IV. Cardiovascular Function During a Stand Test
One of the most important points to make throughout this book is that the human condition on Earth and the human condition in space are not only different, but they are appropriate for their respective environments. Another way to put this is that our bodies function in an "Earth-normal" condition while on Earth and a "space-normal" condition while in space. Everything that we have learned so far indicates that when an astronaut is in space, all the changes that occur in the body are absolutely appropriate for the space environment. Any problems that occur do so only when the astronaut returns to Earth. The body has undergone so many changes while in space that it no longer tolerates the gravitational pull on Earth very well. In particular, the astronauts are known to have great difficulty standing upright and are said to be suffering from orthostatic intolerance (orthostatic = standing upright, intolerance = unable to handle).Standing upright represents a particular challenge to returning astronauts because it is in this position that gravity is exerting most of its influence on the cardiovascular system. Think of an upright human body as a tall column of water (after all, our bodies are composed primarily of fluids anyway). As gravity is pulling down on that column of water, each level, or depth, of water is influenced. In any body of water, the pressure at the surface of the water is equal to atmospheric pressure, but the pressure rises 1 mm Hg for each 13.6-mm distance below the surface. This pressure results from the weight of the water above it and therefore is called hydrostatic pressure.
Hydrostatic pressure also occurs in the vascular system of the human being because of the weight of the blood in the vessels. Hydrostatic pressure exists on Earth because of gravity. When an astronaut has spent time in space, the astronaut's cardiovascular system has adjusted to functioning without gravity. The astronaut loses about 20% of the fluid in his or her body and is feeling fine. Also, the various mechanisms that control our cardiovascular function here on Earth have been greatly relieved of their duties while in space and away from the Earth's gravitational pull. Gravity deals a mighty blow to the system when the astronauts return. Many astronauts cannot tolerate standing for any length of time upon their immediate return to Earth, and, in fact, some astronauts have actually fainted when asked to stand up for even short periods. We know that gravity has a great deal to do with that, but what in the cardiovascular system is specifically causing this phenomenon?
Dr. Blomqvist's team put together a series of cardiovascular function tests to measure the extent of orthostatic intolerance as soon as possible after the astronauts return to earth. The full series of tests was conducted both preflight and postflight. The preflight measurements served as baseline control measurements from which to compare the postflight data. These measurements gave us an indication of what may be happening in the body to produce this "weakness" in astronauts when they stand against the clutches of gravity after being in space. Thus, the entire test is referred to as a stand test. If the mechanisms that produce orthostatic intolerance can be determined, we can then concentrate on developing specific countermeasures (steps that we can tak e to fight back, or counter, these effects). The specific measurements that were made included mean arterial blood pressure (MBP), heart rate (HR), and cardiac output (CO). From the heart rate and cardiac output values, stroke volume (SV) was calculated. Table 6 is a list of the raw data (individual data for each astronaut that has not been analyzed) that Dr. Blomqvist obtained during both the preflight and postflight stand test. It is left to you and your teacher to decide how to best analyze this data.
The stand-test protocol consisted of a 29-minute supine (Lying face up) period during which the astronauts were instrumented with the various electrodes on the chest to measure HR; a blood pressure cuff on the arm and one on the finger to measure blood pressure two different ways; and a stocking plethysmograph to measure leg volumes. after instrumentation was complete, a set of supine measurements was obtained. Next, the astronauts were asked to stand up for a 10-minute period, and during this time measurements were taken again. This protocol was performed on each crew member preflight and then within the first four hours after their return to Earth.
An electrocardiograph was used to actually determine the astronaut's HR. An electrocardiograph measures the electrical activity of the heart and displays it as a set of pulses on an electrocardiogram, that correspond to different parts of the cardiac cycle. HR information can be accurately obtained by measuring the length of a complete cardiac cycle (Figure 30). HR was monitored and recorded continuously in this way.
| Figure 30. (a) An electrocardiogram (ECG) displays pulses that correspond to the different parts of the cardiac cycle. A normal ECG is shown. (b) In an ECG pattern, the P wave results from the contraction or squeezing (depolarization) of the atria, the QRS complex results from the squeezing (depolarization) of the ventricles, and the T wave results from the filling (repolarization) of the ventricles. Heart rate information can also be determined using the time axis corresponding to the ECG pattern. | |
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Indirect arterial blood pressure was measured by two different methods. One method used a typical arm cuff of the kind that is used in your doctor's office. The other method recorded continuous finger blood pressure. Both methods were used to double check the accuracy of the measurement. Also, leg volume was determined using the stocking plethysmograph. Leg volume was measured during the supine resting period and after 6 and 10 minutes of standing. The leg volume data was presented in an earlier section.
Cardiac output was estimated by a special rebreathing technique, which measures pulmonary blood flow. Remember that the lungs work so closely with the heart and blood vessels that certain heart function measurements, such as cardiac output, can actually be obtained from measurements of pulmonary function. That is, measurements that involve breathing can actually yield information about blood flow through the heart. This works because all the blood that flows into the lung's capillaries to be oxygenated equals the amount of blood that flows out of the heart. Blood flow through the lungs is called pulmonary blood flow (PBF). Thus
cardiac output = PBF
| Figure 31. The cardipulmonary rebreathing unit (CRU) and the gas analyzer mass spectrometer (GAMS) being used in space to determine cardiac output. |
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The rebreathing technique uses a system called the cardiopulmonary rebreathing unit (CRU) that is attached to a gas analyzer moss spectrometer (GAMS) that can analyze the chemical content of a gas mixture {Figure 31). During the rebreathing maneuver used to measure PBF, the astronaut breathes in and out of a rebreathing bag, hooked to the CRU, that is filled with a known mixture of very safe test gas that is different than normal atmospheric air. This gas is not only safe, but the different chemicals in the gas can also be easily analyzed by the GAMS. When the astronaut breathes in the test gas and then expires back into the rebreathing bag, the mass spectrometer determines how much of the test gas has been absorbed by the astronaut's lungs. The astronaut repeats this rebreathing maneuver for a specified period of time, breathing in and out of the rebreathing bag. After each breath, the spectrometer determines how much more of the test gas has been absorbed into the astronaut's lungs. The amount of gas that is absorbed by the lungs is proportional to the amount of blood that passes through the lungs. As already mentioned, the amount of blood that passes through the lungs (PBF) is equal to the amount of blood that flows out of the heart (cardiac output). Therefore, this is an indirect way to measure cardiac output (CO).
Finally, stroke volume (SV) was calculated after determining both CO and heart rate (HR) by using our well-known relationship:
CO = SV x HR or SV = CO/HR
At least three measurements were made while the astronauts were in the supine resting position, followed by measurements after 5 and 10 minutes of standing.
The MBP, HR, CO, and SV values that were obtained during the postflight stand test will not tell scientists anything about what is causing the orthostatic intolerance. Instead, these values are only indicators of what is happening within the body and can only suggest what might be causing the problem. They will help scientists determine which cardiovascular control mechanisms to begin looking at in detail. That is, they will help scientists sharpen their hypothes about the actual mechanisms of action behi nd orthostatic intolerance and about what should be done to help the astronauts better tolerate their return to Earth. It must be emphasized here that the astronaut's cardiovascular system does return to normal, in most cases within a few days.