Cardiac arrest in microgravity: how to resuscitate in space

A 52-year-old astronaut is in the fourth month of a mission on a space station in low orbit. During the treadmill exercise session, he says he feels palpitations, lets go of the handles and stays still, floating. A colleague arrives within seconds. He does not respond, is not breathing normally and has no palpable pulse in the neck. The exercise equipment monitor shows a chaotic tracing. There are two more crew members on board, one of them with medical training. The scene is fictional and was written for teaching.

This text uses the scenario as a setting for clinical reasoning. The trap is imagining that resuscitation in space is the same as on Earth. Without gravity, whoever compresses the chest is pushed away, the patient floats and every drop of liquid behaves differently. At the same time, the causes of arrest remain the same, and reasoning about reversible causes is worth as much as on Earth. The text is not an agency protocol. If you play the case of the day on NEXO, you will recognize the challenge: deciding quickly in a setting where the usual rules do not work the same way.

Key points

• There has never been a cardiac arrest in space outside of accidents, but longer missions make the scenario plausible.
• In microgravity, compressions require techniques that anchor the rescuer to the patient or to a surface.
• No manual technique guarantees the depth recommended on Earth.
• Early defibrillation remains central, with the patient restrained and electrically isolated.
• Reversible causes remain the same as on Earth, with different weight for electrolytes and thrombosis.
• Telemedicine helps little in the first minutes, and the decision to stop rests with the most qualified crew member.

Why this is a reasoning scenario

Cardiac arrest is the emergency where time weighs heaviest. On Earth, the sequence is trained until it becomes a reflex: recognize, call for help, compress, defibrillate. In space, each step of that reflex runs into a physical detail. The rescuer needs to think about what changes and what stays the same, and do it without losing seconds.

A European aerospace medicine guideline acknowledges that cardiac arrest in spaceflight is very unlikely today and has never happened, which means there is no real experience (Hinkelbein et al., 2020). The authors note that space tourism and missions to the Moon and Mars should change this picture. That is why the scenario serves to train reasoning, and not to describe something already seen.

What the scene teaches

The scene trains three habits. The first is separating what is principle from what is technique: the principle of compressing fast and hard does not change, but the technique does. The second is keeping the search for reversible causes even in a strange environment. The third is deciding with those present, because outside support arrives late. The post on clinical decision-making discusses how to choose approaches with limited information.

What changes in the body in microgravity

Without gravity, about 2 liters of fluid shift from the legs to the chest and head, and plasma volume falls in the first days (Alzaabi et al., 2025). The heart then works with less load and tends to lose mass over the course of the mission. The same review describes arrhythmias in astronauts, linked to changes in electrolytes and the autonomic nervous system.

Another review on cardiovascular risks in deep space missions lists concerns about arrhythmias and the QT interval, as well as venous stasis, with rare reports of thrombosis (Brojakowska et al., 2026). These findings do not say that arrest is likely. They say which causes should move up the list when it happens.

Why this matters for reasoning

On Earth, the most common cause of sudden cardiac arrest in a middle-aged man is coronary disease. In space, rigorous astronaut screening reduces that risk, and other causes gain relative weight: an electrolyte disorder, an arrhythmia favored by the environment, a clot that traveled to the lung. The post about space medicine, SANS and bone loss explains other adaptations of the body to microgravity.

Pathophysiology: compressing without weight

On Earth, the rescuer uses their own body weight to compress the chest. The arms stay straight, and the torso comes down onto the patient. In space, there is no weight. Each compression pushes the rescuer backward and the patient to the opposite side, if neither of them is secured.

Physics forces a search for another source of force. Either the rescuer holds on to the patient with their legs, or they brace against a fixed surface and use muscle strength. Both solutions have limits, and that is why compression depth is the big problem of resuscitation in space.

What happens to fluids

Without gravity, air bubbles do not rise inside an IV bag. They stay mixed with the fluid and can enter the vein. The European guideline recommends gas-free infusion bags, placed inside a pressurized bag (Hinkelbein et al., 2020). The advantage of the pressurized bag is simplicity, because it does not depend on a motor.

How the condition presents

Cardiac arrest presents the same way in space and on Earth: the person does not respond, does not breathe normally and has no pulse. The difference is the context. The astronaut may be floating, attached to equipment or in a cramped module. The first step is to recognize it and call the other crew members, without wasting time checking the pulse for too long.

Clues that point to the cause

In the scenario, the palpitations before the collapse and the effort on the treadmill point to arrhythmia. The chaotic tracing suggests ventricular fibrillation. Other clues would change the reasoning: shortness of breath and chest pain before the collapse would suggest pulmonary embolism; vomiting or diarrhea in the previous days, an electrolyte disorder; a cabin atmosphere alarm, hypoxia.

What to ask colleagues

While one crew member compresses, another can answer quick questions. Did the astronaut complain of anything in the previous days? Did they take any new medication? Did they vomit? Was there any environmental alarm? The post about signs, symptoms and history shows how to take the history from those who were nearby.

Differential diagnosis: the reversible causes

The systematic review on resuscitation in hypogravity recalls that the classic reversible causes, the so-called 4 Hs and 4 Ts, can also occur in space (Overbeek et al., 2022). The table summarizes the main hypotheses of the scenario.

Hypothesis	Clues in favor in the scenario	What changes in space
Primary arrhythmia	Palpitations; effort; chaotic tracing	Electrolyte and autonomic nervous system changes may favor it
Hypokalemia or hyperkalemia	Recent vomiting or diarrhea; medications	Fluid and electrolyte balance altered during adaptation
Hypoxia	Environmental alarm; more than one crew member with symptoms	Failure of the cabin life support system
Pulmonary embolism	Shortness of breath and chest pain before the collapse	Venous stasis, with rare reports of thrombosis
Tension pneumothorax	Trauma; known lung disease	Diagnosis by examination and ultrasound, if available
Cardiac tamponade	Trauma; distended neck veins	Neck veins already look full due to fluid shift
Poisoning	Exposure to gas or a chemical product	A closed environment concentrates contaminants

The post about differential diagnosis with clues shows how to organize the list when time is short.

Arrhythmia and electrolytes

The review on the cardiovascular system in microgravity describes ventricular extrasystoles, atrial fibrillation and episodes of ventricular tachycardia in astronauts, associated with electrolyte and autonomic nervous system changes (Alzaabi et al., 2025). In the scenario, arrhythmia is the most likely hypothesis. But the reasoning does not stop there, because a potassium disorder may be the cause of the arrhythmia, and only treating the cause prevents recurrence.

Thrombosis

Venous stasis in microgravity is a concern, with rare reports of thrombosis in astronauts (Brojakowska et al., 2026). A massive pulmonary embolism enters the differential of any cardiac arrest, and even more so in an environment that favors stasis. The post about thrombosis after a long trip explains the reasoning about this risk on Earth.

Management: basic support

The European guideline divides resuscitation in space into a chain with basic support and advanced support, as on Earth (Hinkelbein et al., 2020). Basic support starts wherever the astronaut is, without waiting to move them elsewhere.

The Evetts-Russomano technique

The first recommended technique is the Evetts-Russomano one. The rescuer passes the left leg over the patient's right shoulder and the right leg around the trunk, crossing the ankles behind the patient's back. This way the two are locked to each other, and the rescuer compresses with the arms (Hinkelbein et al., 2020). The advantage is starting right away, anywhere.

The problem is depth. In a study in simulated microgravity, compressions with this technique were too shallow for both the 2005 and the 2010 guidelines, with means of 28.5 and 32.9 millimeters (Russomano et al., 2013). The authors suggest specific training and arm strengthening.

The handstand

When the patient is already secured on the resuscitation board, the guideline recommends the handstand technique. The rescuer braces the feet against the opposite wall and pushes the chest with extended arms, like an inverted handstand. This technique had the best depth among the manual ones, a mean of 44.9 millimeters, but still below the Earth target (Hinkelbein et al., 2020). It depends on the distance between the patient and the wall and on the rescuer's height. If that is not possible, the guideline suggests other positions with the patient restrained.

Rotating earlier

The systematic review on hypogravity showed that the compression rate holds, but adequate depth depends on heavier rescuers. The authors observed that rescuers bend their elbows to compensate and recommend changing rescuers more often than on Earth, in less than a minute (Overbeek et al., 2022). A study on Earth showed that elbow flexion helps rescuers with low body mass index reach the recommended depth (Thurlow et al., 2026).

Management: advanced support

Defibrillation

In the scenario, the tracing suggests a shockable rhythm, and defibrillation is the priority. The guideline recommends self-adhesive pads, which do not need contact pressure and leave the hands free (Hinkelbein et al., 2020). The shock should only be given with the patient restrained and electrically isolated, so as not to hit those around or the equipment.

Access and medication

The guideline recommends trying venous access and, if it fails, intraosseous access, which reaches similar drug concentrations in a similar time (Hinkelbein et al., 2020). The 2020 international consensus suggests giving adrenaline as soon as possible in non-shockable rhythms, and after unsuccessful defibrillation attempts in shockable rhythms (Nolan et al., 2020). The same consensus says there is little data on dose and interval, and that the two out-of-hospital cardiac arrest trials used the standard dose of 1 mg intravenously or intraosseously every 3 to 5 minutes (Nolan et al., 2020). No source read tested this dose in microgravity.

Airway

Without a crew member trained in intubation, the guideline suggests a supraglottic device, which is simpler to position (Hinkelbein et al., 2020). In microgravity, secretions and vomit do not run downward, and suction gains importance.

Mechanical compression

Automatic compression devices could solve the depth problem. A review of techniques in microgravity concluded that none of the manual techniques tested guarantees quality resuscitation and that mechanical devices will probably be needed (Vanni et al., 2025). A group built a high-fidelity simulator to study the blood pressure response during mechanical compression in parabolic flight (Lord et al., 2026). A scoping review notes that alternative techniques in extreme environments still have limited evidence (Overbeek et al., 2024).

Telemedicine and the decision to stop

In low orbit, contact with mission control is almost immediate. On Mars, a message round trip would take 6 to 46 minutes, and so telemedicine is unlikely to help in the initial phase of resuscitation (Hinkelbein et al., 2020). The crew must still inform mission control.

The guideline recommends that the decision to end resuscitation be made by the crew member with the highest medical qualification, after consulting telemedicine when possible. If telemedicine is not available or the delay is too long, that crew member makes the decision (Hinkelbein et al., 2020). The post about telemedicine and clinical reasoning discusses the limits of deciding at a distance.

The weight of the decision

Ending a resuscitation is hard anywhere. In space, the team is small, resources are finite and the patient is a colleague. Training before the mission also serves to prepare for this decision. The post about simulation of care shows how to train rare situations.

Step-by-step reasoning in the case

First, recognition: the astronaut does not respond, is not breathing normally and has no pulse. It is cardiac arrest.

Second, the start: the colleague begins compressions on the spot with the Evetts-Russomano technique and calls the other crew members.

Third, defibrillation: the defibrillator arrives, the self-adhesive pads are applied and, with the patient restrained and isolated, the shock is delivered.

Fourth, position: the patient is secured to the resuscitation stretcher, and compressions switch to the handstand technique, with frequent rotation.

Fifth, access and medication: venous or intraosseous access, and adrenaline after unsuccessful shocks.

Sixth, the causes: while resuscitation continues, the team considers electrolytes, hypoxia, thrombosis and the other reversible causes.

Seventh, communication: mission control is informed, and the decision to continue or stop belongs to the most qualified crew member.

Common mistakes

The first mistake is trying to compress without securing the rescuer to the patient. Without a support point, the force is lost.

The second mistake is waiting to get the patient to the stretcher before starting. The Evetts-Russomano technique allows starting right away.

The third mistake is assuming the depth is good. Manual techniques fall below the target, and rotation helps.

The fourth mistake is delivering a shock without isolating the patient. The guideline calls for a restrained and isolated patient.

The fifth mistake is forgetting the reversible causes. The arrhythmia may have a cause that needs treatment.

The sixth mistake is waiting for telemedicine to decide. The first minutes depend on whoever is on board.

The seventh mistake is using a regular saline bag. The bubbles do not rise without gravity. The post about aeronautical cardiology shows how cardiac risk is assessed before the flight.

How NEXO helps train this reasoning

To use the case of the day as training, try three habits. Before the first guess, ask which reversible cause you would treat first. With each clue, ask what the environment changes in the probability. After the result, note which cause you forgot.

Three exercises with cases

List the reversible causes. For each case of collapse, write the 4 Hs and the 4 Ts and mark the ones the clues support.

Change the environment. Picture the same case on a plane, a ship or a space station and see what changes in the approach.

Separate principle from technique. Write down what does not change in any environment and what needs to be adapted.

The post about from basic sciences to the clinic shows how to link mechanism and symptom, and the post about case-based learning shows how cases help cement that habit.

The limits on using the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information under curation and creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

Is there a guideline for resuscitation in space?

Yes. European aerospace medicine societies have published a guideline for resuscitation in microgravity (Hinkelbein et al., 2020).

Can you start compressions without a stretcher in space?

Yes. The Evetts-Russomano technique lets you start on the spot, with the rescuer tethered to the patient (Hinkelbein et al., 2020).

Do the reversible causes from Earth apply in space?

Yes. The 4 Hs and the 4 Ts can also occur in space (Overbeek et al., 2022).

Can the defibrillator be used in microgravity?

Yes. The guideline recommends self-adhesive pads with the patient restrained and electrically isolated (Hinkelbein et al., 2020).

Could mechanical devices help in the future?

Yes. A review concluded that mechanical devices will probably be needed (Vanni et al., 2025).

Final checklist

• I recognized the arrest without wasting time looking for a pulse.
• I started compressions on the spot, with the rescuer tethered to the patient.
• I called the other crew members and asked for the defibrillator.
• I restrained and isolated the patient before the shock.
• I moved to the stretcher and to the handstand technique when possible.
• I rotated rescuers often.
• I tried for venous access and, if it failed, intraosseous access.
• I thought about the reversible causes with the weight the environment gives each one.
• I informed mission control and left the final decision to the most qualified crew member.

Want to practice reasoning with progressive clues? Play the case of the day.

References

1. Alzaabi AA, Alzaabi FM, Siddiqui RW, et al. Review of microgravity's impact on cardiovascular and nervous systems in space exploration. NPJ Microgravity. 2025. PMID 41168174.
2. Brojakowska A, Bisserier M, Eskandari A, et al. Cardiovascular risks and hazards associated with deep space exploration. Commun Med (Lond). 2026. PMID 42337079.
3. Hinkelbein J, Kerkhoff S, Adler C, et al. Cardiopulmonary resuscitation (CPR) during spaceflight - a guideline for CPR in microgravity from the German Society of Aerospace Medicine (DGLRM) and the European Society of Aerospace Medicine Space Medicine Group (ESAM-SMG). Scand J Trauma Resusc Emerg Med. 2020. PMID 33138865.
4. Lord Z, Andrade C, Leroux L, et al. A high-fidelity simulator for evaluation of hemodynamic response during cardiopulmonary resuscitation in hypogravity environments. NPJ Microgravity. 2026. PMID 41741473.
5. Nolan JP, Maconochie I, Soar J, et al. Executive Summary 2020 International Consensus on Cardiopulmonary Resuscitation and Emergency Cardiovascular Care Science With Treatment Recommendations. Resuscitation. 2020. PMID 33098915.
6. Overbeek R, Liebold F, Johnson Kolaparambil Varghese L, et al. Alternative techniques for cardiopulmonary resuscitation in extreme environments - A scoping review. Resusc Plus. 2024. PMID 39286061.
7. Overbeek R, Schmitz J, Rehnberg L, et al. Effectiveness of CPR in Hypogravity Conditions-A Systematic Review. Life (Basel). 2022. PMID 36556323.
8. Russomano T, Baers JH, Velho R, et al. A comparison between the 2010 and 2005 basic life support guidelines during simulated hypogravity and microgravity. Extrem Physiol Med. 2013. PMID 23849595.
9. Thurlow K, Rehnberg L, Ivetić J, et al. Elbow flexion enables rescuers with low BMI to deliver chest compressions in compliance with CPR guideline recommendations. Sci Rep. 2026. PMID 41708689.
10. Vanni V, Hu JL, Currao A, et al. Chest compressions techniques in microgravity: light years from a good quality resuscitation. Resusc Plus. 2025. PMID 40607044.