Effect of dihydroergotamine on +Gz acceleration tolerance.

Dihydroergotamine is a potent vasoconstrictor of the venous capacitance vessels and has been shown to decrease blood pooling in patients with orthostatic hypotension. Acceleration acting on the long axis of the body (+Gz) pools blood into the lower body of the subject. Nine healthy men received an injection of 1 mg dihydroergotamine and 1 ml of saline intramuscularly in a randomised, double blind fashion, and had their Gz acceleration tolerances measured. Dihydroergotamine significantly increased Gz tolerance by 0.24 G and narrowed the difference between the maximum heart rate during the acceleration epoch and resting baseline by 10%. The drug had no effect on blood pressure and heart rate when the subjects changed from the supine to standing position. The increase in Gz tolerance was not sufficient for operational use.

Long-duration +Gz acceleration on cardiac volumes determined by two-dimensional echocardiography.

To enhance protection of humans exposed to long-duration low-gravity environments such as the Space Shuttle and National Aerospace Plane during re-entry or in the short-duration high(-)+Gz environment of fighter aircraft, the effects of +Gz acceleration on cardiovascular hemodynamics must be understood. This study reports the use of two-dimensional echocardiography in normal men during +Gz acceleration. The heart's position in relation to the chest did not change during acceleration up to +7 Gz. The success in maintaining high-quality images during exposures to G-forces of this magnitude may be attributed to the relatively low rate of G onset. End-diastolic volumes (EDV) and stroke volumes (SV) decreased during a +Gz acceleration ramp that increased until the subject experienced peripheral light loss (PLL) (P < .05). An inflated G-suit partially counteracted this effect. By 30 seconds of a +3 Gz acceleration plateau, the protective effects of the inflated G-suit to maintain EDV is lost and the EDV of the inflated G-suit was lower than the EDV of the uninflated G-suit (P < .05).

Unequal narrowing of the visual field in a +Gz environment.

Anecdotally, the existence of individuals who experience an unequal loss of peripheral vision (left versus right) under sustained +Gz acceleration is well known. However, there is little mention or explanation of the etiology in the literature. Only recently has there been a surge of interest in this phenomenon. The operational significance is more important. The most common interpretations for this phenomenon are unequal arterial pressure between the left and right blood supplies to the retina (ophthalmic artery, its branch to the retina, the central retinal arteries, the circle of Willis, and the internal carotid artery), and a difference in the intraocular pressure. Two case histories of unequal peripheral light loss (PLL) are discussed. It is possible for a pilot not to be aware of his or her full sequence of PLL and, in an operational environment, lose half the visual field and not realize it. We need more controlled studies to define unequal PLL and to ensure valid centrifuge subject data.

Female exposure to high G: effects of simulated combat sorties on cerebral and arterial O2 saturation.

BACKGROUND: One of the key factors in maintaining optimal cognitive performance in the high-G environment is the adequate delivery of oxygen to the cerebral tissue. As eye-level blood pressure is compromised at 22 mmHg x G(-1), perfusion to the peripheral cerebral tissues (cerebral cortex) may not be adequate to support the mental demands of flight. This study measured the effect of closed-loop flight simulations (3 min) on cerebral oxygen saturation changes (rSO2), arterial oxygen saturation (SAO2), and heart rate (HR), in both rested (8 h of rest) and sleepless (24 h without sleep) conditions. METHODS: Subjects (16; 8 males and 8 females) were subjected to G-exposures via closed-loop flight simulations in a series of four 3-min sorties flown by subjects on the Dynamic Environment Simulator (centrifuge) in either a rested or a sleepless state. Prior to the centrifuge flight, subjects were instrumented with sensors for measurement of arterial oxygen saturation (SAO2) and regional cerebral tissue oxygenation (rSO2). Subjects wore the standard flight suit, boots, CSU-13B/P anti-G suit, and the COMBAT EDGE positive-pressure breathing for G-protection system. RESULTS: Significant changes in cerebral and arterial oxygen saturation were observed within groups when comparing pretest baselines and minimum values during the test and pre- and post-G rSO2, SAO2, and HR in both the rested and sleepless state, (p # 0.01), respectively, for each group. Comparisons between groups showed women to have significantly smaller regional cerebral cortex oxygen decreases than men (p # 0.01). No significant changes in SAO2, however, were observed between groups. Both men and women showed a slow recovery of rSO2 values to the prebaseline levels. CONCLUSIONS: Sleeplessness had no effect on the rSO2, SAO2, and HR compared with the rested condition. During acceleration, regional cerebral tissue oxygen decreased 13% in men compared with 9% in women. The recovery of cerebral tissue oxygen levels to prebaseline values was retarded somewhat when compared with the recovery response of arterial oxygen saturation.

Female exposure to high G: performance of simulated flight after 24 hours of sleep deprivation.

BACKGROUND: Ground-based research has investigated the loss of cognitive function in the extreme conditions of G-induced loss of consciousness, however, little is known about pilots' abilities to maintain cognitive performance throughout prolonged conscious exposure in the high-G environment. The effects of fatigue and G layoff on performance during exposure to high G are mostly unknown for the female population. METHODS: This research was conducted on the centrifuge Dynamic Environment Simulator. Active-duty personnel (8 male and 8 female) were trained to fly the F-16 simulation while 30 performance measures were recorded. Performance was re-evaluated after 24 h of sleep deprivation. RESULTS: Neither male nor female overall performance was affected significantly by sleep status, although individual tasks showed sensitivity; call-sign reaction time was longer by 33%, and missile survival was less likely. Also, when sleep deprived, perceived effort and physical demand were higher while perceived performance was lower. No differences in performance were found in either gender due to lay-off, although some physiologic deconditioning was apparent. Women commanded and endured the same amount of G load as men, however, on average they could not perform the tracking task quite as well. CONCLUSIONS: Sleep deprivation (24 h) produced sensations of fatigue and frustration, but overall performance was not reduced. The ability of personnel to complete a complex defensive maneuver was reduced when they were sleep deprived. The women that we tested apparently could not optimize the tracking task as well as their male counterparts when Gz was in the simulation. None of these results were sufficient to suggest that women should not be allowed to compete for flying assignments in high-performance aircraft.

Female exposure to high G: chronic adaptations of cardiovascular functions.

INTRODUCTION: Exposure to microgravity is associated with increased leg venous compliance and reductions in cardiac output, baroreflex functions, and tolerance to orthostatism. However, the effects of chronic exposure to high-G environments are unknown. In addition, there is evidence that females have lower orthostatic tolerance than males, although the underlying mechanisms are unclear. Therefore, we tested the hypotheses that high-G training will enhance baroreflex and orthostatic functions and that females will demonstrate similar adaptations compared with males. METHODS: Calf venous compliance, baroreflex function, and orthostatic performance were measured in six men and seven women before and after repeated exposures on the centrifuge (G-training) for 4 wk, 3 times/wk, with gradual levels of G starting with +3 Gz without G-suit protection during week 1 and advancing to +9 Gz with G-suit protection by the end of week 4. Calf venous compliance was measured by occlusion plethysmography using impedance rheographic recordings of volume change. Baroreflex function was assessed from beat-by-beat changes in heart rate (HR) and mean arterial pressure (MAP) that were measured before, during, and after a Valsalva maneuver strain at 30 mmHg expiratory pressure. The orthostatic performance of reflex responses was assessed from beat-by-beat changes in HR, MAP, stroke volume (SV), cardiac output (Q; by impedance plethysmography), and systemic peripheral resistance during the last 10 cardiac beats of a 4-min squat position and during the initial 10 cardiac beats in a standing position. RESULTS: G-training increased calf compliance in both men and women. SV and Q were increased during the squat-to-stand test in the males, but not in the females, following G-training and provided protection against the development of acute hypotension in the men. CONCLUSIONS: G-training caused adaptations in orthostatic functions opposite to those observed following exposure to microgravity environments. However, adaptations to G-training were limited in females, a finding that may provide a physiological basis for their lower simulated combat tracking performance during simulated aerial combat maneuvers compared with males.

Auditory localization under sustained +Gz acceleration.

The ability to localize a virtual sound source in the horizontal plane was evaluated under varying levels of sustained (+Gz) acceleration. Participants were required to judge the locations of spatialized noise bursts in the horizontal plane (elevation 0 degrees) during exposure to 1.0, 1.5, 2.5, 4.0, 5.5, and 7.0 +Gz. The experiment was conducted at the U.S. Air Force Research Laboratory's Dynamic Environment Simulator, a three-axis centrifuge. No significant increases in localization error were found between 1.0 and 5.5 +Gz; however, a significant increase did occur at the 7.0 +Gz level. In addition, the percentage of front/back confusions did not vary as a function of +Gz level. Collectively, these results indicate that the ability to localize virtual sound sources is well maintained at various levels of sustained acceleration. Actual or potential applications include the incorporation of spatial audio displays into the human-computer interface for vehicles that are operated in acceleration environments.

The effect of brief headward acceleration on human G tolerance.

OBJECTIVE: To generate, on a multi-axial centrifuge, a negative to positive acceleration profile that reproduces the physiological reaction and subjective symptoms experienced by agile aircraft pilots. Previous research will be summarized and current status of research described. METHODS: Experiments have been accomplished with the Dynamic Environment Simulator (DES) using different profile generating techniques. The DES was programmed with a set of open-loop profiles that provided five second duration baseline exposures ranging from +1.4 Gz down to -2 Gz followed by rapid transition to positive G levels up 2 to 8 +Gz. Volunteer subjects were instrumented and trained to report visual symptoms. RESULTS: Both profile generation approaches produce the reduced cardiovascular and subjective tolerance in a subset of subjects tested. Accumulated stress and motion sickness are factors in quantitative measures. Use of the anti-G straining maneuver may only temporarily alleviate the symptoms with a second period of visual symptoms even as the G load is removed. Positive pressure breathing for G (PBG) does not appear to exacerbate the effect, but the advantages gained from PBG may be lost when a sustained G pull is preceded by a negative G push. CONCLUSIONS: The utility and limitations of using a multi-axial centrifuge for the study of negative to positive G transitions have been described. Greater study is necessary on multiple factors affecting the effect and large numbers of volunteer subjects are needed.

Haemodynamic effects of ATP in dogs during hypoxia-induced pulmonary hypertension.

Pulmonary hypertension may result from an increase in vascular resistance caused by persistent hypoxia. We have investigated the effects of adenosine triphosphate (ATP), administered into the pulmonary artery, on haemodynamic changes occurring in anaesthetized adult dogs subjected to acute hypoxic pulmonary vasoconstriction. Hypoxia alone (ventilation with 10% O2/90% N2) caused significant increases in mean pulmonary arterial blood pressure (PAP), central venous pressure (CVP), and cardiac index (CI) by 71, 102 and 38%, respectively. ATP (0.03-3.0 micromol/kg/min approximately 0.02-1.65 mg/kg/min), when infused under hypoxic conditions, significantly reduced both mean PAP and systemic arterial blood pressure (ABP) in a dose-dependent manner. The maximum decrease in mean PAP amounted to 20%; mean ABP, on the other hand, was decreased by up to 52% (P<0.01). Heart rate, CI, CVP and pulmonary occlusion pressure were not dose-dependently affected by ATP. Our data indicate that while pulmonary arterial administration of ATP in mature dogs during hypoxic pulmonary hypertension causes dilation in the pulmonary vascular bed, it is even more effective in dilating the systemic vasculature. This result suggests a need for further evaluation and warrants cautious use of ATP in the treatment of hypoxic pulmonary hypertension in adult dogs.

The effects of inflation of antishock trousers on hemodynamics in normovolemic subjects.

Antishock trousers may maintain mean arterial pressure in trauma patients by increasing central blood volume and cardiac output. Hemodynamics, end-diastolic volume, stroke volume, cardiac output, and blood pressure were recorded in eight supine, healthy men in antishock trousers using two-dimensional echocardiography. Two inflation protocols were used. The antishock trousers were inflated to 50 and 100 mm Hg in a random fashion and inflation was maintained for 30 minutes before deflation. End-diastolic volume and blood pressure rose significantly (p less than 0.05) after antishock trouser inflation of 50 and 100 mm Hg. With the 50 mm Hg inflation, the stroke volume and end-diastolic volume fell below baseline over time. This did not occur with the 100 mm Hg inflation. After suit deflation, the stroke volume, end-diastolic volume, and cardiac output increased with 50 mm Hg inflation. The study shows that the antishock trousers alter several hemodynamic parameters. With lower inflation pressures, antishock trousers cause an increase in arterial pressure by increasing peripheral resistance. At higher inflation pressures, the antishock trousers increase cardiac output and as the cardiovascular system adjusts, maintain the pressure by increasing peripheral resistance.

The effects of various antishock trouser inflation sequences on hemodynamics in normovolemic subjects.

In an attempt to maximize the hemodynamic effects of antishock trousers, a pair of such trousers was modified to a five-bladder trouser (one abdominal, two thigh, and two calf bladders) and inflated using three different inflation sequences. These three sequences were simultaneous inflation of all five bladders; concurrent inflation of the calf and thigh bladders and then inflation of the abdominal bladder (standard inflation); and sequential inflation of the calf, thigh, and abdominal bladders. Simultaneous inflation resulted in the greatest increase in blood pressure and the smallest increase in end-diastolic volume, stroke volume, and cardiac output. The standard inflation sequence resulted in slightly higher increases in end-diastolic volume, stroke volume, and cardiac output than the sequential inflation sequence, and a significantly larger increase in cardiac output than the simultaneous inflation sequence. Our study indicates that of the three sequences used, the standard inflation sequence produces the greatest blood return to the heart.