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logic saline will not prevent deformation of intrathoracic structures is documented by thoracic roentgenograms of a dog at I G and at 6 G when in air and when breath- ing air in a water immersion respirator (Fig. 1) [8].

Prevent ion of deformation of intrathoracic structures during exposures to hypergravity environments can, how- ever, be achieved by replacement of the intrapulmonary gases by a physiologic liquid with specific gravity similar to body tissues and with the associated capabili- ties for maintenance of normal alveolar gas exchange.

The feat of physiologically sustained liquid breathing during high G exposures was accomplished on the Mayo human centrifuge by Sass and coworkers using a water immersion respirator and an ex- tracorporeal fluorocarbon oxygenator as- sembly (Fig. 2) 18-IO]. Prevention of physiologically significant deformation of intrathoracic structures and mainte- nance of normal arterial blood gases docu- mented, that, although impractical for operational use, this assembly provides nearly perfect protection against changes in the gravitational force environment, i.e., from the physiologic viewpoint, a nearly perfect anti-G “suit” [9]. The knowledge that liquid breathing, although required for prevention of deformation of the intrathoracic structures, is impractical operationally, has mandated recourse to less effective methods which are admit- tedly incapable of prevention of the pathophysiologic effects of hypergravita- tional environments on the heart and lungs.

R Consequently. protection against blackout in pilots by partial immersion in water was considered in Germany during the 1930s. Subsequent ly , fabrication of the initial G suits i n Canada, Australia. and the United States was initiated circa 1940 [ 1 1 - 141. These suits were based on the belief that prevention of the presumed decrease in arterial pressure at heart level due to cessation of venous return to the heart from dependent regions of the body would be a very effective means of i n - creasing the G tolerance of humans when sitting upright in the conventional pilot position.

The basis for this belief is illustrated in the left panel of Fig. 3. This diagram illustrates the hydrostatic pressure differences in the arterial and venous circulations of a pilot when exposed to gravito-inertial force environments of 1 and 5 G when i n the conventional upright sitting position. At 5 G, a pres- sure of about 250 cm H 2 0 is required in the venous system at foot level to main-

elevunte to Development of Anti-G Suits

If physical deformation

could be prevented

during acceleration,

the pathophysiologic effects

of sustained high G forces

could be practically

eliminated

A I R i M M E R S i 0 N

rain return of venous blood to the heart. Consequently, [he early anti-G suits i n the Allied Nations were designed to support return of venous blood to the heart from the dependent regions of the body, especially the legs and abdomen (Figs. 4 and 5 ) .

The water filled Franks Flying Suit in Canada [ 1 11, Cotton‘s Aerodynamic Suit for Protection of Pilots Against Blackout in Australia [ 12). and the Navy’s gradient pressure and pulsatile pressure suits [ 13,141, all of which were designed to support venous return to the heart from the legs and abdomen as would happen if the lower body were immersed in water, were fabricated with this objective in mind.

Proof on the Mayo human centrifuge that normal arterial pressures were main- tained at heart level during exposures to 5 G, and unexpectedly increased to very high levels at heart level if the exposures were sustained for longer than 5-10 seconds (Fig. 3, right panel), indicated that sufficient venous return for mainte- nance of normal or increased arterial pres- sures at heart level must be maintained under sustained high G, conditions, in- cluding aerial combat maneuvers [ 14- 161.

f I Gv

Y

4

WATER l M M f R S l O N

1. Thoracic roentgenograms of an anesthetized dog studied without thoracotomy while in the left decubitus position during simultaneous pressure recordings from multiple sites within the thorax. Panel A and C during spontaneous respiration in an air environment when stationary at 1 G! and during rotation of the centrifuge at 6 G,, respectively; panel B and C during controlled respiration of air in a water-immersion restraint system when stationary at 1 Gy and during rotation of the centrifuge at 6 G,, respectively. Identification of positions of the tips of catheter-strain gauge pressure transducer systems: PA, pulmonary artery; PAI, for pressure recording; PA2, for injection of circulatory indicators (indocganine green, 69% renovist); IA, left atrium; LPV, left pulmonary vein; Ao, aorta; PI, pleural catheters, PI?; lateral upper border of right pleural spaced PII , lateral lower border of left pleural space. Immersion in physiologic saline solution (right panels) did not prevent downward displacement of the heart and great vessels and concomitant overdistention of the lung in the non-de- pendent right hemithorax. (From Sass and coworkers [SI).

March 1991 IEEE ENGINEERING IN MEDICINE AND BIOLOGY 27

2. Top panel: Photograph of the fluorocarbon oxygenator water immersion respiratory assembly: A) Variable rpm dc motor; B) 8-inch finned aluminum nebulizing wheel directs high velocity spray of fluorocarbon to impact against the four walls of the chamber. The liquid is nebulized in a 100% oxygen atmosphere. Oxygen diffuses into and carbon dioxide diffuses out of the liquid droplets; C) Exhalation chamber. The valve which controls shunt chamber is mounted on outside wall at left end; D) Row of ingress ports for catheters on far side of respirator. The multiple physiologic pressures listed in the legend of Figure 1 were recorded via these catheters (8-10). Bottom panel: Schematic drawing of water-immersion respiratory and fluorocarbon oxygenator assembly. Dog is totally immersed in physiologic saline (37C) which completely fills the rigid chamber. Respiratory rate and tidal volume are controlled by the pump which adds and removes saline to and from the immersion chamber in a sinusoidal manner. Either room air or fluorocarbon, selected by valves, flows in and out of the lungs in response to the alternating positive and negative pressures generated over the entire body surface by the pump. With permission from: Sass DJ, Wood EH, Greenleaf JF, Ritman EL, Smith HC: Effects of breathing liquid fluorocarbons on regional differen- ces in pleural pressures and other physiological parameters. U.S. Government Special Publication SAM-TR-72-15: 1-173 (December) 1972.

This surprising finding back in the 1940s indicated that prevention of loss of vision, i.e., blackout, at levels of acceleration of greater than the normal relaxed G, tolerances of about 4-5 G required some means of producing an increase in arterial pressure at heart level to hypertensive levels throughout such exposures [ 14- 181. Consequently, G suits designed primarily to produce an increase in arterial pressure were designed and tested (during 1942- 43), on the Mayo human centrifuge. These so-called arterial occlusion suits (Figs. 6 and 7) produced increases in Gz tolerances of about 3 G, more than double the value obtained by the presumably ideal water filled Franks suit or by actual immersion of humans in water up to the xiphoid process of the sterum (Fig. 6, left panel) [ 16-20]. Furthermore, voluntary methods of increasing arterial pressure, particularly muscular positive pressure respiratory straining maneuvers, were ex- plored. These provided a self-protective

means of increasing Gr tolerance, a find- ing which was repor ted by von Diringshofen of the German Air Force in the 1930s.

The so-called M- 1 self-protective maneuver , a n d t h e s i m i l a r L-1 maneuver, both of which are still used today, were perfected and tested on the M a y o c e n t r i f u g e i n e a r l y 1943 [ 14 ,17 ,21 ,22 ] . This capabi l i ty to produce an increase in arterial pressure at heart level and the associated increase in G tolerance by use of the L-l maneuver is documented in Fig. 8.

Although human centrifuge studies and confirmational tests in-flight indi- cated that individuals trained in these maneuvers can achieve higher levels of protection than provided by venous sup- port type G suits, the M-1 and L-1 maneuvers were considered at that time to be too distracting and fatigue produc- ing for practical use by fighter pilots. Furthermore, there was lack of pilot ac-

ceptance of the inconvenience of replac- ing their flight uniforms by a G suit, which was somewhat bulky and uncomfortable. In addition, there was the realization that a G suit, which provided 1 G protection would be adequate for the propeller driven planes of the World War I1 era. These factors initiated a radical change in G suit design in 1944.

T h e r e s u l t was t h e s i m p l i f i e d interconnected five-bladder system (Fig. 9, left panel), which was designed to be inflated to a single pressure. This bladder system could be built into any type of garment that would allow reasonably adequate transmission of pneumatic pressures from within the bladder system to the underlying skin surfaces of the calves, thighs, and ab- domen. The system was contained in an individually or lace-adjusted fitted torso and leg segments of a trouser or coverall type garment (Figs. 9 and 10). Human centrifuge and in-flight tests confirmed that inertially controlled automatic inflation of the bladder sys- tem to a pressure of about 50 mm mer- cury per G at accelerations above the 1.5 G valve opening level would pro- vide an average protection of 1.5 to 2.0 G [14,17]. The effectiveness of such a su i t i n producing a n increase in arterial pressure at heart level and an associated increase in G tolerance are documented in Fig. 11. However, the suit of choice by fighter pilots was the considerably less effective skeleton type garment (Fig. 10, right panel), which could be conveniently donned and worn over a standard flight uniform. This suit, designated as model (3-3, which provides an average protection of 1 G is, with minor modifications, still used by current fighter pilots. The fact that until quite recently, use of this G suit, which provides only 1 G protec- tion, practically eliminated aware- ness of the life and plane destroying G induced loss of consc iousness problems that are vitally important in today’s f ighter planes [23], re- quires more of an explanation of its acceptance by pilots than jus t its convenience.

The three major factors, in addition to convenience, which maintained the model G-3 as an adequate G suit for more than three decades after World War I1 are: 1 ) The cerebral ischemic cerebral reserve time; 2) The inability, especially of propeller driven fighter planes, to sustain without loss of al- titude, accelerations in the 5 to 7 Gz G-LOC range for periods longer than this reserve time; and 3) The use of M-1 type positive respiratory pres- sure, muscular straining maneuvers to supplement the protection afforded by

28 IEEEENGINEERING IN MEDICINE AND BIOLOGY March 1991

a G suit. The additive effect of such maneuvers is especially evident when a relatively ineffective G suit, such as the G-3, is used.

erebral Anoxic Reserve Time The fact that in humans no ap- C parent loss of cerebral cognitive

functions occurs for an average period of about 7 seconds after cessation of cerebral blood flow was not generally recognized until the 1943 cervical arterial occlusion studies in volunteer p r i s o n i n m a t e s by R o s s e n and coworkers [24]. These studies were not widely appreciated by most avia- tion physiologists until 40 years later [25-271. That the cerebral reserve time was an important de te rminant of human G tolerance, was documented by measurements of arterial blood pressure at head level obtained during

The early anti-G suits

in the Allied Nations were

designed to support return

of venous blood to the heart from

the dependent regions

of the body, especially

the legs and abdomen

P L R C L R

Ea 41m

3. Left panel: Diagrammatic representation of hydrostatic pressures in vascular system of a 34-year-old man in upright sitting position at 1 G, and during headward acceler- ation of 5 G. (Left) Average position of pilot in present-day aircraft. (Center) Diagrammatic representation of vascular system of seated pilot at 1 G. (Right) Illustra- tion of the 5-fold increase in hydrostatic pressure differences in the arterial and venous circulations imposed by 5 G of headward acceleration. If arterial pressure at the heart was 120 mm Hg, it would be zero at the base of the brain, and 370 mm Hg at the heels. A venous pressure of 250 mm Hg would be required to return blood from the heels to the heart. Right panel: Sequence of physiologic events during exposure of a healthy man to headward (positive) acceleration of 5 G for 15 seconds on a human centrifuge. The recordings were made by two photokymographic cameras, one mounted in the centrifuge cockpit (bottom) and one in a recording room adjacent to centrifuge (top). Black acceleration line indicates the magnitude of headward acceleration in G units. Simultaneous recording of acceleration, indicated as G in top, serves to synchronize the two recordings. Length of black lines designated as PLR and CLR indicates subject’s reaction times to light signals in peripheral and central fields of vision, respectively. Note initial period of progressive failure during which there are, in order of occurrence, an increase in heart rate, decrease in blood pressure at head level, loss of blood volume in the ear as measured by ear opacity, reduction in amplitude of arterial pulse in the ear, and loss of vision (blackout). Then note period of compensation during latter half of exposure, in which blood pressure at heart level increases to hypertensive levels, circulation to the bead improves so that ear pulse recovers, blood returns to the ear, heart rate slows, and vision is restored, in spite of continued acceleration. (Based on data from Lambert and Wood, Fed. Proc. 1946; 5-59 Safe J . 1989; 19:39).

Morth 1991 IEEE ENGINEERING IN MEDICINE AND BIOLOGY

World War I1 on the Mayo human centrifuge (Fig. 12) [15,28]. Note that al- though arterial pressure at eye level fell to zero at the onset of the 5 G, exposure, central vision was maintained for 6 seconds. Slumping in the seat due to muscular relaxation, indicative of loss of consciousness, did not occur until several seconds later. This recording, along with many others, documents that Gz induced zero arterial pressure at the level of the base of the brain for periods of less than 6 seconds, i.e., 1-2 seconds less than the average cerebral reserve time, can be endured by most humans without loss of consciousness. The average 6.5 second anoxic cerebral reserve time explains how and why: 1 ) most pilots can attain short duration ( sec) very high accelerations without loss of consciousness; 2) many fighter pilots, especially during World War 11, believed they had a very high G tolerance and; 3) some pilots believed that a G suit was unnecessary.

imited Power and Consequent Restricted Sustained G Capabilities of Propeller 1 Driven Fighter Planes

The belief by World War I1 fighter pilots that their G tolerance was in the 9 G range resulted from the power limitations of the propeller driven fighter planes of that era. The lack of sufficient power made it prac- tically impossible, without unacceptable loss of altitude during aerial combat, to sustain accelerations of more than 5 to 6 G for longer than several seconds [29]. Since this period is less than the average cerebral ischemic anoxic reserve time, most pilots experienced only transient los- ses of peripheral or central vision during aerial combat maneuvers. When peak ac- celerations of, for example 9 G, were at- tained for several seconds during pullout from high speed dives, pilots did not real- ize that they were “riding the razor’s edge,” i.e., one to several seconds away from G-LOC and the 12 or more seconds loss of control which inevitably follows [ 14,24-271.

se of M-1 Type Positive Respiratory Pressure Muscular Straining Maneuvers U In the early 1970s, Dr. Sidney

Leverett and colleagues at Brooks Air Force Base realized that the 1 G protection afforded by the standard G-3 suit would be inadequate for pilots of upcoming, very high speed and maneuverable fighter planes. This new generation of planes was capable of sustained accelerations in the 9 G, range for many seconds and possibly minutes. A series of multifaceted pio- neering studies on their human centrifuge began, and continues [22,30-331. These investigations confirm the World War I1 studies, which documented the additive protection afforded by combined use of an

2 9

4a 4b

~ Sb

sa 4c

7b

6b

Figures 4-7. Anti-G suits developed and tested prior to and during World War 11. Fig. 4: Abdominal Bladder, J.R. Poppen, U.S.N., 1930s (a); W.R. Franks Water Filled Flying Suit, RCAF, 1940 (b); F.A. Cotton Aerodynamic Suit, WAF, 1941 (c). Fig. 5: Anti-G suits in World War 11. Berger Gradient Pressure Suit, USN Model Z-1, USAAF Model G-1,1942 (a); Ferwerda Pulsatile Pressure Suit, Berger Bros., USN, 1942 (b). Fig. 6: "The Bath- tub," Mayo Aeromedical Unit, "The Protection Afforded the Human by Hydrostatic as Compared to Pneumatic Anti-G Devices, Comm. Aviat. Med. Report No. 207, Nov. 12, 1943 (a); Progressive Arterial Occlusion Suit, Mayo Aeromedical Unit, 1942 (b). Fig. 7: Arterial Occlusion Suit, Mayo Aeromedi- cal Unit, 1943 (a); Cut-a-way Single Pressure Suit, Mayo Aeromedical Unit, USAAF "Evaluation of Anti-G Suits" Report No. 1, Sept. 29,1943 (b).

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anti-G suit and posit ive pressure respiratory straining maneuvers. They also confirm the tactically important fact that, by use of these maneuvers in con- junction with the G-3 suit, highly trained centrifuge subjects can maintain vision during sustained exposures to 9 GI [ 14.- 17,341. Training of fighter pilots in the com-

bined use of the G-3A suit and one or more variants of positive respiratory pressure self-protective maneuvers on the human centrifuge at Brooks Air Force Base (and more recently on a similar machine at Holloman Air Force Base) makes it possible for near- ly all trained pilots to maintain vision and consciousness during sustained exposures to 9 G I [ 3 5 ] . The intense physical effort and the consequent mission limiting fatigue associated with prevention of G-LOC by these means has. however, restricted its tactical utility. Furthermore, in spite of combined use of the currently standard G-3A anti-G suit and assisted positive pressure breathing, i.e., the Combat Edge Technology, inci- dents of G-LOC will continue to occur as long as fighter planes are designed for pilots in an upright sitting or partially supinated position [36]. Consequently, current efforts are underway in several laboratories to replace the G-3A by a more effective G suit 1371. Presumably, use of a more effective G suit con- comitantly with assisted positive pres- sure respiratory straining maneuvers would reduce the probability of in-flight incidents of G-LOC.

evelopment of more eff ectiie G suits Theendofworld WarIIandthepracti- D cal elimination of G-LOC as a tactical

problem by the use of the G-3A suit at that time. halted most efforts to develop better

8. Recordings obtained during 1946 illustrating the protection against blackout by use of successive brief Valsalva maneuvers performed with the glottis open and lips closed. Intrapulmonic pressure was monitored via a catheter held between closed lips using an air-tight strain gauge manometer. This maneuver was subsequently designated as the L-1 and is currently taught to all fighter pilots [35].

G suits, especially in civilian laboratories. Nevertheless, Mr. David M. Clark and Company, with continued guidance from acceleration physiologists, designed and fabricated new. improved anti-G suits during the period from 1946 to 1955 [38,39]. Restudy oftwoofthe most effec- tive of these “new” G suits. the use of a bioassay procedure based on multiple ob- jective circulatory and subjective visual endpoints that provide objective quantita- tive measurements of the protection af-

9. Photographs and diagrams of simplified single pressure pneumatic bladder systems (left and middle panels) and a trouser-type anti-C suit (right panel), the bladder system of which (left panel) was inflated by an inertially controlled valve (extreme right) which opened automatically at 1.5 G and incremented the pressure in the bladder system by about 50 to 75 mm Hg/G above this level.

March 1991 IEEE ENGINEERING IN MEDICINE AND BIOLOGY

forded by the different types of G suits developed during World War 11, and the factors which determine their protective value [ 14-2 1,37-401 is a logical starting place for current efforts in this regard.

The 1947 Air Force CoverciII Suit The United States Air Force model G-4A coverall suit, developed in collaboration with Ernest E. Martin, afforded an average protection against visual symptoms of 1.9 G as compared to 0.8 G provided by the standard Air Force cut-a-way G-3A when tested in the same 13 subjects on the human centrifuge at Wright-Patterson Air Force Base (Fig. 13) 1411. On the basis of subsequent service trials during 1948 by 25 fighter pilots, the G-4A was deemed superior to the G-3A both in protection and from the standpoint of comfort and convenience. Consequently, it was recommended that “the G-4A suit be made a standard item of equipment and the present USAF type G-3A suit be placed on ’limited standard’ basis” [42]. For reasons that are unknown, perhaps pilot preference when not in flight, this recommendation was never implemented.

l t ie Full-Covercige Half-Suit A novel method for applying uniform pneumatic pressure to all surfaces of the body below thoracic level was devised by the David Clark Company in 195 1. The principle used is illustrated by a diagram from a U S . Patent (Fig. 14). An excellent

31

I I

10. Photographs of three types of single pressure anti-G suits used during World War 11. The trouser and coverall type suits (left and middle panels) which provided a protection of about 1.5 G were supplanted by the cut-a-way type suit (right panel) which could be worn over a standard flight uniform. The average protection of about 1 G provided by this suit was less than that provided by more closely fitted suits but was adequate for the propeller driven planes of that era. Because of pilot preference, this suit is still used today.

comparative study of the increases in G tolerance and associated decreases in heart to brain distance and increased venous and arterial pressures produced by inflation of this suit (a lower body full coverage bladder type) and a conventional G-4A five-bladder suit, was reported in 1953 by Sieker and co-workers [43]. The full pressure anti-G suits for the 1 1 ex- perienced subjects that participated were not only found to be more effective but, in general, were more comfortable when in- flated during acceleration than the G-4A. The concluding statement of this Air Force report of 37 years ago includes the recommendation that “development, test- ing and modification of full pressure anti- G suits be continued in an effort to provide a more effective mechanical means for protection against positive accelerations in the upright seated position.” Additional tests of the Clark full pressure lower body suit in 15 experienced centrifuge subjects were completed in the Naval Air Station human centrifuge in 1954 1441. The average protection against visual symp- toms of more than 3.4 G when inflated to 75 mm Hg/G starting at 1.8 G is, to this author’s knowledge, greater than the average protective value for any other pneumatic suit that has been tested before or since. At accelerations of greater than 5 G, however, the discomfort in the upper abdomen and lower thoracic regions caused by inflation of the suit to pressures in excess of 200 mm Hg made respirations difficult or impossible, and limited the tests to levels ranging from 6.2 to 9.1 G for the five subjects that experienced no visual symptoms when wearing the suit.

The possible danger of the not fully un- derstood mechanisms responsible for the many electrocardiographic irregularities recorded during G exposures with this suit precluded tests at higher accelerations in these five subjects. Tests on the other nine

32

subjects that experienced only slight to moderate dimming of peripheral vision during the highest G tested (average was 7 G for all 15 subjects) were also discon- tinued.

In spite of the encouraging tone of this U.S. Navy report and a level of protection that had not been achieved until very recently, no additional centrifuge or in- flight studies of this full coverage pneumatic pressure stratagem have been carried out [45]. This current 1990 study of a lower body, uniform pressure suit does not mention the nearly half-century earlier reports [43,44] or the 1956 U.S. patent (Fig. 14) of a practically identical garment.

Although it is possible for trained and physically fit individuals to maintain vision and consciousness during sustained exposures to accelerations greater than 9 G, using a G suit and self-protective maneuvers, there are potential dangers of severe functional and anatomic injuries during exposures to such extreme in- creases in the gravito-inertial force en- vironment [5,46-491.

athophysiologic limitations of Protection Against Gz Acceleration by External P Garments

The potential danger of anatomic damage due to the very high intravascular pres- sures and the pressure imbalances within the air contains lungs, which are un- avoidably associated with maintenance of vision and consciousness at high sus- tained levels of acceleration, must be kept in mir.d [46-491. In addition, bradycardia and cardiac arrhythmias are frequently as- sociated with pressurization of moderate- ly effective G-suits at 1 G. Furthermore, instances of bradycardia have been reported when using highly effective full coverage suits [17,47-491.

Presumably, the bradycardia and cardiac

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arrhythmias observed under these cir- cumstances are due to depressor reflexes from the aortic arch, which are not op- posed by pressor reflexes from the supe- riorly located carotid sinus receptors. This situation pertains when the G-suit mediated, very high arterial pressure at aortic level, is sufficient to maintain a near normal arterial pressure at head level (as indicated by the maintenance of vision). Hence it results in absence of low pressure stimulated pressor reflexes from the carotid sinus, which usually occur and produce the tachycardia normally ob- served during G, exposure. The pos- sibility should not be disregarded that exacerbations of such cardia arrhythmo- genic mechanisms could provoke life threatening arrhythmias during testing of full coverage suits capable of provid- ing the very high aortic pressures re- quired for maintenance of vision at 9 to 12 GI [47-491. The additional fact that 5 minute exposures of anesthetized dogs to 8-10 G, when immersed i n water to the second rib were uniformly fatal in the six animals studied is a further danger sign [50].

Autonomic blockade by tetraethyl ammonium chlor ide prevents the bradycardia during inflation of G suits i n healthy humans, and supports the likelihood that the aberrations in cardiac rhythm are neurogenic in origin 1511. Further exploration of the causes and possible methods ofprevention of G suit induced cardiac arrhythmias is quite certainly indicated prior to testing very effective G suits at sustained accelera- tions of I O G , and above. There is high likelihood that in-flight incidents of G- LOC will continue to occur in spite of the use of the Combat Edge Technology. In addition, there is a possibility of serious functional or residual anatomic injuries during exposures to high levels of acceleration. Therefore, additional methods for amelioration or prevention of the G-LOC problem should be ex- plored.

evelopment of a Premonitory G-LOC Warning System Based on Real-Time Monitoring of Arterial Opacity Pulses Within the Ear

Recent investigations of fighter plane crashes indicate that since 1978, when the Air Force became cognizant of the cause of these accidents, 18 pilots and many more aircraft have been lost as a result of G-LOC. Each pilot costs our country about $2 million dollars to train. Each F16 or F15 fighter costs $16 to $25 million dollars to build.

Fighter pilots have no warning device that tells them that they are about to lose consciousness. Consequently, if a p ra c t i c a 1 1 y ac c e p t a b 1 e f oo 1 proof premonitory G-LOC warning device

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March 1991

could be developed and used by fighter pi lots t ra ined in i t s forewarning capabilities on a human centrifuge, many incidents of G-LOC could be avoided and; when CLOG did occur, activation of an auto-recovery system would prevent most terrain impact crashes.

A photoelectric earpiece for on-line monitoring and objective recordings of decreases in blood content and amplitude of arterial pulses in the pinna of the ear was perfected and used very successfully for noninvasive measurements of G tolerance during extensive human centri- fuge and in-flight studies during the 1940s [ 14-20]. Decreases in blood con- tent of the ear were closely related to G-induced decreases in arterial pressure at head level, and the consequent decreases in cerebral blood flow that cause loss of vision and consciousness

during sustained exposure high G, ac- celerations. The decrease of systolic arterial pressure to zero at head level during Gz acceleration always resulted in simultaneous loss of the ear pulse in a real-time, beat-to-beat sequence. Fur- thermore, decreases in amplitude of the ear pulses to zero were uniformly fol- lowed within 4 to 8 seconds by loss of vision and/or consciousness (G-LOC)

Our tactical fighter pilots are now re- quired to undergo centrifuge training. However, the only monitoring of physio- logic data consists of electrocardiog- raphy [35] . The use of an ear-pulse monitor during this training would enable the pilot to use the biofeedback of his own ear pulse to confirm, in centrifuge and subsequent in-flight training situations, its premonitory warning capabilities and to develop a “personally tailored” strain-

[14-201.

r

11. Simultaneous recording of radial arterial blood pressure at the level of the head and at the level of the heart during exposure to 3 and 5 G without protection and at 5 G when using an M-21 pneumatic anti-blackout suit.

12. Effect of headward accelerations of increasing magnitude on: Arterial pressure at head level, subjective symptoms, and other hemodynamic variables in a healthy human volunteer. PLR and CLR are abbreviations for peripheral and central light responses, respectively. The degrees of visual impairment are listed at the bottom of each record (PLL indicates loss of peripheral vision; blackout indicates loss of both peripheral and central vision). Note the progressive increase in the magnitude of the alterations in the various hemodynamic variables with increasing levels of acceleration; the correlation between arterial pressure at head level and the changes in ear opacity, ear opacity pulse, heart rate, and the occurrence of visual symptoms; and that after detection of loss of the ear opacity pulse (right-most panel), there would be time to activate an automatic plane control take-over system prior to or coincident with pilot loss of consciousness.

Morch 1991 IEEE ENGINEERING IN MEDICINE AND BIOLOGY

ing maneuver that maximizes the G tolerance. The biofeedback potential of such a system during high Gz centrifuge training would expedite pilots’ capabili- ties to avoid impending G-LOC by ter- minating the exposure or intensifying respiratory straining efforts. This training and, if the ear pulse system was opera- tionally qualified, would enhance most pilots’ abilities to avoid impending G- LOC in-flight. With auto-recovery, it would reduce the number of terrain im- pact crashes when G-LOC did occur [52].

The output, i.e., the electrical signal, from such a photoelectric ear transducer has two components. First, there is the higher frequency (2-5 Hz) changes in out- put associated with each arterial pulse. Second, there are slow (less than 0.5 Hz) changes associated with changes in the total volume of blood in the ear.

Excellent arterial opacity pulses can be obtained using a narrow, 0.5 to 5 Hz, bandpass system. It is thus easy to secure recordings of pulses that are practically free of high frequency noise and inde- pendent of slow frequency changes in total blood content.

The use of currently available, very small solid-state light sources and detec- tors allow a sensor package small enough to be contained within a hearing aid type ear mold such as worn by thousands of people for many hours. These devices are relatively comfortable, individually fitted, and remain in a fixed position relative to the underlying skin surfaces.

If human centrifuge testing supports the feasibility and probable inflight value of such a system, it is technically feasible to eliminate all hard-wired connections to the pilot. In this system, receiving and transmitting capabilities can be incorpo- rated in the within-the-ear system. Signals would be received remotely by a minia- ture data processing infrared-linked as- sembly in the pilot’s helmet.

This data processing system should be programmed so that a loss of the ear pulse sustained for three seconds would activate a premonitory G-LOC auditory or visual warning system. The system would ac- tivate an automatic plane control takeover system in the absence of an appropriate response by the pilot (and assuming real- time flight parameters indicated potential terrain impact). Use of transducers in both ears, and the use of independent data processing systems, would increase the fail-safe characteristics of this device. In- corporating it into an integrated helmet system with the latest noise cancellation, three-dimensional sound localization and other technologies would be a potentially highly valuable developmental effort. Admittedly, however, neither the Combat Edge Technology andor a physiological- ly based G-LOC avoidance monitoring system will eliminate all incidents of G- LOC.

evebpment of a Horizontal Prone Position Couch with a F&rOptic, Video Assisted Omni-Directional Pilot Surveillance System

D Recourse to a fully horizontal position is

33

Figure 13. Photographs of an improved, more closely fitted coverall-type anti-G suit developed in 1946-47 by the David M. Clark Company and the Aeromedical Laboratory at Wright-Patterson Air Force Base. This suit provided about twice the protection afforded by the standard cut-a-way suit [41,42]. However, because of pilot preference for the cut-a-way suit which could be easily put on over a standard flight uniform and removed after flight, this more effective suit never achieved widespread use.

the only certain and biophysically safe method of eliminating G-LOC in-flight. Consequently, if pilot capability to withstand sustained GZ accelerations in the 9 to 15 G range is tactically impor- tant, development of a horizontal posi- tion cockpit with pilot acceptable surveillance and flight controls should be undertaken [23]. Development of an advanced horizontal position (preferab- ly prone) cockpit with optical electroni- cally a ided omni-directional pilot surveillance capabilities would allow full exploitation of all aspects of the advanced high speed maneuvering capabilities of current and future fighter planes. Just as important, such a system would allow our fighter pilots to discard all anti-G equipment and associated dis- t r a c t i n g and f a t i g u e producing respiratory straining maneuvers with consequent improvement in their men- tal and physical efficiency particularly during prolonged combat maneuvering [39] . Quite cer ta inly, a squadron equipped with a pi lot acceptable horizontal prone position cockpit sys- tem would have a spectacular advantage in air-combat situations [53] .

hysidogic and Anatomical Considemtions Human centrifuge tests in Ger- P many in the 1930s documented

that subjects in the horizontal prone or supine position could withstand sus- tained accelerations of 15 G without loss of consciousness. Similar results during sustained accelerations using the horizontal prone position or increasing angles of backward tilt indicate that a fully horizontal prone or supine position is required for elimination of G-LOC during sustained exposures to accelera- tions in the 10 to 15 G range. Chimpan- zees can survive 60 second exposures to

40 G when in the fully supine or partially prone positions [7].

The ventral surface of the heart and the dorsal, internal surface of the sternum, are in close anatomical ap- posi t ion. Consequent ly , displace- ment of the heart within the thorax and dis tor t ion of associated lung parenchyma i s minimized during high G exposures when in the prone position [23,54].

In contrast, because the dorsal sur- face of the heart is widely separated

from the parietal surfaces of the paraver- tebral gutters and is contiguous to the protruding surfaces of the thoracic ver- tebrae, displacement of the heart within the thorax and consequent compression of lung parenchyma in the dorsal regions of the thorax concomitantly with distortion and distention of ventral lung parenchyma are inescapable effects of high Gx ex- posure in the supine position [54]. Consequently, as demonstrated by F l a n a g a n G r a y a n d c o - w o r k e r s , humans can withstand Gx accelerations

7s. 4.

z4.5.

Figure 14: Diagrams from U.S. patent of the David Clark Company full pressure half suit. This suit provided a uniform pressure over the entire surface of the body below the thorax. The average protection of more than 3.4 G afforded experienced relaxed subjects on the Navy human centrifuge a t Pensacola (when inflated to about 75 mm Hg/G starting a t 1.8 G [44]) has not been exceeded since that time by any other suit unless supplemented by respiratory positive pressure muscular straining maneuvers [17,39].

34 IEEE ENGINEERING I N MEDICINE AND BIOLOGY March1991

of 30 G when in the prone position and immersed in a physiologic saline solution [4].

ractical Aspects Bodily comfort, upward and rear- P ward surveillance, in addition to con-

venient flight controls and instrument viewing, are mandatory requirements for pilot acceptance of a horizontal prone position cockpit. Based on the preliminary findings of W. Clark and co-workers [56], Hertzberg and Colgan at Wright-Patte- rson Air Force Base developed a prone position couch with a counter-weighted head support and controls designed for flight in the prone position. This system was tested for comfort in 1948 with sub- jects who rested on the couch for eight hour periods at 1 G. The consensus of the opinions of 18 Air Force pilots who par- ticipated in these studies was that this assembly provided greater comfort for longer periods of time than conventional airplane seats [56].

Satisfactory flight control was reported by pilots flying a specially modified B- 17 bomber and an extended nose F-80E jet- powered fighter while in this prone posi- tion assembly. The prone position couch was reported to be perfectly comfortable [56-581. The forward and downward visibility during flight was excellent and lateral vision adequate. However, lack of visibility in the upward and rearward directions precluded operational use of this system. Consequently, development of a pilot surveillance system that will provide adequate visibility in all direc- tions, particularly in the upward and rear quadrants, is a primary prerequisite for a prone position cockpit to achieve opera- tional status.

The continuing loss of life and plane destroying dimensions of the current G- LOC problem indicate that the limited rear- ward and upward pilot surveillance problem should be solved. The use of a rearward focussed fiber-optic system (Fig. 15), if coupled with a similar upward focussed sys- tem, is one possible approach to this prob- lem.

Dr. Harald von Beckh’s opinion of the need to develop a horizontal position cock- pit should be remembered [53] and can be paraphrased as follows:

‘The urgency to provide fighter aircraft with horizontal seats cannot be overem- phasized. It is hoped that a potential ad- versary does not build such a seat fKst. A squadron equipped with a horizontal prone position cockpit with an optical- electronic aided omni-directional pilot surveillance system would have a spec- tacular advantage in air-combat situa- tions, and could literally fly circles around their adversaries.”

Quite certainly, flight in a fully horizon- tal position is the biomedically safest and

I I EYEPIECE FIBER OPTIC BUNDLE I I

FLEXIBLE METAL SHEATHING 4

W h I V E LENS

BM(w ms Figure 15. Diagram of a rearward focused fiber-optic system which, if coupled with a similar upward focused system, is one possible approach for overcoming the limited rearward and upward pilot surveillance disadvantage of a very high G tolerance prone position fighter plane cockpit.

most certain stratagem for elimination of the G-LOC problem in current and future high performance fighter planes.

cknowledgment The expert assistance of Mrs. Beth A Allred in the syntactical and il-

lustrative aspects of this report have made this manuscript possible. This work h a s been supported by DARPNSTO Contract No. N66001-87- C-0079.

E.H. Wood is one of the early pioneer investigators in the cause and methods of preventing the pathophysiological effects of positive acceleration. From 1942 to 1946, he collaborated with the Aerospace Medical Research Laboratory team at the Mayo Human centrifuge and in instru- mented aircraft, which resulted in the G- activated anti-G suit and in the M-1 self-protective maneuver still used today be every fighter pilot. He culminated his distinguished career as Professor of Physiology and Medicine, Mayo Medical School, and Head of the Mayo Clinic’s Biodynamics Research Unit. There he in- itiated the development of high speed, synchronous, volumetric imaging tech- niques for non-invasive studies of simul- taneous , spat iaUtempora1, anatomic/functional relationships of moving organ systems, particularly the heart, lungs and circulation. He is current- ly reviewing the physiology and methods for preventing acceleration-induced loss of consciousness. The research is sup- ported by a DARPA/NOSC contract.

Dr. Wood received the BA degree in liberal arts from Macalester College, St.

Paul, MN in 1934. He earned the MS degree in physiology in 1939, and the PhD and MD degrees in 1941 from the Univer- sity of Minnesota. He can be reached at the Mayo Medical School and Founda- tion, Rochester, MN 55905, where he is Emeritus Professor of Physiology and Medicine.

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3 3 . W h i n n e r y J E : Defin ing risk i n aerospace medical unconsciousness research. Aviat Space Environ Med 60:688-694, 1989. 3 4 . W o o d E H : G-induced loss of con- sciousness and its prevention. USAFSAM- TR-87-41, September 1988. 35. Gill ingham KK, Fosdick JP: High- G training for fighter aircrew. Aviat Space Environ Med 59:12-19, 1988. 36. Whinnery JE: On the theory of ac- celeration tolerance. Report No. NADC- 88088-60, February 15, 1988. 37. Balldin U, Dahlback G, Larsson LE: Full coverage anti-G suit and balanced pressure breathing. FOA Report C 50065-5.1, February 1989. 38. Clark DM: Anti-blackout suit develop- ment. Bureau of Aeronautics Report, Contract NOa(s) 8528, June 28, 1946. 39. Wood EH: Evolution of anti-G suits, their limitations, and alternative methods for avoidance of G-induced loss of conscious- ness. Final Report, DARPAISTO Contract No. N66001-87-C-0079. May 1990. 40. Code CF, Wood EH, Sturm RE, Lambert EH, Baldes EJ: The sequence of physiologic events in man during exposure to positive acceleration. Fed Proc 4: 14-15, 1945. 41. Martin EE: Evaluation of the anti-G suit. Wright Field Memorandum Report No. TSE.4.4-689-2B. November 20, 1947. 4 2 . M a r t i n E E : Service test report on USAF type G-4A pilots' pneumatic suit, anti- G. Memorandum Report, U.S. Air Force, Air Materiel Command, Wright-Patterson AFB, Dayton, Ohio, MCREXD-689-2E, January 1950. 43. S ieker NO, Martin EE, Gauer OH, Henry JP: A comparative study of two ex- perimental anti-G suits and the standard USAF G-4A suit. Wright Air Development

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48. Wood EH, Lambert EH: Objective documentation and monitoring of human Gz t o l e r a n c e when unpro tec ted and when protected by anti-G suits or M-I type straining maneuvers alone or in combination. SAFE Journal l9:39-48, 1989. 4 9 . W h i n n e r y J E : T h e e l e c t r o c a r - diographic response to high +Gz centrifuge training. Aviat Space Environ Med 61:716- 21, 1990. 50. Hallenheck GA, Baldes EJ, Code CF: The effect of immersion in water on the tolerance of dogs to centrifugal force. Com- mittee on Aviation Medicine Report No. 278, March 14. 1944. 51. Brown GE, Jr. , Wood EH, Lam- bert EH: Effects of tetra-ethyl-ammonium chloride on the cardiovascular reactions in man to changes in posture and exposure to centrifugal force. J Appl Physiol 2: 1 17- 132, 1949. 52. Wood EH: Biofeedback avoidance of G-LOC by ear pulse monitoring. Aviat. S p u c e En1, iron. M e d . ( in press ) . 5 3 . v o n B e c k h NJ: T h e d e v e l o p m e n t and a i rborne testing of the PALE sea t . Naval Ai r Development Center Report No. NADC-8 1200-60, June I O , 198 1 . 54. W o o d E H : H y d r o s t a t i c h o m e o s - t a t i c e f f e c t s d u r i n g c h a n g i n g f o r c e e n v i r o n m e n t s . A I' i a t S p a c e E n v i r o n M e d 6 1 : 3 6 6 - 3 7 3 , 1 9 9 0 . 5 5 . C l a r k W G , H e n r y J P , G r e e l e y PO, D r u r y DR: S t u d i e s o n f l y i n g in t h e p r o n e p o s i t i o n . N a t i o n a l R e - s e a r c h C o u n c i l , D i v i s i o n of M e d i c a l S c i e n c e s , R e p o r t No. 4 6 6 , A u g u s t 20 , 1 9 4 5 . 56 . H e r t z h e r g H T E , C o l g a n J W : A p r o n e p o s i t i o n b e d f o r p i l o t s . A e r o m e d i c a l L a b o r a t o r y , E n g . D i v . M e m o R e p o r t N o . M C R E X D - 6 9 5 - 7 I D , W r i g h t - P a t t e r s o n A F B , O h i o , 1 9 4 8 . 5 7 . S t a n l e y E M : F l i g h t t e s t r e p o r t . F 8 0 - E a i r c r a f t . C o n t r a c t A F 3 3 ( 0 3 9 ) - 7 3 6 0 , D o c u m e n t N o . 3 6 . S t a n l e y A v i a t i o n C o r p o r a t i o n , B u f f a l o , N e w Y o r k , S e p t e m b e r 1 9 , 1 9 5 0 . 5 8 . A n d e r s o n D A : W i l l p r o n e p o s i t i o n l i c k h i g h - G l o a d s ? AL, ia t W e e k , D e c . 2 9 , 1 9 5 2 .

18:30-40, 1988.

36 IEEE ENGINEERING I N MEDICINE AND BIOLOGY March 1991