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| Figure 27. A plethysmography device measures leg circumference change. |
An important way to monitor the head ward shift of fluids in the body, then, is to measure the changes that take place in the size or volume of the legs. Static leg volume (static = still, not moving) measurements in space are one of the easiest measurements to carry out. There is no need for special electronic equipment or complicated procedures. The measurements, however, must be made very carefully so that we can obtain the most accurate measurements possible. Changes in leg circumference (the distance around the leg, also known as girth) are measured by using a technique known as plethysmography plethora = fullness). This is a very fancy word for a very simple technique that measures the "fullness" of the leg.
The technique uses an expandable leg "stocking" that the astronaut wears
over the full length of the leg (Figure 27). Attached to the stocking
are nonexpendable longitudinal measuring tapes placed at eight different
heights along the leg. Before the mission, at various times during the
mission, and after the mission, measurements are made by wrapping the
measuring tapes around the circumference of the leg and recording the
length of the tape. In space, as the leg becomes thinner, the tape
measurement decreases. When the astronauts return to Earth, the leg
circumference increases, and therefore the tape measurement increases.
| Figure 28. Segment locations and the equations for the calculation of leg volumes (c = circumference, h = height, r = radius). |
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How then is the volume of the leg determined from the tape measurements? The volume is calculated using the assumption that each leg segment (between each tape) approximates a truncated (cut off at the top) cone. The basis for this assumption is illustrated in Figure 28. The volume of each segment of the leg is calculated using the equations shown in Figure 28. As we know, the leg segments that are identified are not shaped exactly like a truncated cone, but, in many cases, they are similar enough to feel confident that we are coming close to the actual volumes that we are interested in. If the leg that we are measuring is shaped very irregularly, this technique would not work very well. In that case, the segments would be made smaller so that each segment would contain less of the natural curve of the leg. Also, the thickness of the tape-measure fabric must be corrected for in the final calculation to make sure that the circumference measurement would not be affected by the tape thickness.
As you can see, the measurement of the volume of the leg is a relatively easy one to make. But it is the rate of change of the leg fluid volume that we are interested in. That is, how fast does the fluid leave the leg area? Therefore, in space, the leg volume measurements must be taken early and over a period of time to understand how fast the fluid leaves the lower part of the body. Although leg volume measurements have been made on astronauts in space for many years, Dr. Blomqvist's team was able to take the earliest measurements of such change. In fact, the inflight measurements were made on the first day in space. This is important because we know the fluid begins to shift "upward" while the astronaut is waiting in the shuttle to launch, and we also know that there is a dramatic fluid shift immediately after the astronaut arrives in space (Figure 29). For missions up to two weeks, the leg volume continues to decrease until the astronaut returns to Earth.
| Figure 29. Leg volume measurements preflight, insight, and postflight. (FD = flight day) |
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Now, let's attach the results from Figure 29 to the fluid shift phenomenon, which we know takes place during the early part of the mission. If the body eliminates the excess fluid by the end of the first day, then why does the leg volume continue to decrease? This is a very good question. Dr. Blomqvist hypothesizes that the leg-volume decrease that occurs after the first day or two is not a reflection of the fluid shift but instead is a reflection of the fact that the muscles are not being used in space. That is, the astronauts begin to lose muscle strength because they are not using their muscles to stand up. As muscles become weaker, they lose their mass. Therefore, the decrease in leg volume after the first day in space is probably due to the decrease in muscle strength and mass. This hypothesis is further supported by the postflight data shown in Figure 29. During the postflight measurement, the leg volume is shown to increase, but not to the preflight level. This is probably because the astronaut's muscles have become smaller due to their lack of use in space.