V. Cardiovascular Function During Exercise

An exercise stress test can also be used for research purposes using healthy individuals to determine the limits of aerobic capacity. The anal common denominator in athletic events is what the muscles can do. But a primary measure of muscle performance is endurance. To a great extent, endurance depends on nutritional support for a muscle. The most important determinant of nutritional support to a muscle is how well the heart and lungs can supply the muscle with oxygen when the muscle energy demands are at their maximum level.

What happens to the ability of the cardiopulmonary system to supply the body with needed oxygen under physically stressful conditions in space? This question has tremendous implications for future missions, particularly as we are faced with the need to build a space structure for humans to live in while working on the next generation of space life sciences studies. We must determine if humans are able to work physically hard in space to build those structures and to work in them for a period of months. In addition, we would like to know how much an astronaut's work and exercise capacity is affected by orthostatic intolerance after the astronaut returns home.

The ability of the body to consume oxygen is a determinant of how much exertion a person can withstand. That is, the oxygen uptake capability of the body will determine how much endurance a body has and therefore how hard a body can work. (When the body is exercising at its maximum capacity, the oxygen uptake is known as VO₂ max where V = velocity or rate and O₂ is, of course, the symbol for molecular oxygen.) The oxygen uptake depends on two things: how well the respiratory system can ventilate (bring air into) the lungs so that there is enough oxygen to feed the blood, and how fast the heart can pump oxygenated blood to the muscles. How fast the heart can pump blood to the muscles also depends on two things: the heart rate and the stroke volume. Remember, heart rate x stroke volume = cardiac output. Therefore, a measurement of oxygen uptake (VO₂ max), cardiac output, and heart rate will give researchers the data to determine the heart's pumping ability and the respiratory system's ability to bring air into the lungs during exercise.

Dr. Blomqvist's team examined the exercise capacity of the astronauts before the flight, while in flight, and after the flight by putting the astronauts through a maximal exercise routine, or by assigning them the most exercise they can handle. He was interested in knowing if the changes that occur to the cardiovascular system in space affect the ability of the astronauts to exert themselves. The question he asked was whether or not the almost complete absence of gravity caused any impairment of the cardiac pump function and/or of major cardiovascular regulatory mechanisms under conditions of stress (exercise). In particular, his experiment examined cardiac output (CO), heart rate (HR), stroke volume (SV), and oxygen uptake at maximal exercise levels (VO₂ max).

Figure 32. A marathon runner has one of the highest levels ofendurance of any human being. Their hearts are up to 40% larger and arecapable of pumping 40% more blood than other very athletic individuals.

Each person's maximal exercise level is unique to just that person (Figure 32). Fitness level, age, sex, medical history, and chemical and hormonal variations are all factors that influence how much power (the amount of work performed over a certain period of time) a person can produce when exercising to the maximal limit. Therefore, to understand each astronaut's maximal exercise capacity, the level of power produced by each astronaut was measured at the same time that the cardiovascular measurements (VO₂ max. CO, HR, and SV) were taken. The astronaut's power output was determined preflight as a control (or baseline) value to serve as a point of comparison for the inflight and postflight values.

Power output is determined by measuring how much work the astronauts performed while exercising at their maximum level and then dividing that work by the time it took to do it. In physics terms,

Note that distance/time = velocity. For instance, velocity can be expressed in miles/hour.

Typical units that are used to describe work are: kilocalories, ergs, joules, BTUs (British thermal units), and foot-pounds.

Typical units that are used to describe power are: kilocalories/minute, ergs/second, joules/second (also known as watts), BTUs/hour, and foot--pounds/second. (Just as a note of interest, another unit that is probably the most commonly recognized power term by the general public is horsepower. Actually, horsepower was suggested as a unit to represent the power actually delivered by a horse, but one horsepower is really equal to 550 foot-pounds/second as well as 746 watts.)

Notice that the power units are basically the same as work units, except that they are divided by a time unit. Thus, again, power = work/time.

Figure 33. An astronaut can achieve maximal exercise levels using a bicycle ergometer in space.

How is power determined for each of the astronaut's exercise sessions? When exercising, the astronauts use a bicycle ergometer, which is simply a computerized exercise bike. The computer on this bicycle ergometer can determine the amount of power generated by the astronaut based on the force that the astronaut applies to the pedals (force) and how fast the astronaut is pedaling (velocity). The power (force x velocity) is recorded in units of watts.

As mentioned previously, the maximal exercise test was carried out by the astronauts before flight in order to establish their control or baseline measurements for comparison with inflight and postflight performance. The astronauts reached their maximal exercise level by using a bicycle ergometer that has been specially designed for use in space (Figure 33). Can you imagine trying to stay seated on an exercise bike without gravity? The space version of this piece of hardware has been equipped with special shoulder pads that are attached to the bike and that hold the astronaut down during exercise. Electrodes were attached to the astronaut to obtain heart rate information through use of an electrocardiograph. The astronaut also used the rebreathing technique (using the CRU and the mass spectrometer) described in the last section for the determination of cardiac output. Stroke volume was calculated using our famous equation, CO = SV x HR.

Oxygen uptake (VO2 max) was measured using a technique that is similar to the rebreathing technique used to determine cardiac output (we covered this in the last section). While exercising, the astronauts breathed in and out of a tube that was hooked to a gas analyzer mass spectrometer (GAMS). The GAMS was used to determine the fraction, or concentration, of various chemicals in the air. This GAMS was able to measure the concentration of oxygen in the air (the amount of oxygen molecules in a fixed volume of air) that the astronaut breathed in and compare that with the concentration of oxygen that was breathed out. The idea is simple: the concentration of oxygen breathed in minus the concentration of oxygen breathed out equals the amount of oxygen that was consumed by the astronaut, or in - out = consumption

Thus, the spectrometer measured how many liters of oxygen were consumed by the astronaut out of the total liters of air that the astronaut breathed in, to yield the number of liters of oxygen/liters of air.

At the same time that the oxygen was measured, the rate of ventilation was determined using a turbine flowmeter that was inside the tube. As the astronaut breathed in and out, the flowmeter measured how many liters of gas were breathed per minute. Thus, it measured the flow rate of air into and out of the astronaut's in units of liters of air/minute.

Let's do a some dimensional analysis to figure out the units for oxygen uptake:

Let's take a look at the results. The inflight results were somewhat surprising since they do not differ significantly from the preflight values (Table 7). This suggests that the maximal systemic (referring to the blood flow around the body) oxygen transport mechanisms are well maintained in space. This means that the heart's pumping capability is preserved in space even though the cardiovascular system is changing and adapting to the space environment. It also means that the pulmonary contribution to oxygen delivery around the body is still in good shape.

Now, look at the postflight data. This is where the effects of space flight become clear. It is when the astronauts return from space that the cardiovascalar deconditioning becomes evident. As you can see, there was a significant decrement in performance postflight compared to preflight values. First of all, the astronauts were generally not able to reach the maximum power output during exercise postflight compared to preflight. Also, oxygen uptake during maximal exercise (VO₂ max) was significantly lower than preflight values. Why? Look at the cardiac output and stroke volume values. The heart is simply pumping much less fluid out with every heartbeat. This is certainly caused in part by the lower volume of blood and fluids that are flowing in the astronauts' bodies and, probably, by the smaller size of the heart.

The interesting point about the cardiac output reduction is that of the two variables that determine cardiac output (HR and SV), heart rate is not significantly different, but stroke volume is. The fact that heart rate remains about the same indicates that, although the body is trying to operate with much less fluid, at least the heart-pumping action has not been significantly affected by space flight. Therefore, the heart can still pump as fast under extreme conditions of stress as it did before the astronaut went into space. However, the fact that the stroke volume changes significantly suggests that the heart cannot pump as much blood with each beat during the postflight exercise stress compared to preflight.

Overall, it can be said that even though the heart pump seems to be operating fine, the body's ability to transport and use oxygen changed while the astronaut was in space. Not only was the stroke volume greatly reduced, but the oxygen uptake capability of the body (VO2 max) was also reduced. One reason seems to be that the body has less fluids to circulate around the body. Without an adequate oxygen delivery system, the body cannot exert itself as it did before traveling into space. The good news is that the astronauts regain their endurance after a few days or maybe weeks of being back on Earth.