The heart is a muscle; in fact, it is the most active muscle in our bodies. All our other muscles are able to relax at one point or another, but the heart muscle can never completely relax. There are times when the heart muscle must work harder than other times, but as long as we are alive, it never gets a break from its constant responsibility of pushing the blood ever forward. The size of the heart is relatively small, about the size of an average adult fist, which seems unusual in light of the constant "exercise" it gets. The size of the heart and its chambers remains relatively constant throughout our lifetime, as long as we stay healthy and participate in a moderate amount of physical activity, and as long as we stay within the grip of gravity. It must be noted, however, that the heart chambers of marathoners can enlarge about 40% in response to an increased workload, and, along with enlargement of the chambers, the heart mass enlarges and the cardiac output increases 40% or more as well.
Here on Earth, gravity is both a help and a burden to the heart depending on what direction the blood must flow. Gravity assists the heart pump by pulling the flow of blood downward, but when the blood must flow upward, gravity must be overcome. So, what happens when an astronaut escapes the gravitational pull of Earth and travels into space? Gravity is no longer a participant that can affect the flow of blood throughout the body. Dr. Blomqvist and his team designed part of their experiment to quantify the effect of removing gravity on the size of the heart.
The left ventricle of the heart is surrounded by the most powerful part of the heart muscle. This is because the left ventricle is the chamber responsible for providing enough force to propel the blood out of the heart and into the long network of blood vessels around the body. The left ventricle is, in a sense, the most important chamber of the heart, and therefore is the most sensitive indicator of how our hearts are functioning. Any change in the size or volume of the left ventricle indicates a change in the work that the heart must perform for our bodies. Dr. Blomqvist's study was designed to measure the size of the left ventricle to obtain an indication of changes in the overall size of the heart, and therefore changes in the workload of the heart. But how does he measure the size of the left ventricle? Let's take a look.
|Figure 24. Echocardiography in space.|
Images of the size of the heart chambers give investigators information about the mechanical functioning of the heart. Echocardiography is a method that is used to obtain images of the heart, to watch how it functions, and to determine its size (Figure 24). This technique uses high-frequency sound waves (ultrasound waves that are above hearing range) that are transmitted into the body and that bounce off of the heart and are reflected back to a receiver outside of the body. The principle used here is similar to the principle behind an echo, which is a reflected sound wave. Let's look a little more closely at a type of echo that we are more familiar with in order to understand echocardiography a little better.
If a person, we'll call him Ron, stands on one side of a canyon and shouts "Hello!" toward the other side, an echo will be produced. The sound wave that is produced when Ron shouts travels through the air at the speed of sound (about 741 miles an hour) until it hits the opposite wall of the canyon, where it is reflected back toward Ron. Once the sound wave has been reflected it becomes an "echo." The echo travels back to Ron at the same speed of sound, and, after a brief period of time, Ron hears the echo. We have all probably participated in this type of echo experience.
If Ron has a stopwatch and a small electronic calculator, he can use this echo to calculate how far it is to the other side of the canyon. He simply measures the time it took for his shout to be echoed back to him. Suppose this was 4 seconds. He consults his handy conversion table, which tells him that 741 miles per hour is equivalent to 1087 feet per second. He then multiplies 1087 feet per second by 4 seconds and discovers that his "hello" traveled 4348 feet. Since this represented two trips across the canyon, one for the sound wave going and the other for the echo returning, he then divides by 2 and obtains 2174 feet as the distance to the other side of the canyon.
Ultrasound imaging (and echocardiography in particular) uses this same principle . Ultrasound waves are produced by a machine and directed into the body. The sound travels through the tissues at a constant speed until it encounters a reflecting surface, in our case, the heart. Once the sound waves bounce off the heart, some of the sound beam is reflected back toward the source; there it is received by the ultrasound scanner, which has been keeping track of the time and converting it to a distance in the same manner as Ron and his stopwatch. However, instead o f giving out the distance as a number, the scanner shows it as a dot on something like a TV screen. The position of the dot is proportional to the distance the echo traveled. This enables us not only to measure the distance but to get a visual picture of it as well. The pictures are taken, enabling a scientist to follow the changes in the heart over time, much like a movie.
|Figure 25. The principle behind the construction of the three-dimensional image of the heart.|
This echocardiograph method was used by Dr. Blomqvist's team to obtain two-dimensional pictures of the heart. Images were obtained rapidly and in succession so that changes in the heart could be detected during all parts of the cardiac cycle. The images provided information on changes in heart size as the heart goes through its entire cycle. Dr. Blomqvist's team also developed a method to obtain a three dimensional model of the heart. This method can construct a model of the heart from a set of "sliced" images that were taken at different angles along the heart. These slices could be connected together to reconstruct the three-dimensional heart (Figure 25). This three-dimensional echo reconstruction technique was not used in space, but instead was used before and after the flight. The two-dimensional data that was gathered in space was videotaped for later detailed ground analysis; that is, data was not downlinked to the ground for immediate analysis.
As mentioned, echo images were obtained preflight, at various times inflight, and post flight. From these images, the investigators measured the length of the short-axis dimension of the left ventricle at a point in the cardiac cycle when the ventricle is at its largest size, the end of diastole. Remember, the diastolic period involves the filling of the heart. The end of the diastolic period is when the largest amount of blood is in the ventricle and it is immediately before the powerful systolic contraction. The volume of the left ventricle during this point, which is called the left ventricular end-diastolic volume (LVEDV), was either determined through calculation (by cubing the length of the short axis, which was done for the inflight data) or determined by using the three-dimensional echo reconstruction technique described above (which was done preflight and postflight). Either technique will give very accurate volume measurements. Because the left ventricle is the largest chamber of the heart, any change in the volume of this chamber will reflect a change in the volume of the total heart.
|Figure 26. A graphic representation of the percent increase and subsequent decrease of the LVEDV in space compared to preflight levels on Earth. (FD = flight day)|
The results indicate that the LVEDV, and therefore the total heart volume, increases dramatically when the astronaut first arrives in space (Figure 26). The LVEDV then begins to slowly decrease as the astronaut's body adapts to the space environment, ending up smaller overall compared with its size on Earth. These results were not surprising since similar measurements had been made on previous shuttle missions. The results, however, are difficult to explain.
Let's try to break the explanation into two parts. First, when the heart initially becomes larger in space, it is probably because of the increased volume of blood flowing into the heart. Remember, the body experiences an immediate shift of blood and fluids toward the upper part of the body. (In fact, this upward" shift of fluids begins while the astronaut is in the launch position waiting for the shuttle to take off.) Once in space, it takes the body about a day to eliminate this "flood" of fluids. This seems to correspond directly with the time in which we find the heart to be the largest: the first day. After the first day, the total amount of body fluids decreases below normal and continues at this level for the remainder of the mission.
This brings us to the second part of the explanation, which involves the heart shrinking in size after the first day in space. Once the excess blood and fluid have been eliminated, the heart no longer has to work so hard. This lower volume of fluids, along with the virtual absence of gravity, may be the factor responsible for the decrease in heart size that occurs after about the first day in space. Gravity, or the lack of it, is included in this part of the explanation because it no longer is a burden to the cardiovascular system. The heart has an easier time pumping the blood around the body when it does not have to pump the blood against the gravitational pull. A final point to make is that the weight-bearing muscles in the legs do not have to bear weight in space. That is, astronauts do not have to use the muscles in their legs the same way they do here on Earth. The less a muscle is used, the less oxygen is needed to fuel its activities. The less oxygen that is needed by the muscles, the less rapidly the blood must flow to deliver the oxygen to those muscles.
Dr. Blomqvist and his team of researchers will continue to investigate this very interesting change that occurs in space. The systems within the body are so interrelated that many other factors must be looked at in relation to this observed change in left ventricular volume. The physiological puzzle is just beginning to be put together. So many other pieces have yet to be found.