What policies and capabilities are in place to induce hypothermia for the purpose of reducing hypoxic brain damage in EVA depressurization?

Question

Depressurization incidents during EVA will have hypoxia as the ultimate cause of death or permanent brain damage. If an astronaut’s brain is deprived of oxygen, irreparable damage occurs after about 4 minutes. .. at body temperature. But under conditions of hypothermia (low body temperature), damage occurs much slower.

To illustrate the potential, Hypothermic anesthesia lowers the body temperature to 68-77 °F (20-25 °C) and allows blood flow to the brain to be stopped for up to an hour. https://en.wikipedia.org/wiki/Deep_hypothermic_circulatory_arrest

https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.sciencedirect.com%2Ftopics%2Fmedicine-and-dentistry%2Fdeep-hypothermic-circulatory-arrest&psig=AOvVaw0D4ZkKY1kdEkCSi1gvAmRQ&ust=1669911847262000&source=images&cd=vfe&ved=0CBEQjhxqFwoTCNiL3cWo1vsCFQAAAAAdAAAAABAJ

There is evidence from rescued avalanche suffocation victims that hypothermia is neuro-protective. https://www.resuscitationjournal.com/article/S0300-9572(16)30114-9/pdf#:~:text=Hypothermia%20is%20one%20factor%20in,make%20a%20comparison%20to%20drowning

There are anecdotal cases of downing victims immersed in cold water who have been resuscitated after 30 minutes. The longest time from hypothermic cardiac arrest to return of spontaneous circulation is almost 7 hours. https://www.sciencedirect.com/science/article/pii/S0300957214005243

Back to the astronauts: Most possible fatal events during EVA have cerebral hypoxia as the ultimate cause of death. During EVA depressurization, rescue and successful resuscitation could be enhanced by inducing hypothermia during the emergency retrieval and repressurization.

EVA suits have a Liquid Cooling and Ventilation Garment (LCVG) which has a 2.7 liter water reservoir. It also has a 7000psi emergency supercritical O2 tank. Either of these could be incorporated into an emergency refrigeration device, utilizing latent heat of vaporization.

The lungs have a wetted gas exchange area about the size of a tennis court. While “breathing vacuum”, water would evaporate to try maintain a partial pressure of 47mmHg. This would remove significant heat from pulmonary blood flow and potentially cause rapid core cooling.

Imagine an EVA depressurization, leading to complete loss of pressure. The astronaut would lose consciousness within 15 seconds. The only hope for survival would be re-pressurization. 4 minutes is not very long for the EVA partner to manhandle the unconscious astronaut into an airlock. If an emergency refrigeration device and evaporative pulmonary cooling could lower the body temperature to 30*C during the rescue, this could increase the time to brain death for a few more minutes. Nothing to loose.

This is an unusual proposal, but it is for a very unusual situation. In a hospital, Hypoxia is treated with oxygen and ventilation, which is not available during the EVA rescue. Crash Hypothermia induction using a custom designed total body refrigeration suit is not usually available as an emergency alternative. But this alternative could be available in an EVA suit.

Question: What policies and capabilities are in place to induce hypothermia for the purpose of reducing hypoxic brain damage in EVA astronauts? If not, why not?

Answer 1

Question: What policies and capabilities are in place to induce hypothermia for the purpose of reducing hypoxic brain damage in EVA astronauts? If not, why not?

It is very hard to demonstrate a negative, but I am going to go out on a limb and say: there are (almost certainly) no plans and it (probably) has never been seriously considered.

NTRS finds one study - from 1967! - that mentions the possibility of this treatment, but summarises it as "might be operationally impossible in the foreseeable future". It does, interestingly, support the potential for using a cooled space suit to induce it. It doesn't seem to have been particularly picked up after that.

It's easy enough to look at something and say "well that would give you a better chance in X situation, why not do it". But three things to consider with this as with anything else:

1. Would this actually work?

Let's accept for the moment that you could rapidly induce hypothermia using the suit. I am not entirely sure that being exposed to vacuum isn't going to give you very terminal hypothermia anyway, mind you.

The WP outline for DHCA suggests it is normally a gradual process not a shock one, and often involves directly cooling blood rather than just the body. So I have my doubts it would work, but let's assume it would do something.

2. What would you do next, if it does work?

Okay - so now rather than a dead astronaut (bad) or one surviving with minor hypoxia (good), you have one who is severely hypothermic and needs to be removed from the suit and progressively warmed in a controlled fashion, as well as restoring oxygen, as well as the traumatic injuries from severe vacuum exposure (complicated). Is this a capability we have available on a space station? If we can put someone in a hypothermic state but not recover them again, we've not got anywhere.

(Can we even get them back inside effectively and promptly? I have no idea if EVA contingency plans lay out methods for carrying an unconscious astronaut.)

3. Does this capability have costs, or introduce other problems?

As Cadence notes in the comments, this is where we start thinking about tradeoffs.

Any development to support this (very unusual) circumstance is time and money that could be spent on something else - like a backup oxygen supply or a more reliable suit seal. Likewise, any physical hardware build into the suit over and above what is there gives an added weight/volume/complexity penalty, which might more usefully be spent on other things.

Even if it is only a software switch to set existing cooling systems onto "induce hypothermia now", you now have introduced something that would definitely be a Bad Day if triggered accidentally...

Answer 2

Answer: No, the capability is not in place to induce therapeutic hypothermia to reduce hypoxic brain damage in a EVA depressurization, or for treating post hypoxic cerebral edema. But maybe it should be considered.“… hypothermia would apparently be of great value in the treatment of posthypoxic cerebral edema in space … adequate total body cooling might be attained with a water-cooled space suit.” https://ntrs.nasa.gov/citations/19670023576

This could potentially slow hypoxic brain damage to allow more favorable prognosis once the astronaut can be re-pressurized.

However, existing LCVC suits do not have the capacity to induce rapid hypothermia. Maybe they could, with minor modification. The Apollo A7L suit could dissipate 2000 BTU/hr. https://en.wikipedia.org/wiki/Liquid_cooling_and_ventilation_garment. This is the equivalent of 500,000 calories, enough to reduce 70kg of astronaut by 7 °C in an hour. Not fast enough.

However, if the 2.7 kg of water in the suit’s LCVC reservoir https://www.hq.nasa.gov/alsj/ALSJ-FlightPLSS.pdf were dumped into the depressurized suit, flash evaporation would provide 1,450,000 calories of evaporative cooling. This is more than enough refrigeration to induce hypothermia of 30 °C, and thereby double the astronaut’s hypoxic survival time. Since the “dump” is both triggered by, and powered by, the vacuum in the depressurized suit, a fail-safe mechanism should be practical.

Basic design research would need to be done on human-size mammals, such as pigs.

Exposure of the astronaut to vacuum would not add to the cooling.

“Explosive depressurization” can produce ebullism (called “blood boiling” in SF circles). This is caused by inert gasses like nitrogen bubbling out of solution. The bubbles block circulation and prevent the effectiveness of pulmonary evaporative cooling. However, EVA astronauts pre-breath O2 to flush nitrogen. This markedly reduces the risk of ebullism. Chimpanzees similarly prepared by pre-breathing O2 “have tolerated exposures to ambient atmospheric pressures of less than 2 mm Hg for up to 210 seconds with a return of apparently normal psychophysiologic function after recompression.” https://ntrs.nasa.gov/citations/19670023576

An EVA astronaut exposed to “explosive depressurization” after pre-breathing O2 would likely survive several minutes of “breathing outer space” a la David Bowman. If emergency therapeutic hypothermia of 30 °C was induced by a combination of water dump into the suit and pulmonary evaporative cooling, they would likely survive 8 minutes before re-pressurization rather than 4 minutes without hypothermia.

Once inside the airlock, this induced hypothermia could be treated in a controlled, pre-programmed way with heated oxygen. https://ccforum.biomedcentral.com/articles/10.1186/s13054-020-2839-1 The astronaut would receive warmed 100% O2 at normal atmospheric pressure, using airway and Ambu Bag if necessary. Slow re-warming could be provided using the LCVC. Or the LCVC could be used to maintain hypothermia to treat post-hypoxia cerebral edema. “… hypothermia would apparently be of great value in the treatment of posthypoxic cerebral edema in space … adequate total body cooling might be attained with a water-cooled space suit.” https://ntrs.nasa.gov/citations/19670023576