Becoming an astronaut is a highly selective process. Only the best, the fittest, are chosen to be astronauts and to go on missions. In 2024, 8,00 people applied to become an astronaut, only 10 were chosen. To be selected, the candidates have to be in perfect physical health. There are vision and height requirements, they need to have excellent heart health and no high blood pressure. Once chosen, the process continues. Astronauts go through one of year of intensive physical training including wilderness survival, underwater training and learning to cope with low gravity. Despite being in tip top shape and low risk, it is still possible that an astronaut could suffer sudden cardiac arrest (SCA) while in space. If that happens, could they be resuscitated? How would cardiopulmonary resuscitation (CPR) be performed in zero gravity? How can astronauts (and those of us on Earth) lower our risk for SCA?
Sudden cardiac arrest is a common problem and is often the first manifestation of heart disease. It is an abnormal heart rhythm most often caused by ventricular fibrillation (an irregular heart rhythm from the lower chambers of the heart). When the heart’s ventricles are fibrillating, they cannot pump blood to the brain and vital organs. If not treated promptly, this leads to death. Sudden cardiac arrest affects more than 300,000 people in the US each year with a death rate of about 90%. Treatment of SCA involves prompt initiation of CPR and performing defibrillation. Defibrillation is an electric shock to the heart that restores the heart to normal rhythm. The shock is usually provided by an Automatic External Defibrillator (AED). Cardiologists can determine which patients are at high risk for SCA including those with prior heart attack, previous SCA, congestive heart failure and low ejection fraction (below 30%). However, these high-risk patients only account for a small proportion of the SCA total (about 10% of the total number of arrests). In contrast, the majority of SCA occurs in asymptomatic, low-risk people in the general population (more than 50% of the total). It is very hard to predict, or prevent, these events in the general population.
By any measure, astronauts would fall into a low-risk category. Still, the risk is not zero and SCA could occur while on a mission. If SCA happened in space, how would CPR be performed? Traditional CPR relies on gravity to be effective and weightlessness poses a problem. NASA recommends that a rescuer stand behind a victim, wrap their arms around the person (in a bear hug) and squeeze (almost like the Heimlich maneuver, but applying continuous compressions to the chest not the abdomen). The rescuer and the victim can be moved to a room with medical equipment and the victim strapped down. The rescuer then does compressions in a handstand position, with hands on the patient’s chest and legs braced against a wall. This method can’t provide the compression depth that is recommended, so it would not be as effective as CPR on Earth. One way around this would be to use a LUCAS automated chest compression system, a machine used in emergency medicine with a plunger that automatically does chest compressions to the right depth and at the right rate. Current space vehicles do not have this device (due to size and weight considerations) but do have AEDs.
If you are an astronaut or a low-risk citizen of Earth, what can you do to lower the risk for SCA? Over the past twenty years, the incidence of SCA has gone down, but there is room for improvement in preventing it. The CARES (Cardiac Arrest Registry to Enhance Survival) network is a national registry tracking SCA. It currently covers about 40 states. CARES showed that from 2021 to 2024, overall survival improved from 9% to 11%, public AED use increased from 10% to 13%, but bystander CPR remained steady at 41%. However, we can do better. A large study of SCA was published this year covering 500,000 participants, average age 56 and 50% were women. The study showed that the American Heart Association’s Life’s Essential Eight were strongly related to risk for SCA. The eight risk factors are:
Diet- high in fat, low in fruit and vegetable intake
Activity- sedentary lifestyle
Smoking
Sleep- less than seven hours per night
Obesity- high body mass index (BMI), high waist circumference
High Cholesterol
Diabetes
High Blood Pressure
In addition, the study found several novel risk factors for SCA. These are depression, social isolation, low educational level, increased arm fat, reduced grip strength (a marker of frailty) and air pollution. The study concluded that SCA could be reduced by 40% to 60% if these risk factors were controlled or eliminated.
With this information, we can now respond to SCA anywhere in the universe and begin to prevent its occurrence. So, if you are planning on being a space tourist (or if you want to reduce your risk for SCA here on Earth), work on controlling these risk factors. And maybe bring your own LUCAS device on board, just in case.
