Anesthesiologists are specialists in administering drug-induced comas in operating rooms for hours at a time, or in intensive care units for days at a time. Is suspended animation for extended periods—months or years—possible with today’s science as we know it? Suspended animation is defined as “the temporary (short- or long-term) slowing or stopping of biological function so that physiological capabilities are preserved.” Suspended animation is also a “hypometabolic state characterized by the slowing of life processes by external means without termination.”
Suspended animation in fiction
The science fiction notion of suspended animation is frequently portrayed in movies to show astronauts “asleep” during prolonged interstellar journeys. Examples include the films Planet of the Apes (1968), 2001: A Space Odyssey (1968), and Alien (1979), in which space travelers are awakened prior to arrival at their destination after long flights. In The Empire Strikes Back (1980), Han Solo is temporarily frozen in carbonite, depicted as a means of placing a prisoner into suspended animation for travel to another planet.
In literature, “Sleeping Beauty” describes an evil fairy placing a curse on a princess, causing her to sleep for one hundred years until she is awakened by a kiss from a prince. “Rip Van Winkle” is a short story about a man who stumbles upon fairies and consumes their moonshine. He then falls into a twenty-year sleep and awakens after the American Revolution to find his village and country forever changed by the passage of time.
Hypothermia (low temperature) as a preservative
Our current understanding of the challenges of preserving life with suspended animation point to hypothermia is the most promising strategy. In true suspended animation, all metabolism in the body would stop. You can accomplish this by flash freezing to –140 °C (–220 °F). Human embryos are stored this way by freezing them in liquid nitrogen. It’s impossible to immerse a whole person into a vat of liquid nitrogen—the temperature insult would kill them.
Humans who’ve fallen into icy lakes have been revived from hypothermic coma with their brain, heart, and lung function intact. Anna Bågenholm, a Swedish radiologist, survived a 1999 accident that left her trapped under a layer of ice for 80 minutes in freezing water. She found an air pocket under the ice, and then experienced circulatory arrest in the cold water. Her body temperature decreased to 13.7 °C (56.7 °F) during that time. She was eventually rescued and transported to a hospital intensive care unit (ICU), where she woke up ten days later. She made a full brain recovery, but was paralyzed from the neck down. Her brain was able to recover because her metabolism slowed down to approximately ten percent of its baseline, and required less oxygen at the low temperatures. For every one degree Centigrade that brain temperature is decreased from normal, the brain’s metabolic rate decreases by 7%.
Patient cooling has been useful in cardiac surgery. For decades, coronary artery bypass grafting surgery has been done by anesthetizing the patient, stopping the heart, placing the patient on a heart-lung machine, and cooling the patient by 4 – 5 degrees Centigrade, to decrease the oxygen demand on the heart and brain during the surgical repair.
Hospitalized patients who survive cardiac arrest are sometimes placed in a hypothermic coma in an ICU, using the cold temperature to preserve the post-arrest brain function by lowering the oxygen demand of the brain. In the HACA (Hypothermia After Cardiac Arrest) trial published in 2002, induced hypothermia was associated with improved survival and neurologic outcomes among inpatient cardiac arrest patients with a shockable rhythm. The HYPERION (Therapeutic Hypothermia After Cardiac Arrest in Nonshockable Rhythm) trial, published in 2019, demonstrated improved survival with favorable neurologic outcomes using hypothermic coma to 33 degrees Centigrade for 24 hours on patients with a nonshockable rhythm. The TTM2 (Targeted Hypothermia Versus Targeted Normothermia After Out-of-Hospital Cardiac Arrest) study, published in 2021, looked at patients with coma after out-of-hospital cardiac arrest. Targeted hypothermia in this study did not lead to a lower incidence of death at 6 months when compared to targeted normothermia.
Profound hypothermic arrest experiments on animals
The New York Times published the article “Zombie Dogs” in 2005. In a series of experiments at the University of Pittsburgh, dogs were placed in suspended animation by draining the blood out of their bodies, and injecting a low temperature intravenous solution into their circulation. After three hours of being clinically dead, the blood of each dog was re-transfused into their circulatory systems, and the animals were revived by delivering an electric shock to their hearts. Their hearts began pumping blood once again, and the dogs returned to life.
In 2006, doctors from Harvard placed 40 pigs in suspended animation with a similar technique. The pigs were anesthetized and major blood loss was induced by a scalpel-induced injury to their iliac vessels, as might happen in a major car accident or shooting. After the pigs lost half their blood, their remaining blood was replaced with a hyperkalemic (high in potassium) saline solution which was chilled to 10 degrees Centigrade. After the damaged blood vessels were repaired, the blood was returned into the circulation and the animals rewarmed. The survival rates for slow, medium and fast rewarming from the hypothermic arrests were 50%, 90%, and 30%, respectively. All the surviving animals were neurologically intact, displayed normal learning capacity, and had no long-term organ dysfunction.
In 2014 at the University of Maryland, College Park, Dr. Rhee and Dr. Samuel Tisherman kept animal bodies in suspended animation for hours at a time by draining the animal’s body of its blood and cooling it more than 20 degrees Centigrade below normal body temperature. Once the surgical injury was fixed, blood was retransfused once again back into the circulation, and the body was slowly warmed. “As the blood is pumped in, the body turns pink right away,” said Dr. Rhee. At a certain temperature the hearts began beating again on their own. The animals in their experiments showed few ill effects once they woke up. “They’d be groggy for a little bit but back to normal the day after,” said Dr. Tisherman. Can this technique be successfully applied to humans? Further research is needed.
Drug-induced suspended animation?
Are there anesthetic drugs which can be used for prolonged suspended animation? Anesthesiologists perform lengthy anesthetics using medications such as sevoflurane gas and/or the intravenous sedative propofol. Anesthesiologists also order prolonged ICU sedation regimens for patients on ventilators, using drugs such as propofol and/or dexmedetomidine to keep patients unconscious for days. Are any of these drugs promising for prolonged sedation on long missions to outer space? No. Don’t expect a prolonged general anesthetic to solve the problem of lengthy space travel. During an anesthetic, patients are in a pharmaceutical coma but require intravenous fluids over prolonged times so they do not dehydrate, and their bodies still require caloric intake so their muscles and organs don’t atrophy and lose function. As well, patients under prolonged general anesthesia require vigilant monitoring of heart rate, blood pressure, oxygen saturation, electrocardiogram, ventilation, and temperature. An astronaut sedated with propofol or sevoflurane for months or years would continue to age, and would dehydrate and lose protein mass after days, weeks, or months of general anesthesia.
Hibernation: Bacteria and bears
Cryogenics refers to the behavior of materials at very low temperatures. Bacteria are masters of utilizing cryogenics to prolong survival. Bacteria preserved in ice have been known to come alive after millions of years of suspended animation. Bacteria have only one cell, and the preservation of bacterial potency after years of “sleep” is a process far simpler than trying to cryopreserve a human.
Short term versions of suspended animation exist on Earth. Bears hibernate during winter months. Hibernation is characterized by low body-temperature, slowed breathing, slowed heart rate, and low metabolic rate. Black bears cut their metabolism by seventy-five percent when they hibernate. Their normal body temperature is 37.7° to 38.3 °Centigrade (100° to 101 °Fahrenheit), but during hibernation their lowered body temperature stays above 31 °C (88 °F). Hibernation conserves their energy during the cold season when food is scarce. Before entering hibernation, an animal needs to store enough energy to last through the duration of their dormant period, possibly the entire winter.
Torpor: Is hypometabolic “human hibernation” possible?
Torpor is the physiological basis for hibernation in animals. Torpor is marked by a decrease in metabolism, body temperature, heart rate, and respiratory rate. Human hibernation or torpor would be a reduction of our metabolic rate, heart rate, and breathing rate, accompanied by a controlled decrease of body temperature. Most body functions would be suppressed to a fraction of normal, and metabolic pathways would become reorganized—for example, there would be a shift from glycolysis to lipolysis. Humans lower the metabolic rate and body temperature during traditional sleep, but we cannot spontaneously enter torpor, and there is no known way to turn humans to torpor. To induce torpor, scientists will need a deeper understanding of the neurological and endocrine changes which induce torpor in animals. The road map to learning how to induce torpor in astronauts will include experiments to induce torpor in animals such as rats or pigs which normally don’t develop torpor.
The future
If travel to distant stars is ever possible by sub-light-speed velocities, such trips would take many years. Proxima Centauri, the nearest star to Earth, is about 4.24 light-years away. With our current technology it would take centuries—7230 years—to reach the destination. The fantasy of suspended animation by hypothermia or torpor is appealing, but supporting humans for prolonged interstellar travel is likely impossible. The idea of having an astronaut lie supine, pull a glass window over his or her face, and to fall peacefully asleep for a lengthy space trip seems a futuristic dream. For now, suspended animation for spaceflight seems as implausible as time travel or faster-than-light speeds.
Suspended animation as an adjunct for emergency surgeries on humans may someday become plausible. Stay tuned. Smart minds are working on the possibilities.