Effect of 30-min +3 Gz centrifugation on vestibular and autonomic cardiovascular function.

INTRODUCTION: Repeated exposure to increased +Gz enhances human baroreflex responsiveness and improves tolerance to cardiovascular stress. However, it is not known whether such enhancements might also result from a single, more prolonged exposure to increased +Gz. Our study was designed to investigate whether baroreflex function and orthostatic tolerance are acutely improved by a single prolonged exposure to +3 Gz, and moreover, whether changes in autonomic cardiovascular function resulting from exposure to increased +Gz are correlated with changes in otolith function. METHODS: We exposed 15 healthy human subjects to +3 Gz centrifugation for up to 30 min or until symptoms of incipient G-induced loss of consciousness (G-LOC) ensued. Tests of autonomic cardiovascular function both before and after centrifugation included: 1) power spectral determinations of beat-to-beat R-R intervals and arterial pressures; 2) carotid-cardiac baroreflex tests; 3) Valsalva tests; and 4) 30-min head-up tilt tests. Otolith function was assessed during centrifugation by the linear vestibulo-ocular reflex and both before and after centrifugation by measurements of ocular counter-rolling and dynamic posturography. RESULTS: Of the 15 subjects who underwent prolonged +3 Gz, 4 were intolerant to 30 min of head-up tilt before centrifugation but became tolerant to such tilt after centrifugation. The Valsalva-related baroreflex as well as a measure of the carotid-cardiac baroreflex were also enhanced after centrifugation. No significant vestibular-autonomic relationships were detected beyond a vestibular-cerebrovascular interaction reported earlier in a subset of seven participants. CONCLUSIONS: A single prolonged exposure to +3 Gz centrifugation acutely improves baroreflex function and orthostatic tolerance.

Effect of acute exposure to hypergravity (GX vs. GZ) on dynamic cerebral autoregulation.

We examined the effects of 30 min of exposure to either +3GX (front-to-back) or +GZ (head-to-foot) centrifugation on cerebrovascular responses to 80 degrees head-up tilt (HUT) in 14 healthy individuals. Both before and after +3 GX or +3 GZ centrifugation, eye-level blood pressure (BP(eye)), end tidal PCO2 (PET(CO2)), mean cerebral flow velocity (CFV) in the middle cerebral artery (transcranial Doppler ultrasound), cerebral vascular resistance (CVR), and dynamic cerebral autoregulatory gain (GAIN) were measured with subjects in the supine position and during subsequent 80 degrees HUT for 30 min. Mean BP(eye) decreased with HUT in both the GX (n = 7) and GZ (n = 7) groups (P < 0.001), with the decrease being greater after centrifugation only in the GZ group (P < 0.05). PET(CO2) also decreased with HUT in both groups (P < 0.01), but the absolute level of decrease was unaffected by centrifugation. CFV decreased during HUT more significantly after centrifugation than before centrifugation in both groups (P < 0.02). However, these greater decreases were not associated with greater increases in CVR. In the supine position after centrifugation compared with before centrifugation, GAIN increased in both groups (P < 0.05, suggesting an autoregulatory deficit), with the change being correlated to a measure of otolith function (the linear vestibulo-ocular reflex) in the GX group (r = 0.76, P < 0.05) but not in the GZ group (r = 0.24, P = 0.60). However, GAIN was subsequently restored to precentrifugation levels during postcentrifugation HUT (i.e., as BP(eye) decreased), suggesting that both types of centrifugation resulted in a leftward shift of the cerebral autoregulation curve. We speculate that this leftward shift may have been due to vestibular activation (especially during +GX) or potentially to an adaptation to reduced cerebral perfusion pressure during +GZ.

Tactile situation awareness system: proprioceptive prostheses for sensory deficiencies.

UNLABELLED: Pilots and astronauts do not experience spatial disorientation in normal day-to-day terrestrial activities. On the ground, the perception of position and motion is determined by central nervous system integration of concordant and redundant information from multiple sensory channels (somatosensory, vestibular and visual) which collectively yield veridical perceptions. In the acceleration environments experienced by pilots and astronauts, the somatosensory and vestibular senses frequently present false but concordant information concerning the direction of gravity or down. When presented with conflicting sensory stimuli, it is normal for pilots and astronauts to experience episodes of disorientation. Visual instruments and displays developed over the past 70 yr have not solved the problem. A simple solution to maintain spatial orientation is to provide true information using the same sensory channels we use so successfully on Earth. METHODS: The Tactile Situation Awareness System (TSAS) developed by NASA and the U.S. Navy uses a matrix of mechanical tactile stimulators (tactors) applied on the torso and limbs to convey orientation cues (e.g., gravity vector) in an intuitive fashion to the skin. A series of in-flight experiments to validate and test a variety of tactile displays and concepts has been carried out in both helicopters and fixed wing aircraft. RESULTS: Pilots were able to fly complex maneuvers with no instruments or outside visual references (blindfolded) with less than 20 min of training. Recovery from unusual attitudes solely by tactile cues was trivial. Lab tests have shown the TSAS improves performance under conditions of high workload. CONCLUSIONS: When orientation information is presented via intuitive tactile displays spatial orientation is easily maintained in altered sensory conditions including unusual acceleration environments.

The experience of nausea during sustained hyper-gravity flight with negligible angular velocity.

BACKGROUND: The purpose of this study was to investigate nausea and vomiting during hyper-gravity flight at a slow rate of turn. During head-movements under these conditions, the semi-circular canals of the vestibular system function normally whereas the otoliths experience a G-excess effect, displacing further and moving faster than in a 1 G field. HYPOTHESIS: Nausea and vomiting are greater during hyper-gravity flight compared with historical data collected in a 1 G field at a similar rate of turn. METHODS: There were 27 subjects who were exposed to 1.8 G (hyper-G) during a slow rate turn on three NASA KC-135 flights. Subjects participated in one of three experimental periods, each period consisting of two 5.5-min hyper-G runs: 1) rest, with only incidental head movements; 2) active roll and pitch head movements; or 3) passive roll head movements. Subjective symptom data were collected pre-flight, following each experimental period and post-flight using a standardized nausea questionnaire. Electrogastrograms were obtained from eight subjects. RESULTS: Seven subjects (26%) vomited during the flight. Nausea initially increased but was then stable throughout the flight. Nausea appeared highest during active head movements. In subjects who vomited, greater gastrointestinal distress and somatic distress were reported. CONCLUSIONS: The levels of nausea and vomiting observed during hyper-G cannot be explained by Coriolis cross coupling and are likely due to the G-excess effect on the otoliths. The nausea profiles observed in individuals who vomit during hyper-G appear similar to those previously observed during nausea produced by an optokinetic drum stimulus in a 1-G field.

The human vertical optokinetic afternystagmus and the effect of gravity during pitch tilt.

BACKGROUND: Findings concerning human vertical optokinetic afternystagmus (VOKAN) are often not consistent and in some cases even contradictory. METHOD: VOKAN was studied in nine normal subjects using corneo-retinal potential technique and an infrared video camera detection apparatus (ISCAN). VOKAN responses were measured at three angles of pitch head (and body) tilt: 0 degrees (upright), 90 degrees (supine) and 135 degrees (45 degrees below horizontal). The optokinetic stimulus was stripe movement with a velocity of 60 degrees/s. RESULTS: Upward slow phase velocity (SP-Up) VOKAN was evoked in 6/9 subjects (67%) at 0 degrees tilt and at 90 degrees tilt, and in 4/9 (44%) at 135 degrees tilt. Downward slow phase velocity (SP-Down) VOKAN was elicited in 3/9 subjects (33%) at 0 degrees tilt; in 2/9 subjects (22%) at 90 degrees tilt and in 0/9 subjects (0%) at 135 degrees tilt. CONCLUSION: The pitch tilt, as a gravity effect, influences significantly on the appearance and the duration of VOKAN.

Vertical optokinetic nystagmus and after-responses during backward tilt.

METHOD: Vertical optokinetic nystagmus (VOKN) and after-responses were detected in nine subjects using the corneo-retinal potential (CRP) technique and an infrared video camera detection apparatus (ISCAN) simultaneously. The ISCAN method produced a much smaller inter-subject variability, a higher linear regression coefficient (0.94) when vertical eye position was regressed against vertical target position (+/- 30 degrees, 5 degrees increments). Detected by ISCAN, VOKN responses were measured at 3 angles of pitch head (and body) tilt: upright (0 degrees), supine (90 degrees), and declined 45 degrees below horizontal (135 degrees). Two stripe velocities (40 degrees.s-1 and 60 degrees.s-1) were used. RESULTS: In six of the nine subjects (67%) and in 40/54 conditions (74%) for all subjects, upward (slow phase velocity up) VOKN gain was greater than downward VOKN gain for both stripe velocities at all tilt angles. The gain for both upward and downward VOKN decreased as stripe velocity increased from 40-60 degrees.s-1, which suggests that both upward and downward VOKN systems were starting to saturate. Across subjects, a mean up-down asymmetry index, I, increased monotonically as the subjects declined. The slope of the monotonic function was greater for 60 degrees.s-1 stripe velocity than for 40 degrees.s-1 stripe velocity. The mean of all subjects' individual asymmetry ratios (ASYM) also increased as tilt increased but the slope of the best fitted regression line was not statistically significantly different from zero (p > 0.05). CONCLUSION: Optokinetic after-responses observed in the present study were of two types: 1) resetting of the eye by a drift, with nystagmus superimposed, from a beating field (eye position) that occurred during optokinetic stimulation; and 2) resetting of the eye without nystagmus superimposed. Upward VOKN produced the greatest number of after-responses. The beating field of VOKN was not correlated with pitch tilt.

Visual vestibular interaction in the dynamic visual acuity test during voluntary head rotation.

BACKGROUND: Although intact vestibular function is indispensable to maintaining spatial orientation, no good screening tests of vestibular function are implemented in the aviation community. High frequency voluntary head rotation was selected as a vestibular stimulus to isolate the vestibulo-ocular reflex (VOR) from visual influence. METHOD: A dynamic visual acuity test that incorporates voluntary head rotation was evaluated as a potential vestibular function screening tool: 27 normal subjects performed voluntary sinusoidal head rotation at frequencies from 0.7-4 Hz under 3 different visual conditions: visually-enhanced VOR, normal VOR, and visually suppressed VOR. Standardized Bailey-Lovie chart letters were presented on a computer monitor in front of the subject, who then was asked to read the letters while rotating his head horizontally. The electro-oculogram and dynamic visual acuity score were recorded and analyzed. RESULTS: There was no significant difference in gain or phase shift among 3 visual conditions in the frequency range 2.8-4 Hz. The dynamic visual acuity score shifted less than 0.3 log MAR at frequencies under 2.0 Hz. CONCLUSION: The dynamic visual acuity test at frequencies around 2 Hz can be recommended for evaluating vestibular function.

Vestibulo-ocular response of human subjects seated in a pivoting support system during 3 Gz centrifuge stimulation.

The vestibulo-ocular reflex (VOR) and spatial orientation perceptions were recorded in 15 subjects during 3 Gz centrifuge runs. These data were obtained to study two issues: (1) to gain insight into reports of asymmetrical disorientation and disturbance during acceleration and deceleration of centrifuge runs like those used to train pilots on the procedures to counteract G-induced loss of consciousness (G-LOC); (2) to study the effects of sustained vertical linear acceleration on the vestibular system. The centrifuge angular velocity profile consisted of a 19 s angular acceleration to 3 Gz that was sustained for 5 min during a period of constant angular velocity, and a 19 s deceleration to 1 Gz. The runs were repeated three times with the subject facing the motion and three times with the subject's back to the motion. The VOR and spatial orientation perceptions from the eight subjects who completed all six runs were analyzed. The total VOR response during acceleration and deceleration was composed of interacting angular (AVOR) and linear components (LVOR). Asymmetries in pitch orientation perception between centrifuge acceleration and decleration were not matched by asymmetries in the total VOR slow phase velocity. During the constant velocity high-G phase of the run, sustained up-beating LVOR (Lz nystagmus) was present in 14 of the 15 subjects. Significant individual differences in Lz nystagmus were found, but the maximum Lz response in our 15 subjects was probably of insufficient magnitude to degrade visual scan of cockpit instruments. Mean magnitudes ranged from 0 to 10 deg/s at 90 s from onset of centrifuge run.

Visually triggered migraine headaches affect spatial orientation and balance in a helicopter pilot.

The authors present a case of an attack helicopter pilot with recurrent spatial disorientation (SD) flying with night vision goggles (NVG's), diagnosed as having visually triggered migraine headaches. Serial Dynamic Platform Posturography testing during an acute migraine attack demonstrated balance dysfunction under visual and somatosensory deficient conditions, correlating with headache intensity. Vestibular symptoms are associated with migraine, and may be triggered by visual stimuli. NVG scintillations in susceptible individuals may act as a visual trigger for migraine. Migraine phenomenon may be a contributing factor to SD, especially during NVG operations. The association of visual and vestibular dysfunction with migraine and aeromedical disposition of migraine in aviators is discussed.

The "push-pull effect".

The purpose of this study was to prove or refute previous authors' suggestions that tolerance to +Gz is reduced when preceded by 0 Gz or -Gz. Six men and six women were subjected to one session of acceleration stresses that varied between -2 and +2.25 Gz on the NAMRL Coriolis Acceleration Platform (CAP). At the beginning and end of each session, we exposed the relaxed subjects to identical control segments that were comprised of +1 Gz for 30 s, followed by +2.25 Gz for 15 s, and then return to +1 Gz. Subjects were also exposed to three experimental segments that were comprised of 0, -1, or -2 Gz for 10 s, followed by +2.25 Gz for 15 s, and then return to +1 Gz. Subjects verbally reported any decrements in peripheral vision during exposure to +2.25 Gz. Blood pressure (BP) was reduced during each 15-s period at +2.25 Gz. The minimum BP was progressively lower during the 15-s period as the preexposure experimental conditions became more negative (+1, 0, -1, and -2 Gz). Episodes of peripheral vision loss increased as the preceding -Gz became more negative. BP during exposure to +Gz was significantly affected by the preceding 10-s exposure to -Gz, and is indicative of reduced +Gz tolerance. As this "push-pull effect" may result in unexpected incapacitation, it has important implications for aviation safety.

Spatial disorientation and dysfunction of orientation/equilibrium reflexes: aeromedical evaluation and considerations.

Loss of spatial awareness has been implicated as a direct causal factor in 4-10% of serious aircraft mishaps and 10-20% of fatal aircraft mishaps (7). Spatial disorientation in flight usually results from misperception of visual, vestibular, or proprioceptive cues. Pathologic causes have rarely been implicated. A student naval aviator with recurrent loss of spatial awareness due to a defective vestibulo-ocular reflex (VOR), presumably from vestibular neuronitis in adolescence, is reported. His chief complaint, an inability to focus on the instrument panel during turbulent instrument meteorological conditions (IMC), resulted in spatial disorientation and adverse flight attitude. A simple test of visual-vestibular interaction, the dynamic visual acuity test, could identify a defective vestibulo-ocular reflex in aviation personnel. An absent or defective vestibulo-ocular reflex has potential for disorientation in instrument flight. A comprehensive vestibular function test battery is indicated in individuals with recurrent or overwhelming spatial disorientation who fail a screening dynamic visual acuity test. The aeromedical disposition of vertigo and dysequilibrium is discussed.

The dynamics of spatial orientation during complex and changing linear and angular acceleration.

The dynamics of spatial orientation perception were examined in a series of experiments in which a total of 43 subjects were passively exposed to various combinations of linear and angular acceleration during centrifuge runs. Perceptual effects during deceleration were much stronger than effects during acceleration. The dynamics of spatial orientation perception differed substantially from changes in the vestibulo-ocular reflex (VOR). VOR was fairly well predicted by a current model, but our experiments revealed perceived change in attitude (roll, pitch, yaw tilt position in space) and perceived angular velocity in space that was not reflected by parallel changes in the plane or magnitude of the VOR. This series of experiments establishes several facts concerning spatial orientation perception beyond the predictive domain of any current model. New concepts are needed and several are suggested to deal with changing reactions to complex combinations of linear and angular accelerations.

Steady state and transient G-excess effects.

Acceleration forces of flight are associated with a number of spatial orientation illusory effects. This note focuses on two effects, both called the "G-excess effect." A distinction between steady-state and transient G-excess effects is important because prescriptions for preventive management of effects in flight will differ.

B virus (Herpesvirus simiae) infection in humans: epidemiologic investigation of a cluster.

A cluster of four cases of symptomatic B virus infection in humans occurred in Pensacola, Florida, in March 1987. Three cases occurred in persons who worked with monkeys at a research facility, and the fourth resulted from apparent autoinoculation through use of a nonprescription skin cream. Contact tracing identified 159 persons who may have been exposed to B virus (21 had been exposed to monkeys at the facility and 138 had been exposed to one or more of the case-patients), but no further cases were identified. Comparisons of restriction endonuclease patterns from B virus isolates linked two of the three cases in monkey handlers to one clinically ill monkey and the other to a second, healthy monkey. Three risk factors for human infection were identified: nonuse of mechanical or chemical restraints for monkeys before handling, nonuse of available protective gear, and direct viral inoculation through the application of a topical medication.