Anesthetic Concerns of Spaceflight

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THE missions of crewed spacecraft will evolve from brief forays close to the home planet into years-long voyages to Mars and, eventually, colonies independent of Earth. A need for surgical interventions and critical care is inevitable in these operations. Anesthesiologists are acutely aware of the fact that, although a given surgical procedure may be relatively simple, the required anesthetic care is, in certain cases, extremely complex. This principle becomes particularly evident as one considers strategies for the delivery of anesthetic and critical care in space. In this issue of Anesthesiology, Keller et al.  1confront some of these problems through their evaluation of airway management techniques in simulated microgravity.

These investigators used water immersion as a model of microgravity, a reasonable choice because immersion facilities are readily available. Limitations of this method include the possibility of “cheating” on the simulation of microgravity through swimming movements, the presence of sensory cues for spatial orientation, and damping of reactive motions (e.g. , a diver who pushes off a pool wall will come to a halt in a few feet rather than gliding the length of the pool). Spaceflight conditions are more perfectly reproduced by parabolic flight in aircraft, but this method yields periods of microgravity that last only 25 s. The best approach to these studies is through formal investigations during spaceflight. A limited number of such studies have been performed, but none has emphasized airway management. The future may hold more opportunities for these “real world” studies.

Keller et al.  1found that direct laryngoscopy of an unrestrained patient is difficult. Their conclusions are valid for the specific techniques that were investigated. At least one other approach to laryngoscopy in microgravity is possible: My colleagues and I have evaluated laryngoscopic techniques during parabolic flight, and we found that grasping the head of an unrestrained patient with one’s knees affords a quick, stable, albeit somewhat distant, view of the glottic opening (fig. 1).

Further innovations, evaluated with rigorous methods similar to those developed by Keller et al. , 1will optimize procedures for in-flight medical care.

As a further prelude to the article by Keller et al. , 1I would like to outline other challenges to be overcome before surgical, anesthetic, and critical care can be delivered beyond our planet.

Anesthetic complications that arose during the recent Bion 11 mission indicate that a cautious approach should be taken to in-flight and post-flight anesthetic care. This project, conducted jointly by Russia and the United States in 1997, flew two primates for 14 days in a dedicated “biosatellite.” An anesthetic was delivered to the animals on the first post-flight day to perform minor surgical procedures such as muscle biopsies. Despite competent care by a team of veterinarians from both countries, one animal died during anesthesia and the other experienced serious complications. In previous Bion flights, no animal received an anesthetic before the seventh post-flight day, and no anesthetic complications occurred. Clearly, our knowledge of the risks of anesthesia associated with spaceflight is imperfect.

The goal of crew medical selection and health-maintenance programs is to fly fit people. Consequently, the type of disease observed in space is likely to arise as a physiologic reaction of a normal person to an abnormal environment. For example, microgravity induces disorders of calcium metabolism that increase the risk of nephrolithiasis. 2Therefore, a need for the capability to perform urologic procedures should be anticipated. In addition, trauma is inevitable. Guidance regarding likely patterns of disease can be obtained from analysis of operations in analog environments such as ballistic missile submarine cruises 3and from longitudinal studies of astronaut health. A wide spectrum of diseases is encountered in these settings, indicating that versatile medical capabilities will be needed during extended space missions.

Cardiovascular deconditioning, disruptions of central modulation of baroreceptor reflexes, nonresponsiveness of autonomic function to cardiovascular stresses, and decreases of intravascular volume have been frequently observed during and immediately after spaceflight. 4The implications of these observations on the conduct of anesthetic care are profound. For example, because spinal anesthesia seems to carry a risk of cardiovascular collapse due to autonomic dysfunction, 5an astronaut might be at increased risk for this complication.

Succinylcholine can produce hyperkalemic cardiovascular collapse in bed-ridden individuals. 6,7Extended exposure to microgravity conditions seems to carry at least a theoretical risk of eliciting changes in the neuromuscular junction similar to those induced by bed rest. Until more is known about the effects of spaceflight on the neuromuscular junction, depolarizing neuromuscular blockers should be avoided during and immediately after flight.

The majority of crew experiences “space motion sickness” during the first few days of flight. An unlucky few are never free of symptoms throughout multiweek flights. Some suffer a relapse on return to Earth. In-flight investigations have revealed marked decreases in gastric motility early in the course of space motion sickness. 8In addition, crew members frequently report symptoms of gastroesophageal reflux during flight. These observations suggest an increased risk of aspiration, both in-flight and post-flight.

In microgravity, fluids, and gases do not separate on the basis of differing densities. Consequently, a vial of a drug or a bag of intravenous fluid contains something akin to foam (fig. 2).

This seemingly trivial methodologic detail is a significant obstacle to the delivery of health care in microgravity. Each fluid transfer involves a procedure that, if not performed properly, produces a useless mix of gas and fluid. Furthermore, many devices that depend on gravity-induced separation of gases and fluids (e.g. , anesthetic vaporizers) malfunction in microgravity.

Spacecraft contain tightly sealed environments. Drugs used in flight should not be capable of unintentionally reaching crew through the cabin atmosphere or water reclamation systems. Dumping of oxygen from respiratory support equipment into the cabin atmosphere must be avoided to control fire hazards. The safe use of gaseous anesthetics in spacecraft will be difficult. Minimal-flow systems that have been developed for xenon anesthesia 9may prove useful.

The bill associated with placing 1 kg of material (e.g. , 1 l of crystalloid solution) into Earth orbit is currently $22,000 USD. Furthermore, stowage space aboard spacecraft is extremely limited. Clearly, the inventory of medical supplies that is flown must minimize mass and volume. In addition, chosen pharmaceuticals should have long shelf-lives at room temperature.

Currently, the crew complement of each mission includes a Crew Medical Officer (CMO). The CMO may have no biomedical background whatsoever before entering the Astronaut Corps, and the formal training a CMO receives totals approximately 80 h. CMO background requirements and training will need to be modified before advanced medical care in space is possible.

Telemedicine technology has obvious applicability to space operations. However, during a journey to Mars, the speed of propagation of electromagnetic transmissions is such that as much as 40 min may be required to receive an answer to a question. Given the fast pace of events during anesthetic procedures, telemedicine technology will need to be supplemented with advanced on-board information systems.

Expertly performed local and regional anesthesia may avoid many of the problems associated with conducting a general anesthetic in space. However, the on-board anesthetist is unlikely to be an expert, regional blocks sometimes fail, and anesthetic complications do occur. Consequently, although local and regional anesthesia may be preferred approaches, they cannot be the only options.

Many techniques are available to perform total intravenous anesthesia. Adaptation of these methods to microgravity conditions seems entirely feasible. Total intravenous anesthesia is a promising means of providing general anesthetics in space.

Because the number of potential landing sites is large, the medical infrastructure necessary to support missions is widely dispersed. Surgical procedures may need to be performed immediately after landing. Advances in postflight anesthetic care should be thoroughly communicated to any practitioner who might participate in such a drama.

Significant difficulties must be overcome before anesthetic and critical care can be delivered safely in-flight and post-flight. The same can be said of virtually any other aspect of medical care. Meeting these challenges is the task of the nascent field of space medicine. Anesthesiologists have much to contribute to these efforts.