Anesthesia and Surgery in Space: Reply

As pointed out by Drs. Covington and Moon,¹  in response to our recent article,²  mitigating the risk of decompression sickness is of utmost importance for future missions to the Moon or Mars. This risk is proportional to the ratio R between the partial pressure of nitrogen in the tissues and the pressure of the spacesuit.³  For future space exploration missions, an acceptable risk of decompression sickness is reached for an R less than 1.3 to 1.4.⁴  Whereas the atmosphere inside the International Space Station is normoxic and normobaric (14.7 psi and 21% oxygen), the American spacesuit circulates pure oxygen pressurized at 4.3 psi (about 0.3 atm).³  As a consequence, astronauts prebreathe oxygen for 4 h to wash out nitrogen from body tissues (to reduce the partial pressure in the tissues) before depressurization, with the option to enhance denitrogenation with physical exercise.^3,4  For a Mars mission that will potentially have almost daily extra-vehicular activities, 4 h of prebreathe time for each spacewalk will be operationally impractical.

Several solutions exist: lowering the ambient pressure of the habitat (at the cost of increasing the fraction of inspired oxygen and flammability) or raising the suit pressure (at the cost of increasing suit rigidity and the metabolic cost of movement).⁴  Pretreatment with hyperbaric oxygen before decompression may also be effective in reducing the incidence of decompression sickness, although the evidence remains limited to animal models.⁵  It is estimated that an atmosphere in the habitat of 8.0 psi and 32% oxygen would eliminate the need to prebreathe.⁴  Another solution might come from innovative hybrid suit design, combining “mechanical counter-pressure” and gas pressure.⁴  Finally, “smart suits” could progressively lower their internal pressure during the extra-vehicular activity to improve flexibility while keeping the R ratio within acceptable bounds.⁴ 

The challenge of spacesuit design, and more globally of manned spaceflight, elegantly illustrates the interactions between physiology and engineering in designing life support systems, very much like the applied physiology and physics that underpins submarine and diving activities, pressurized airplanes, high-altitude mountaineering, but also the delivery of anesthesiology and critical care on Earth.

The author has received speaker honoraria from GE Healthcare (Limonest, France) and consulting fees from Philips Healthcare (Eindhoven, The Netherlands).