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centrifuge
The effects of acceleration on the human body, particularly the cardiovascular system, are serious enough that prospective space tourists need centrifuge experience and other tests before flying. (credit: USAF)

Human factors in commercial suborbital flight: What does acceleration do to the human body?

Because human tissues are viscoelastic (material properties that vary with strain rate), the response of the body to acceleration varies with duration of exposure. In general, acceleration pulses of 0.2 seconds or less are considered to be “impacts,” while acceleration durations of more than perhaps two seconds are considered to be “prolonged.”

During impact accelerations, acceleration tolerance increases as the exposure duration decreases. Consequently, the best indicator of injury potential for impact accelerations is “delta-V”, or impact-related speed change. For prolonged acceleration exposures, body fluid shifts become relatively important, and tend to dominate the deleterious effects of acceleration.

Much of the modern understanding of acceleration effects comes from the pioneering work of the late John Paul Stapp during his career in the United States Air Force. An annual conference devoted to improving the understanding of crash injuries is named after Dr. Stapp. As is the case with many pioneers, his personality was, to say the least, interesting, and tended toward irascible. Many of his remarks have become the stuff of legend. For example, during Congressional testimony in which he was asked to defend the experimental use of pigs and chimpanzees, he is quoted as saying “You wonder why I use hogs and chimpanzees? Well, man is somewhere between the hog and the chimpanzee. Some people are more like hogs; others are more like chimpanzees.” [Ref. 1] After observing that more USAF personnel were being killed in automobile crashes than in aircraft crashes, Stapp played a pivotal role in the adoption of automotive restraints and many thousands of lives have been saved as a direct result of his efforts.

We will initially consider the physiological effects of prolonged acceleration. Then, after some consideration of the effects of impact accelerations, we will consider the implications of these effects in various failure scenarios which may be relevant to commercial suborbital spaceflight.

The effects of prolonged acceleration

Prolonged linear acceleration effects are usually simulated in a laboratory environment by the use of large-radius centrifuges. The first reference I have found related to human centrifugation described a four-meter radius unit spun at 50 revolutions per minute located in Berlin’s Charite Hospital beginning in 1818. Interestingly, the device was used to subject mentally ill people to accelerations of up to 5 gravities, although the proposed therapeutic effect escapes me totally [Ref. 2].

Much of the modern understanding of acceleration effects comes from the pioneering work of the late John Paul Stapp during his career in the United States Air Force. As is the case with many pioneers, his personality was, to say the least, interesting, and tended toward irascible.

A typical sitting human’s blood pressure, measured at the level of the heart, ranges from a gauge pressure of about 120 mm-Hg (systole) to about 75 mm-Hg (diastole). Although these pressure units are considered archaic in physics and engineering, they are still used in medicine. Sea level atmospheric pressure is about 760 mm-Hg. The systolic blood pressure maximum is attained as the main chamber of the heart (the left ventricle) completes its contraction and ejection of blood into the main artery (aorta). The diastolic or minimum pressure is attained just before the next contraction of the heart. It is a function of both heart rate and the peripheral flow resistance as the blood flows out into the body. An additional circuit from the right side of the heart pumps blood to the lungs through the pulmonary artery. Systolic pressure on this side of the circulation is typically about 20 mm-Hg and the diastolic pressure is about 7 mm-Hg.

Why is this relevant to any consideration of acceleration effects? A significant fraction of the total blood flow (defined as the cardiac output) is directed to the brain and is necessary for the brain to function. In a sitting human, that blood must be pumped uphill with a corresponding loss of pressure. At the level of the brain (perhaps 45 cm above the heart), the arterial blood would have a hydrostatic pressure drop of about 35 mm-Hg. The blood flow through the brain is related to the pressure driving the flow. If arterial pressure at the level of the brain drops, so does blood flow through the brain. The pressure drop of a fluid column, including a blood column, is proportional to the height of the column, the fluid density, and the acceleration of gravity. Under acceleration, the effective gravitational term is altered.

Assume that our sitting human is accelerated upwards. This is the so-called “eyeballs down” acceleration or plus Gz. If the acceleration level is, for example, three times the normal acceleration of gravity (three times 9.8 meters/second2), a column height between the heart and the head of 45 cm will lead to an acceleration-induced pressure drop of about 105 mm-Hg at the brain instead of the typical one gravity drop of about 35 mm-Hg. If the flow resistance through the brain remains unchanged, then the brain blood flow would be reduced proportionally at +3 Gz compared to +1 Gz.

As the acceleration continues, the blood flowing downhill into the body from the heart, particularly into the abdomen and the legs, will tend to pool there because the normal venous return to the heart is impaired by the increased distance-acceleration product. This reduces the venous pressure at heart level from a typical value of perhaps 7 mm-Hg. The reduced venous pressure reduces the return flow of blood into the heart, so the blood pumped out of the heart is correspondingly reduced. The heart’s output per beat is reduced because of the reduced filling, and the output pressure in the arteries is reduced. This reduces the arterial pressure at brain level even more, and the process snowballs.

Because the arterial pressure in the retina of the eye (the sensory portion) is typically less than that in the brain, vision fails before consciousness is lost, the so-called “gray out” phase.

If the preceding were exactly true and no additional mechanisms were to come into play, the +3 Gz would become fatal as the brain blood flow spiraled to zero. However, the human body has some beautiful compensatory mechanisms that can be triggered. For example, as the heart’s output volume per beat is reduced, the heart rate, expressed in beats per minute, tends to increase. In addition, there are pressure receptors in various locations in the circulatory system. They exist as part of several feedback mechanisms which tend to partially abate the effects of the acceleration. The most dominant of these mechanisms, the carotid sinus reflex, requires about 5 seconds to fully engage. If the onset of acceleration is sufficiently slow, the +Gz tolerance can be increased by up to about one G by this reflex. Any detailed discussion of these various compensatory mechanisms is far beyond the scope of this series of columns. Interested (or masochistic) readers can refer to any of the many medical textbooks of circulatory physiology for more details.

Still, the arterial pressure at the brain level is reduced as is the corresponding blood flow rate. A direct indication of this effect is the associated visual changes. Because the arterial pressure in the retina of the eye (the sensory portion) is typically less than that in the brain, vision begins to degrade at retinal systolic pressures somewhat below about 50 mm-Hg. Blood flow to the retina is reduced to symptomatic levels before brain blood flow. As a consequence, vision fails before consciousness is lost. This is the so-called “gray out” phase as peripheral vision progressively fails, central vision fails, and then consciousness is lost.

This circulatory effect is not the only problem faced by our sitting human as he or she is accelerated upwards. The more dense tissues of the body tend to be driven downwards. As a consequence, the liver sinks deeper into the abdomen, and the heart and great vessels also descend in the chest. The net effect of this process is to displace the diaphragm downward. This makes breathing progressively more difficult as +Gz acceleration increases. In addition, any useful activities performed by the arms, such as reaching for switches, etc., becomes progressively more difficult. At +2 Gz, a person experiences a distinct feeling of heaviness. By +3 to +4 Gz, there is a marked dragging sensation in the chest and abdomen, and it requires great effort to move. By +6 Gz, it is extremely difficult to reach overhead. Depending on a person’s physical condition and stature, consciousness is generally lost at between +3 and +5 Gz in a sitting position. As blood pools in the legs, muscle cramping in the calves can occur. In fact, some of the blood can leak out of the smaller vessels and cause petechiae (broken capillaries) in the feet and legs.

Impacts on spacecraft design

What is the spacecraft designer to do? There are two approaches to abating these effects. The first is to decrease the uphill distance between the heart and brain by tilting the seat back, and the second is to apply counter pressure against the legs and abdomen to retard blood pooling there. The counter pressure can be generated intrinsically by grunting and straining the voluntary muscles to temporarily raise blood pressure and reduce blood pooling, or it can be generated extrinsically by a so-called G-suit.

If the rider is stretched out so the feet, head, and heart are all at the same level, the vehicle diameter will have to be more than 1.8 meters or so. This puts constraints on vehicle mass, air drag during the early part of the flight, and loading under cabin pressure.

Taking the tilted seat back approach to the maximum, the human will have the acceleration directed back to front (“eyeballs in”) or +Gx instead of eyeballs down or +Gz. With the heart and brain at more or less the same level, the hydrostatic pressure loss at the brain is largely abated. By bringing the legs closer to the heart level, blood pooling in the lower extremities is reduced. However, this doesn’t completely solve the problem. In general, the tolerance limit of +3 to +5 Gz is increased to perhaps +8 to +10 Gx. In this instance, tolerance is usually limited by chest pain and shortness of breath.

From the point of view of the designer, a sitting 1.8-meter (six-foot) human has a height of about 142 cm (56 inches). If he or she is going to ride in a vehicle in which the primary acceleration is along the vehicle’s long axis and is +Gx for the seated occupant, the vehicle diameter will have to be at least 1.5 meters (5 feet). If the rider is stretched out so the feet, head, and heart are all at the same level, the vehicle diameter will have to be more than 1.8 meters or so. This puts constraints on vehicle mass, air drag during the early part of the flight, and loading under cabin pressure during ascent as discussed in a previous column.

A typical winged suborbital vehicle concept would entail longitudinal acceleration during the motor burn of about one Gx to about 4 Gx during a burn of perhaps 3 to 3 1/2 minutes up to an altitude of about 50,000 meters. The ballistic phase of the flight would provide several minutes of microgravity. During re-entry to sensible atmosphere, dynamic loads would maximize at about 4 G (+3.7 Gz and –1.5 Gx).

What are some of the medical implications related to suborbital passengers as compared to crew? Military pilots of high performance aircraft are exposed to monitored acceleration both for familiarization and for screening purposes. Commercial spacecraft passengers are to have some type of medical screening yet to be determined. FAA’s Office of Commercial Space Transportation (AST) recommends collection of a medical history and having a physician sign off on the passenger.

I submit that this is inadequate. All passengers should be monitored and exposed to an acceleration profile similar to the proposed flight both for familiarization and for medical screening. The overall effects of +Gz, and, to a lesser extent, +Gx, are very well documented in healthy populations and result in markedly increased heart workload and oxygen demand.

Heart rate increases and the vascular return pressure is reduced (decreasing preload) under acceleration. This basically starves the pump by decreasing filling of the atria during diastole. Muscle straining, grunting, activation of an anti-G suit, all increase resistance to circulation, which tends to drive up the systolic pressure. If this process isn’t in exact synchrony with G loading, the heart afterload (peripheral flow resistance) fluctuates—possibly wildly.

The effects of these events on the heart are not necessarily detected on an electrocardiogram, whether it is obtained during resting conditions or during exercise. Treadmill exercise protocols do not drop preload or cardiac output. Peripheral resistance to blood flow is decreased and output from the heart increases with aerobic exercise on a treadmill. Therefore, a treadmill test (stress electrocardiogram) will not assure that an individual can safely experience prolonged acceleration stresses because it does not develop analogous stresses.

If a passenger candidate develops electrocardiographic abnormalities during centrifugation, the centrifuge can be stopped and, most likely, the irregular heart rhythm will revert to normal beating. If this problem manifests itself early during the acceleration phase of a flight, it can be life-threatening unless the flight is aborted.

Abnormalities of heart beat (arrhythmias) occur during acceleration. In a series of 1,180 centrifuge training sessions involving professional aeromedical course attendees at the USAF School of Aerospace Medicine, 47 percent resulted in arrhythmias. Of these, 4.5 percent resulted in (or should have resulted in) termination of the sessions. “Centrifuge training can provoke serious dysrhythmias in ostensible healthy individuals,…” [Ref.3]. These arrhythmias can occur in prescreened individuals. For example, in a series of 195 male fighter pilots, Hanada and coworkers found relatively harmless physiological variant responses in one-third to one-half of the subjects, but also found a rate of 2.6 percent ventricular tachycardia, 1.5 percent paroxysmal supraventricular tachycardia, and 0.5 percent paroxysmal atrial fibrillation, all of which they considered indications to stop centrifuge training and initiate further medical studies [Ref. 4].

If a passenger candidate develops electrocardiographic abnormalities during centrifugation, the centrifuge can be stopped and, most likely, the irregular heart rhythm will revert to normal beating. If this problem manifests itself early during the acceleration phase of a flight, it can be life-threatening unless the flight is aborted. Remember that the reported incidence of these potentially dangerous findings is perhaps 4 to 5 percent of otherwise healthy people. Absent monitoring electrocardiographic activity of all passengers during flight, there is no effective way to gain the information necessary to abort the flight. The potential result is a dead passenger if he or she goes into complete cardiac arrest during the motor burn of up to several minutes. Remember that the vehicle is committed to the ballistic phase of the flight for a possibly prolonged period of time as well.

A lot of older men take aspirin daily to reduce potential clotting in diseased arteries. If that person’s blood pressure increases significantly during acceleration, the result can be bleeding. If that occurs in the brain, it is called a hemorrhagic stroke and can be catastrophic.

Previously, I alluded to the various compensatory mechanisms to maintain blood flow to the brain. As a general rule, the effectiveness of those mechanisms is reduced with increasing age. The acceleration tolerance characteristics of the population matching the demographics of potential paying suborbital spaceflight passengers have not been well defined.

What about negative accelerations, either –Gz (“eyeballs up”) or –Gx (“eyeballs out”)? The basic reasoning described above holds except the blood pressure in the brain tends to be higher than in the heart. Rather than the risk being loss of consciousness from inadequate brain blood flow, one risk is of hemorrhagic stroke from blood leaking out of vessels in the brain. Besides, for negative accelerations, the passenger will be hanging from his or her safety harness rather than be pushed into the seat with attendant discomforts associated with the concentrated loads under the straps.

The next column will deal with short duration accelerations or impacts and expose the reader to an extreme skydiving experience.

References

  1. Man in Flight: Biomedical Achievements in Aerospace. E. Engle & A. Lott, Leeward Publications Inc., Annapolis, Maryland, 1979, page 103.
  2. Op. cit., page 195.
  3. Incidence of Cardiac Dysrhythmias Occurring During Centrifuge Training. I. McKenzie & K. Gillingham, Aviation, Space, and Environmental Medicine, Vol. 64, No. 8, pp. 687-691, Aug. 1993.
  4. Arrhythmias Observed During High-G Training: Proposed Training Safety Criterion. R. Hanada et al., Aviation, Space, and Environmental Medicine, Vol. 75, No. 8, pp.688-691, Aug 2004.

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