The Need for a Systems Approach in Spaceflight Health and Medicine

Much is known about the effects of space flight on the human body. Many early predictions of dire consequences proved not to be the case. In later flights, medical issues began to appear. It is a little-recognized theme of human spaceflight that each time humans undertake longer or more ambitious missions, new problems arise in the realm of human physiology. Despite vast experience and great diligence, we are regularly taken by surprise. American astronauts did not experience space motion sickness until early in the Apollo program, when longer flights and larger spacecraft provided the opportunity for unrestrained head and body movements. ISS flights of even longer duration yielded episodes of reactivation of some latent viruses (viral shedding). More recently, visual impairment and deep vein thrombosis have occurred. This list is not exhaustive, and it is possible that considerations of medical privacy prevent us from knowing of others. When we consider a human mission to Mars - far exceeding in distance and duration any human flights to date - we have to wonder what new unexpected events will occur. The distances involved will preclude direct and immediate assistance from mission control, or even an emergency return. What can be done to avoid more surprises - or to accept that surprises will occur but be best prepared for them - while planning for human expeditions to Mars? Anticipation and prediction of new problems must become part of the medical planning for future missions. We propose that some of these unexpected issues will arise by secondary effects and interactions between systems. Deep fundamental understanding of individual physiological systems will get us only so far. This must be complemented by an understanding of the interactions between systems. Once the obvious primary risks have been mitigated, it is the secondary interactions that will present problems, because they are unexpected, often subtle, and therefore largely ignored. There are several interactions that can already be postulated. As one example, spacecraft cabin CO 2 is typically higher than on earth, for operational and logistical reasons. Elevated CO 2 has well-known primary effects such as lethargy, headache, and irritability. These can be tolerated and partially mitigated. However, one of the ways that the body compensates for elevated CO 2 is by drawing calcium from the bones to buffer the increased acidity of the blood. Astronauts already suffer from loss of bone density as a result of gravity unloading. Another compensatory mechanism is to increase perfusion to the brain, in order to counter the decreased oxygen supply due to breathing of increased CO 2 . Astronauts already suffer from fluids (blood, lymph, CSF) shifting upward in the body due to lack of a hydrostatic gradient in space. These three factors (and others) might exacerbate each other if their interactions are not considered. A systematic modeling and review process should be implemented in order to identify other plausible interactions and their consequences.