|Figure 18. To measure central venous pressure (CVP), a catheter is used.|
The system for the measurement of CVP provides a means for directly measuring the pressure in the large veins near the heart. Before a mission, a medical doctor inserts a catheter (a thin, soft plastic tube) into an arm vein and advances the tube through the veins to a point just outside of the right atrial chamber of the heart (Figure 18). The position of the catheter in the body is verified by taking an X-ray. The catheter is attached to monitoring systems outside of the body that measure and record the changes in CVP. Let's look a little closer at how the CVP system works.
In order to keep the catheter tubing open, a solution of salt water and heparin is pumped continuously through the tubing. Heparin is a substance that stops the blood from clotting; this is important because clotting would cause the catheter to clog. The catheter is directly attached to a CVP recording unit that is strapped to the astronaut's hip. A transducer is placed under the arm of the astronaut at the same level as the catheter tip placement at the opening of the heart.
The actual pressure that is detected by the tip of the catheter travels through the fluid in the catheter line to the transducer. When the pressure reaches the transducer, it is transformed into a proportional voltage signal that is fed to the CVP recording unit. The transducer is not directly attached to the catheter tubing because the salt water in the catheter would conduct the electricity from the transducer right into the body. This would cause the electrical activity of the heart to go completely crazy, endangering the astronaut. Therefore, the transducer and the catheter are kept absolutely separate.
|Figure 19. The physiological monitoring system (PMS) acquires blood pressure and heart rate information while collecting central venous pressure signals. The astronuats operate the system themselves.|
The CVP system also provides an electrical signal to the physiological monitoring system (PMS) and the CVP measurement is recorded on the cassette dote tape recorder (CDTR), which is inside the PMS (Figure 19). The function of the PMS itself is to acquire blood pressure and heart rate information from the astronaut at the same time that it is collecting and recording the CVP measurements. The PMS consists of an automatic arm cuff to measure arterial blood pressure, chest electrodes to acquire heart rate information from a recording of the electrical signals of the heart called an electroccardiogrom (EGG), and electronics to process the blood pressure and heart rate data. The PMS is used to gather continuous heart rate data and periodic blood pressure data for as long as the CVP catheter is in place. The PMS displays the data on a numerical display unit, records the data on the CDTR, and has the capability to transfer information to the experiment microcomputer onboard the spacecraft for downlinking to groundbased systems back on Earth. The term "downlink" refers to the transmission of data from space via satellite communications to Mission Control and thus to the scientists following the experiment on Earth. For Dr. Blomqvist, this afforded him the opportunity to receive the data while the experiment was taking place in space, so he could make sure that the experiment was proceeding smoothly or determine whether any changes in the protocol were needed at any given moment.
As stated above, investigators believed that the degree and speed of fluid redistribution in the body could be determined by observing the changes in CVP. The CVP system records pressure measurements for the entire shuttle launch sequence while waiting on the launch pad, during the actual launch, and up to about 24 hours while actually in space. After about 24 hours in space, the astronaut removes the catheter by slowly pulling it out of the arm vein. Later, on the day the shuttle returns to Earth and just following the space flight, a medical doctor inserts another catheter back into the arm vein in order to measure the CVP changes associated with the body's readaptation to the normal gravity of Earth.
|Figure 20. Astronaut orientation during launch.|
CVP measurements were made by Dr. Blomqvist's team on three crew members. The CVP responses of all three crew members were essentially the same. The fact that the same results were observed in all three astronauts gives Dr. Blomqvist's team some very strong results that can be "trusted." One of the "hallmark" features of a good science experiment is that the results of the experiment should be reproducible. That is, the scientist should be able to obtain the same results over and over again in order to be able to "trust" the results. However, these strong results contradicted some of what researchers believed should happen to the CVP in space. That is, when the astronauts were on the launch pad waiting for their journey to begin, and during the actual launch, the CVP responded exactly as expected. However, when the astronauts arrived in space, the CVP responded in an fashion to what was predicted by Dr. Blomqvist and his team. Let's find out what happened.
Dr. Blomqvist and his team expected the CVP to increase as the astronauts waited on the launch pad. This is because the astronaut is oriented in a supine position (on his back) with his feet elevated and head tilted somewhat downward (Figure 20). It was predicted that the fluids would begin to shift away from the feet and toward the heart area. This should cause the CVP, the pressure just outside of the right atrium, to rise because of the rush of fluids to that area. This part of the prediction came true for all three astronauts.
Then, it was predicted that the CVP would increase even more during the
actual launch phase. During launch (Figure 21), the forces that are
experienced by the astronauts are equivalent to about three times gravity
(or 3 g). Since the astronauts are in a supine position, the direction of
this great force is front to back, compressing the chest area. The effect
of this force is the same as if a large person were sitting on your chest
as you are lying down. It was believed that the result of this mechanical
compression force would be to increase the CVP. This part of the
prediction also came true for all three astronauts.
Finally, it was predicted that the CVP would remain elevated when the astronaut arrived in space because, as we've already learned, the fluid shift to the head would continue due to the nearly complete absence of gravity. This part of the prediction did not come true for any of the three astronauts. Let's discuss specifically what happened by examining an analog tracing of the CVP response that was obtained for one of the astronauts (Figure 22).
|Figure 22. Analog tracing of the central venous pressure response to various phases of the mission including the preflight control measurement, sitting on the hunch pad, during the actual launch phase, and immediately upon entering the virtually gravity-free environment of space. The reference to "zero-G" in the figure acually refers to microgravity.|
|Figure 23. Central venous pressure measurements were obtained for three astronauts (indicated by different colors). Notice the dramatic COP changes that occurred during launch as well as when the astronauts arrived in space.|