(CNN) Doctors can't make house calls to the International Space Station, but even if they could, Earth's routine practices like setting up an IV, taking a blood sample or even keeping a wound clean are incredibly difficult.

Consider heart attacks. Even the healthiest astronauts could suffer cardiac arrest due to the harmful nature of space itself. But CPR as we know it on Earth isn't possible on the space station because it's missing one key ingredient: gravity.

Trying to perform chest compressions when you're floating around only results in two people pushing away from each other. So what's the solution?

Five methods have been tested to find the best replacement for CPR in space, according to Dr. Jochen Hinkelbein, executive senior physician at the University Hospital of Cologne in Germany. Hinkelbein recently presented his paper on the topic at the Euroanaesthesia conference in Geneva.

Simulating zero gravity during short flights on reduced-gravity aircraft and in underwater environments, each method has been tested to see whether it could achieve the right compression rate and depth to revive someone.

The methods include the standard side straddle, the waist-straddling maneuver, the reverse bear hug method, the handstand and the Evetts-Russomano method. The first two involve the use of a restraint system, and the reverse bear hug is exactly what it sounds like, with compressions.

The handstand is also what it sounds like: achieving compressions by placing one's feet on the wall of the cabin and the patient's back against the opposite wall.

The Evetts-Russomano method involves the person performing CPR placing their left leg over the right shoulder of the patient and their right leg around the patient's back under the left arm. Crossing the legs this way makes forceful compressions possible.

The best and most effective method: the handstand. It also proved to be the most sustainable for a period of time. The Evetts-Russomano method came in second but isn't sustainable for more than three minutes. (However, it was the best option in the event that there isn't enough room for a handstand.)

"Medical emergencies in an astronaut during a long-term space mission represent a potential life-threatening situation for both the victim as well as the complete crew and endanger the complete mission," Hinkelbein said. "There is absolutely no help from outside possible. The most life-threatening problem is cardiac arrest needing for cardiopulmonary resuscitation. Performing the CPR technique properly may save the life of the astronaut in space as well as the mission."

If the simple act of CPR as it is performed on Earth is impossible in space, what else may be medically impossible?

Emergencies in space

After more than 50 years of human spaceflight, researchers know some of the risks posed to the human body by being in zero gravity. Space motion sickness happens in the first 48 hours, creating a loss of appetite, dizziness and vomiting.

Over time, astronauts staying for six months on the station can experience the weakening and loss of bone and atrophying muscles. Astronauts also experience blood volume loss, weakened immune systems and cardiovascular deconditioning since floating takes little effort and the heart doesn't have to work as hard to pump blood. Scott Kelly and other astronauts in their late 40s and 50s have also complained about their vision being slightly altered. Some of them have required glasses in flight.

"You can lose about a percent of bone mass every month, and that's the typical situation astronauts are in," astronaut Mark Kelly said. "Without constant pounding on the ground, you lose bone mass. If we want to send people to Mars someday, this is something we're going to have to learn to overcome. If the human body were to stay in space for 10 to 20 years, evolutionarily, over a long period of time, we would probably lose our skeleton in space because you don't need it. We'd probably just become big bags of meat."

Complex medical procedures haven't taken place on shuttle missions or the space station yet. Most astronaut fatalities have occurred during training or the launch and landing of spacecraft.

But there are situations that can help determine protocol for space missions: medical plans for people living in polar bases, stationed in submarines or scaling the world's tallest and most dangerous peaks.

From Antarctica to Mars

Dr. Scott Parazynski has flown five shuttle missions for NASA, summited Mount Everest and overseen health care for the National Science Foundation's Antarctic Program. Providing medical support in Antarctica was among the more challenging feats, he said. For more than eight months of the year, medical evacuations are almost impossible.

Residents used antibiotics to treat things like appendicitis and pressed for a medical evacuation only if pharmaceuticals didn't work. This is when telemedicine, the remote diagnosis and treatment of patients through communications technology, came in handy.

"We did a number of things at South Pole Station when we didn't have a subspecialist," Parazynski said. "We could get a consultation with a clinical expert and perform an echocardiogram, an ultrasound of the heart. Looking over the doctor's shoulder at the station's video feed, they could guide them into how to steer the probe to get the view we needed to make sure the heart was working the way we expected."

Parazynski is a strong supporter of telemedicine and telementoring, especially when it comes to teaching how to deliver anesthesia and provide other types of specialized diagnotistic and therapeutic procedures.

"Those same kinds of capabilities will one day benefit us when we return to the moon and hopefully on to Mars," Parazynski said.

But on Mars, delays in communication of up to 20 minutes each way would make telemedicine challenging and even impossible. When Mark Kelly was on Earth and his twin, Scott, spent a year on the space station, they were able to overcome the slight delay to talk every day.

"You get two weeks out from Earth, going 30,000 mph, that delay will be so noticeable," Mark Kelly said. "Twenty-five percent of the way there, there is no way to have a phone conversation."

Parazynski said that the use of optical transmissions, or messages sent as light pulses, might speed things up.

The future of medicine in space

Looking at how emergency medical procedures are carried out in these extreme environments can also help decide what medical equipment would be the most useful, the qualifications for any chief medical officers on board and even the types of conditions they may encounter, said Dr. Matthieu Komorowski, consultant in intensive care and anesthesia at Charing Cross Hospital in London.

Komorowski also presented on emergency medicine in space at the Euroanaesthesia conference this month.

Even though the healthiest candidates are chosen as astronauts, a long-term deep-space mission would pose risks to anyone. Radiation will be one of the biggest obstacles to overcome, possibly through shielding technology, but other threats may also arise.

"Traumatic conditions with blood loss are a big concern, but if someone needs a transfusion, there won't be any blood bank on Mars," Komorowski said. "The expected lack of blood products could be mitigated using whole fresh blood transfusion, which is commonly used in military operations. In this case, blood compatibility could become a selection criteria for a mission to Mars."

For a Mars mission, basic medical skills training would need to be extended to the whole crew, he believes.

"Imagine what would happen if the crew physician himself suffers from an injury or severe illness and has to be treated by a someone with no medical knowledge," Komorowski said.

A "super surgeon" or doctor who can perform multiple specialized procedures and surgeries would be helpful but is virtually impossible, especially given all of the other responsibilities they would need to perform as an astronaut, Parazynski said.

Komorowski proposed that artificial intelligence tools, such as decision support systems, could help the crew diagnose and treat medical conditions. Parazynski added that the ability to teleoperate surgical robots and do interventional procedures would also contribute to this idea of support from beyond the spacecraft.

But zero gravity will have to be factored in.

"Surgery in space would be very difficult," Parazynski said. "Blood wouldn't pool in the surgical wound, and you would have to manage blood loss and contamination of the wound. The air in a spacecraft is full of hair follicles and dead skin floating around. Keeping a wound clean is a real challenge up there."

Even simple procedures like drawing blood or infusing medication in an IV drip are more difficult in space. As a chief medical officer on shuttle missions, Parazynski performed many blood draws. He would have to adhere all of the supplies -- including bandages, gauze, wipes and other hardware -- to the wall with duct tape. A blood pressure cuff would have to be wrapped around the IV bag to force it to move and form a drip.

Unlike in an ambulance or an operating room, supplies for medical procedures and emergencies are stored in multiple areas and lockers. Easy access will have to factor into the design in the future, Parazynski said.

The stability of medicine is also impaired in space, so it would have to be specially packaged to help it last longer. The astronauts may also have to create intravenous fluid because trying to store it would take up too much room. And a 3-D printer, like the prototype currently on the space station, could be used for on-demand production of medical equipment, such as surgical tools or even surgical implants, Komorowski said.

After landing on Mars, things would only become even more difficult.

"In low-Earth orbit on station, if you have a major medical emergency, it's fortunate that you can de-orbit quickly," Parazynski said. "On Mars, if you're unlucky and develop disease that doesn't have a solution out there, you've been a pioneer."

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Although a mission to Mars isn't possible right now, it's not impossible in the future, and that's why researchers and scientists are analyzing every aspect to remove obstacles and reduce risks along the way.

"Humans are explorers," Mark Kelly said. "We don't have a choice. It's not an option to send robots to Mars. Ultimately, we have to go and put humans on the surface of these planets. It might take us a really long time because of the challenges to do these difficult things. It's in our DNA to explore, and we're not going to stop."