space.

Dr. Bonventre: You mentioned waste product recirculation. Are the engineering systems in space efficient in terms of reprocessing waste products?

Dr. Clark: The urine recycler, officially called the water recovery system, was an amazing engineering feat. The water recovery system flew on the space station at the end of 2008, and it enabled the recovery of condensate collected from humid air and from urine. That system uses a lot of advanced technologies, such as filters and molecular sieves, that have allowed the space station to go from a three-person crew to a six-person crew. It was vital to the development of a partially closed loop space habitation. The astronauts weren’t just bringing drinking water up and then dumping it overboard; they were recycling and reusing. It’s incredibly important to survival in space and to survival on earth. Many places on earth don’t have adequate water supplies, so there is a great value in the ability to process nonpotable water so that it meets health standards.