The severity of space motion sickness had been reported to decrease with successive flights. First flight experiences influence behavior and use of countermeasures such as in-flight exercise, fluid loading, and G-suit use. Because of the possible influence of previous flight experience on response to subsequent flight, only first time flight experience was analyzed.
Nearly all lab parameters showed predictable changes following shuttle flights. More than half of the approximately 90 analytes were statistically significant at the 0.05 percent level. These data demonstrate the importance of examining data for clinical relevance when a statistically significant result is reported. Although the t-tests were statistically significant at the 0.05 percent level, all lab parameters remained within normal ranges. For example, the normal range for serum glucose is 78 to 110 mg/dl. The group mean glucose value at L-3 (3 days before launch) was 92 mg/dl. The group mean glucose value at R+0 (landing day) was 101. Both of these group mean values are well within normal ranges. The difference between the group means is 8.7 mg/dl, that is, the R+0 group mean is 8.7 units greater than the L-3 mean value. Although this difference is significant at the 0.05 percent level (exact P value = 0.0001 percent), it is not clinically important. There is no health impact from this small temporary increase in serum glucose.
There are many factors that may influence postflight lab parameters. In-flight factors include space motion sickness, fluid shifts, neurovestibular and cardiovascular changes associated with weightlessness, diet, medications usage, in-flight illness or injury, fatigue, circadian dissynchrony, exposure to environmental toxins, and mission duration. Factors encountered during reentry include exposure to G-forces following prolonged weightlessness, heat stress caused by the suits worn, and fluid loading. Postflight factors include varying time lag between landing and collection, crew activities scheduled in the postflight period, and consumption of fluids and food after landing, but before sample collection.
Many of the differences between preflight and postflight lab parameters can be explained by physiologic stress, hemoconcentration, and relative hypovolemia, which are experienced in the short-term by space participants. An important confounding factor is that the preflight serum samples were collected following a 12 to 14 hour fast. Postflight serum samples were collected after a meal (breakfast) and fluid loading and possibly food intake between landing and blood draw.
In conclusion, the differences between preflight and postflight measurements can be attributed to any of the factors described above. However, the differences appear to be too small to be clinically significant. Future analyses will examine the recovery curve between R+0 and R+3 because crew members typically return to duty and return to flight status after R+3.
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