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

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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?

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    $\begingroup$ Instead of of an additional system to induce hypothermia for the purpose of reducing hypoxic brain damage in EVA astronauts there should be another system for oxygen supply in case of an emergency. To cool down the hypoxic astronaut in less than 4 minutes before any damage to the brain would be close to impossible. To recover a hypothermic astronaut successfully a team of very skilled physicians as well a lot of medical equipment is needed. $\endgroup$
    – Uwe
    Nov 30, 2022 at 1:20
  • $\begingroup$ @Uwe ... I'm sure there is presently a protocol for rapid return of a hypoxic astronaut to the pressurized ISS environment along with resuscitation. This protocol would have a higher success rate if hypothermia is induced, using existing capabilities (like the LCVC), as long as implementation did not delay the protocol. The question is, if these capabilities are not part of the protocol, why not? $\endgroup$
    – Woody
    Nov 30, 2022 at 1:36
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    $\begingroup$ Do you have any evidence that doctors on the ground induce hypothermia for hypoxia? It doesn't seem to be included in the Wikipedia article, and both the others seem to be hypothermia as a result of the circumstances. If it isn't part of medical protocol for doctors on the ground, as I suspect it's not, then why would it be in space? $\endgroup$
    – Erin Anne
    Nov 30, 2022 at 7:06
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    $\begingroup$ @Woody here, have your exasperation ellipses back ... surgery that requires reducing blood flow to the brain isn't the same as a depressurization event! Are there medical entities that recommend hypothermic treatment for simple hypoxic hypoxia, which is what you're proposing? If not, that's the answer to your question. $\endgroup$
    – Erin Anne
    Nov 30, 2022 at 19:50
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    $\begingroup$ Think of it from a contingency-planning perspective. In your hypothetical oxygen-loss event, the suit could carry machinery that enables it to rapidly induce hypothermia, which might help. Or, it could carry a redundant oxygen supply, which might help. Which would be more useful in most circumstances? Which would have a better success rate? Which would be less complex (i.e., prone to failure in itself)? Which is the more mature technology? Which weighs less? $\endgroup$
    – Cadence
    Nov 30, 2022 at 22:53

2 Answers 2


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.

  1. 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.)

  1. 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...

  • $\begingroup$ exposure to vacuum will not give you terminal hypothermia. There is no conduction, convection or radiation (inside of suit is at body temperature). $\endgroup$
    – Woody
    Dec 1, 2022 at 17:44
  • $\begingroup$ the object is therapeutic hypothermia (around 30*C), not "severe hypothermia". The astronaut should NOT be removed from the suit !!! Explosive decompression results in spontaneous explosive suit diarrhea. This can cause potentially fatal atmospheric contamination for the other crew members. Who draws the short straw for decontamination? Anyway, you want to leave the LCVC on for controlled rewarming, or maintenance of therapeutic hypothermia. $\endgroup$
    – Woody
    Dec 1, 2022 at 17:50

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

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    $\begingroup$ Honestly not trying to be argumentative, just an (admittedly off the cuff) engineering consideration were you to pursue this through whatever the processes are for proposing a thing to NASA: EVA suits already have had issues with water ingress, so you'd want to anticipate the fears/objections that would come up about adding that capability. Obvs that's too far for a StackExchange answer though, so here's an upvote. $\endgroup$
    – Erin Anne
    Dec 2, 2022 at 0:43
  • $\begingroup$ @ErinAnne Thanks for upvoting a really weird idea. Yes, free liquid water during an EVA spoils your day. Fortunately, the water only needs to be released when the suit is depressurized which never happens except ,,, duh... during depressurization. Normal suit pressure could keep a safety plug safely closed. $\endgroup$
    – Woody
    Dec 2, 2022 at 1:22
  • $\begingroup$ @ErinAnne An alternate to using water is to use the 500 bar emergency tank of supercritical O2 for crash refrigeration. I don't know how to convert that to BTUs of refrigeration $\endgroup$
    – Woody
    Dec 2, 2022 at 1:26
  • $\begingroup$ @ErinAnne I sent a proposal to NASA. Thanks for the suggestion. See what they say. Maybe just laugh? $\endgroup$
    – Woody
    Dec 2, 2022 at 1:40
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    $\begingroup$ The heat transfer capability of the Liquid Cooling and Ventilation Garment (LCVG) to the bare skin is very limited. Cooling by the LCVG could not be faster than a total immersion of the astronaut into water of 0 °C, but the immersion is not fast enough anyway, that is why the heat transfer via the blood is used for medical use of hypothermia. If a body is immersed into cold water, the blood circulation through arms and legs is reduced very much to preserve heat in the torso and head. So arms and legs transfer much less heat to the water. So free liquid water in the suit does not help. $\endgroup$
    – Uwe
    Dec 2, 2022 at 11:34

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