Gz Exposure Research Articles

Echocardiographic Evaluation and Follow-Up of Cardiac and Aortic Indexes in Aviators Exposed to Acceleration Forces

Military jet fighter pilots are routinely exposed to acceleration (+Gz) forces. This recurrent exposure may influence various cardiac parameters. A few previous studies have evaluated the impact of exposure to acceleration forces on cardiac morphology and function, but these studies were mostly based on small cohorts, and subjects did not undergo baseline echocardiographic examinations before +Gz exposure. Ninety-six jet fighter pilots with high +Gz exposure underwent echocardiographic evaluation before and 7 to 12 years after repeated +Gz exposure. Echocardiographic parameters were recorded using M-mode echocardiography and included left ventricular diameter at end-systole and end-diastole, interventricular septal thickness, thickness of the posterior wall, aortic root diameter and aortic valve opening, diameter of the left atrium, and left ventricular mass. Medical records of the subjects identified were evaluated for the development of adverse events. The average age at the time of the initial echocardiographic examination was 19.2 years. All subjects were healthy, without cardiovascular risk factors, and had no prior exposure to acceleration forces. The average flying period on jet planes at the time of follow-up examination was 1,812 hours. The mean follow-up period was 9.13 years. All parameters evaluated by M-mode echocardiography were not significantly changed from the baseline examination. No adverse events occurred during the follow-up period. Exposure to acceleration forces has no significant impact on cardiac and aortic morphology.

Hyperbaric oxygen preconditioning ameliorates high positive acceleration exposure-induced impairment of learning and memory in rats

Objective To explore the effect of hyperbaric oxygen (HBO) preconditioning on learning and memory damage induced by high positive acceleration( + Gz) exposure in rats.Methods Thirty-two male Sprague-Dawley rats were randomly divided into four groups with 8 in each group: control group( Con), + Gz group,HBO group and HBO-+ Gz group.Rats of Con group were given 5d( 1 ATA ,21% O2, 1h/d); Rats of + Gz group was exposed to + 10Gz for 5 min; HBO group were only given 5d (2.5 ATA, 100% O2,1 h/d); HBO-+ Gz group were given HBO 5 consecutive days,and then suffered +Gz exposure.Morris water maze was used to observe the navigation and probe capabilities of rats.Results In the spatial acquisition test,there exist significant difference among these groups(F(3.28) = 5.325, P＜ 0.01 ).Compared with the control group, the escape latency increased significantly in the + Gz group and HBO-+ Gz group (P＜0.05) while had no difference in HBO group.HBO-+ Gz group had significantly shorter escape latency than + Gz group (P＜0.05).In the probe test,compared with the control group, + Gz group and HBO-+ Gz group had a longer percentage in the target quadrant( (43.71 ± 3.29 ) %vs (28.65 ±1.00)%, P＜0.05;(43.71 ±3.29)% vs (37.17 ±0.98)%, P＜0.05)),and HBO-+Gz group was better than + Gz group.Conclusion HBO preconditioning may have a protective effect on the impairment of learning and memory caused by + Gz exposure in rats. Key words: Hyperbaric oxygen; High positive acceleration; Congnition; Morris water maze

Electroacupuncture pretreatment ameliorates hypergravity-induced impairment of learning and memory and apoptosis of hippocampal neurons in rats

High-sustained positive acceleration (+Gz) exposures might lead to impairment in cognitive function. Our previous studies have shown that electroacupuncture (EA) pretreatment can attenuate transient focal cerebral ischemic injury in the rats. In this study we aimed to investigate whether EA pretreatment could ameliorate the impairment of learning and memory induced by a sustained +Gz exposure. Using the centrifuge model, rats of experimental groups were exposed to +10Gz for 5 min. Morris water maze was used for assessing the cognitive ability, and the apoptotic hippocampal CA1 pyramidal neuronal cells were evaluated by caspase-3 activity and TUNEL staining. Our results showed that +Gz exposure significantly caused pyramidal neuronal damage, increased neuronal apoptosis and caspase-3 activity in hippocampal CA1 region, as well as resulted in an impairment of spatial learning and memory, as compared to the sham group animals. Furthermore, the EA pretreatment significantly attenuated the neuronal apoptosis, preserved neuronal morphology and inhibited the caspase-3 activity in hippocampal CA1 region resulted from +Gz exposure. The EA pretreatment also ameliorated the learning and memory function in rats exposed to +Gz. These findings indicate that EA pretreatment provides a novel method to prevent the cognitive damage caused by +Gz, which could significantly protect neuronal damage and impairment of learning and memory.

Discrepancy between blood flow and tissue‐oxygen in the brain during acute cerebral hypoperfusion in anesthetized rats

Acute cerebral hypoperfusion is experienced on head‐to‐foot inertial force (+Gz). However, no report has been found about redistribution of blood flow inside the brain. Then we estimate blood flow (BF) and tissue‐oxygen (PO2) level responses to +Gz stress in the cortex and hippocampus. In 13 male Sprague‐Dawley rats were anesthetized with urethane, cortical and hippocampal blood flows or these PO2 levels were measured by a laser blood flow meter with the glass fiber prove or a PO2 monitor with the polarographic oxygen electrodes, respectively. +Gz stress of 3.0 G by a centrifuge decreased arterial pressure at a level of the brain to 39.3 ± 13.5 mmHg, cortical and hippocampal BF to 77.0 ± 17.2% and 66.6 ± 15.1 %, respectively (NS), and these PO2 to 77.8 ± 14.7 % and 58.9 ± 19.7 %, respectively (p<0.05). After +Gz exposure, PO2 returned significantly slower than BF in both of the hippocampus and the cortex. These results indicate that acute cerebral hypoperfusion may induce more ischemic damages in the hippocampus than the cortex.

Flying Experience and Cardiovascular Response to Rapid Head-up Tilt in Fighter Pilots

Fighter pilots report G tolerance increases with regular exposure. Our previous work has shown that the cardiovascular system of +Gz-adapted fighter pilots responds differently to orthostatic challenges than that of non-pilots. A +Gz training effect in pilots after repetitive +Gz exposure has also been shown. Individual pilot factors such as flying experience may have a role in +Gz adaptation. In this study, we investigated the relationship between flying hours (a marker of cumulative +Gz exposure) and mean arterial blood pressure (MAP) response to head-up tilt (HUT; a marker of enhanced cardiovascular performance). There were 14 male fighter pilots who participated: 9 had over 1000 h jet flying experience and 5 had less than 500 h. Subjects underwent rapid (approximately 4 s) +75 degrees HUT. Beat-to-beat MAP was measured noninvasively. For each subject, change in MAP from resting values was obtained for the first 30 heart beats of the HUT period. MAP responses to tilt were compared between the experienced and less experienced pilots, and the averages of the MAP deviation values were plotted against flying hours. There was a strong correlation (r = 0.87, P < 0.01) between the MAP response to tilt and jet flying hours. Comparison of the MAP responses to tilt indicates that the experienced pilots increased MAP more (+8 +/- 1.7 vs 5 +/- 1.5 mmHg) and maintained MAP at a higher level during the HUT than the less experienced pilots. The results suggest that flying experience in the high +Gz environment is strongly correlated with enhanced cardiovascular performance under conditions of accelerative stress.

Cardiovascular Training Effects in Fighter Pilots Induced by Occupational High G Exposure

A fundamental difference in the cardiovascular response to acceleration between a group of fighter pilots (FP) and a group of non-pilots (NP) has been demonstrated previously. This study investigated the longitudinal effects of repetitive occupational +Gz exposure on the cardiovascular response to acceleration. There were 6 FP and 6 NP subjects who underwent rapid +75 degrees head-up tilt (HUT) on two separate occasions. The FP group were tested after a non-flying period of 5 wk (Test 1), and tested again after a period of repetitive exposure to high +Gz missions (Test 2). The NP group did not fly at all between Test 1 and Test 2. Mean arterial pressure (MAP), heart rate (HR), stroke volume (SV), and total peripheral resistance (TPR) were all determined non-invasively. SV was determined using impedance cardiography and calculated via the Kubicek equation. For each variable, resting values and the response to tilt for both HUT tests within and between each group were compared. In the FP group, resting MAP was higher (86 mmHg) in Test 2 compared with Test 1 (78 mmHg). Between groups, FP resting MAP was only different from the NP resting MAP in Test 2. The FP HR response to HUT increased significantly between the two tests. These findings suggest a +Gz-induced cardiovascular training effect in the FP group. Repetitive exposure to +Gz results in an increased resting MAP and an elevated HR response to tilt, which may provide benefits to operational fighter pilots.

Cardiac Ischemia Model for +G&lt;SUB&gt;z&lt;/SUB&gt; Using Miniature Swine and Baboons

Military aircrew with minimal coronary artery disease (MCAD) may be restricted from flying high-performance aircraft due to possible ischemia during high +Gz. An animal model is presented to provide ischemia data for a more informed decision. There were 18 swine that were placed on a high cholesterol/high fat diet for up to 57 wk. Five control swine were maintained on a standard swine diet. Also, nine male baboons had a constrictor placed around the left anterior descending coronary artery. Two baboons were sham-operated controls. The unanesthetized swine and baboons were infused with Tc-99m at the end of +Gz exposure and scanned for myocardial perfusion. Five swine died unexpectedly before +Gz exposure with moderate-to-severe CAD. Dysrhythmias during +Gz were seen equally in both the control and experimental swine and in the baboons before and after stenosis, with or without propranolol. During +Gz, ECG ST-T wave changes suggesting ischemia were observed in the cholesterol swine but not the control swine, and in the baboons before and after stenosis, with or without propranolol. There was a positive relationship between a normal/abnormal ECG and a normal/abnormal myocardial perfusion scan in the swine and a weak relationship in the baboon before stenosis, but somewhat better after stenosis. Coronary histopathology showed normal vessels from the control swine and stenoses ranging from 0-95% from the cholesterol swine. Baboon stenosis averaged 37.6 +/- 15.0%. In the swine and the baboon extended high levels of +Gz, were associated with evidence of myocardial ischemia.

A new evaluation method for +Gz tolerance with loratadine by using a near-infrared spectroscopy

BackgroundLoratadine (Claritin®), an over the counter antihistamine in U.S. and UK, is acceptable for use without adverse side effects by aircrew with mild or moderate allergic or other situations requiring an antihistamine. Although +Gz (head to foot direction) tolerance testing for aircrew with loratadine has not been documented in the published literature, it is commonly accepted that loratadine dose not effect +Gz tolerance. The purpose of this study was to offer and validate a new evaluation method for +Gz tolerance testing with loratadine by using a near-infrared spectroscopy (NIRS).MethodsA double-blind, placebo-controlled, randomized, crossover protocol was used to administer 10 mg of loratadine or placebo in nine healthy subjects. The subjects didn't wear anti-G suit. The +Gz exposure profiles consisted of, in series, a gradual onset ran (0.1 G·sec-1) to the subject's visual end-point (peripheral light loss) or loss of consciousness (GLOC), and rapid onset run (1.0 G·sec-1) to the subject's same end-point. In this study, G-level tolerance was defined as the +Gz level at visual end-point and/or at GLOC. As a subject's G-duration tolerance, we measured the total time (seconds) during rapid onset run. Otherwise, to confirm the effect of loratadine on +Gz tolerance, we measured the cerebral NIRS variables (hemoglobin concentration changes and tissue oxygenation index) as a new quantitative method for +Gz tolerance during a centrifuge experiments.ResultsNo significant differences were observed in +Gz tolerance (+Gz level, duration time and NIRS variables) between subjects taking loratadine and placebo.ConclusionOur results demonstrate that loratadine has no detectable effect on +Gz tolerance by using a new method with cerebral NIRS variables and the traditional method with +Gz level and duration time. This study represents the first use of a quantitative parameter such as cerebral NIRS variables to assess the effects of a drug on acceleration tolerance.

Frontal cortical oxygenation changes during gravity-induced loss of consciousness in humans: a near-infrared spatially resolved spectroscopic study

Gravity (G)-induced loss of consciousness (G-LOC), which is presumably caused by a reduction of cerebral blood flow resulting in a decreased oxygen supply to the brain, is a major threat to pilots of high-performance fighter aircraft. The application of cerebral near-infrared spectroscopy (NIRS) to monitor gravity-induced cerebral oxygenation debt has generated concern over potential sources of extracranial contamination. The recently developed NIR spatially resolved spectroscopy (SRS-NIRS) has been confirmed to provide frontal cortical tissue hemoglobin saturation [tissue oxygenation index (TOI)]. In this study, we monitored the TOI and the standard NIRS measured chromophore concentration changes of oxygenated hemoglobin and deoxygenated hemoglobin in 141 healthy male pilots during various levels of +G(z) (head-to-foot inertial forces) exposure to identify the differences between subjects who lose consciousness and those who do not during high +G(z) exposure. Subjects were exposed to seven centrifuge profiles, with +G(z) levels from 4 to 8 G(z) and an onset rate from 0.1 to 6.0 G(z)/s. The SRS-NIRS revealed an approximately 15% decrease in the TOI in G-LOC. The present study also demonstrated the TOI to be a useful variable to evaluate the effect of the anti-G protection system. However, there was no significant difference found between conditions with and without G-LOC in subjects with terminated G exposure. Further studies that elucidate the mechanism(s) behind the wide variety of individual differences may be needed for a method of G-LOC prediction to be effectively realized.

Failure of vascular autoregulation in the upper limb with increased +Gz acceleration

Forearm pain occurring during high +Gz exposure has been linked with vascular distension from elevated transmural pressure of hydrostatic origin and is exacerbated by positive pressure breathing (PBG). We postulated that at high vascular transmural pressure vascular autoregulation might be overcome and be associated with worsened pain. Six volunteers were studied at +4, +5, +6, and +7 Gz on a human centrifuge. Forearm vascular resistance (FVR) was assessed by Doppler ultrasound resistive index (RI), and superficial forearm venous pressure (FVP) was measured via an indwelling catheter. Pain rating was assessed by numerical scale. The left arm was located at heart level (control position), or on the throttle (test position). Runs were completed with and without positive pressure breathing for G protection (PBG); subjects wore full coverage anti-G trousers and chest counter-pressure. In the test position, pain increased with increasing acceleration (P < 0.0001), and was more severe with PBG at +5 Gz and +7 Gz (P < 0.05). FVP rose substantially more in the test than control position (238 +/- 17 mmHg vs. 61 +/- 8 mmHg at +7 Gz, P < 0.0001) but the presence or absence of PBG had no effect on the FVP increase during acceleration in either position. In the test position, RI fell with increasing acceleration above +5 Gz (P < 0.0001), and the fall was greater with PBG (P < 0.05). Forearm pain was thus associated with a decrease in FVR and an increase in vascular transmural pressure. PBG exacerbated forearm pain and prompted a greater fall in RI, but had no effect on FVP response. These findings support FVR but not forearm venous distension in the aetiology of +Gz arm pain.

Thoracolumbar Pain among Fighter Pilots

High +Gz forces place high stress on the spinal column, and fighter pilots flying high-performance fighter aircraft frequently] report work-related thoracic and lumbar spine pain. The aim of this study was to determine whether +Gz exposure causes work-related thoracolumbar spine pain among fighter pilots. A questionnaire was used to establish the occurrence of thoracic and lumbar spine pain during the preceding 12 months and during duties over the whole working career among 320 fighter pilots and 283 nonflying controls matched for age and sex. Thirty-two percent of the pilots and 19% of the controls had experienced pain in the thoracic spine during the preceding 12 months (odds ratio [OR] = 2.3; 95% confidence interval [CI] = 1.5-3.5; p = 0.002 for the pilots). Among the pilots, the OR increased up to 6.1 (95% CI = 1.6-23.1; p = 0.0007) with the number of +Gz flight hours. There was no difference between the groups with regard to lumbar pain during the preceding 12 months, but over their whole working careers fighter pilots (58%) had experienced lumbar pain during their duties more often than controls (48%) (OR = 1.8; 95% CI = 1.3-2.6; p = 0.002). The greater the number of +Gz flight hours, the greater the occurrence of lumbar spine pain when on duty (OR = 26.9; 95% CI = 6.2-116; p = 0.0001 for the most experienced fighter pilots). The same was not true with regard to the number of +Gz flight hours and lumbar pain during the preceding 12 months. Age had no effect on pain in the thoracic or lumbar spine. Fighter pilots flying high-performance aircraft have more work-related thoracic and lumbar spine pain than controls of the same age and sex. The difference is explained by the pilots' exposure to +Gz forces.

SELF-REPORTED AEROBIC ACTIVITY AND +Gz-LEVEL TOLERANCE IN RELATIVELY INEXPERIENCED CENTRIFUGE SUBJECTS.

979 It has been suggested that a.) aerobic exercise is ineffective in improving +Gz-tolerance in experienced centrifuge subjects, b.) excessive aerobic training can be a risk factor for untoward responses during +Gz exposure, and c.) aircrew exposed to the high +Gz should limit vigorous aerobic training. Physiologic evidence may contradict these findings by showing that intense aerobic exercise, at least acutely, improves cardiovascular response and orthostatic tolerance in some instances. The effect of aerobic training activity (self-reported from interview) on +Gz-level tolerance during centrifuge exposure was examined. To remove the influence of "centrifuge riding proficiency", all subjects were relatively inexperienced to centrifuge performance. Average weekly aerobic activity estimates were quantified in Met-hr·wk−1 and subjects categorized as engaging in no/low (Group 0, n=25), moderate (G1, n=19) or high (G2, n=13) levels of weekly aerobic activity. Subjects engaged in 2 gradual-onset rate (GOR) centrifuge exposures (with/without the anti-G straining maneuver (AGSM) to aid tolerance) where maximal +Gz level achieved was measured, and a rapid-onset rate (ROR) exposure to +9Gz for 15 s (with AGSM and anti-G suit pressurization to aid tolerance) where subjects' exposure completion (pass/fail) was measured. No group differences (p>.05) in body composition, age or weekly strength training activity existed but groups differed (p≤.05) in Met-h·wk−1 of aerobic activity engaged in, as expected. The groups didn't differ (p>.05) in +Gz levels tolerated during the GOR exposures (mean±s Gz) both with (G0=4.9±1.0, G1=5.0±0.7, G2=5.3±1.1) and without (G0=7.3±1.2, G1=7.5±1.3, G2=8.0±1.1) AGSM. The groups did differ (p≤.05) in +9Gz ROR tolerance where a larger portion of G2 (13/13), relative to G0 (14/25) and G1 (14/19), were able to tolerate (pass) the exposure. These data support further examination of the discouragement of vigorous aerobic training in high +Gz aircrew while considering the use of experienced vs. inexperienced centrifuge subjects.

Chronic Back Disability: USAF Officers at Separation and Exposure to +Gz

This is the second of two studies done to determine whether moderate Gz exposure facilitates back disability. Cumulative trauma (repeated moderate to high G) exposures could be a precipitant of back pain in the military population. There is at present no reliable method of predicting who will suffer from back pain. We looked at officer back disability rates at separation from the Air Force (1972-1993). We compared non-rated Air Force officers to pilots and navigators exposed to low G and moderate to high G. We found no significant differences between aircrew and non-aircrew individuals until 1985, when the rates for aircrew fell below those of non-rated officers. Moderate G exposure does not seem to be a predictor of subsequent back disability. We recommend a larger prospective study of all rated and non-rated officers. We recommend that each separating officer undergo a detailed physical examination and answer a detailed back questionnaire.

Renal blood flow in miniature swine during +GZ stress and anti-G suit inflation.

The effects of +GZ acceleration stress on renal blood flow were studied in chronically instrumented, conscious miniature swine. Renal blood flow was determined using the radiolabeled (46Sc, 85Sr, 141Ce, and 125I) microsphere technique (29 animals) and Doppler-principle flow probes (1 animal). The results obtained with the microsphere technique indicate that +GZ exposure with anti-G suit support caused decreases in renal blood flow throughout the kidney, and that renal blood flow remains decreased for 1-10 min after +GZ stress. Medullary blood flow appeared to recover after +GZ exposure more rapidly than cortical blood flow. The flow probe data also indicate that +GZ stress with or without anti-G suit support results in decreases in renal blood flow, and that anti-G suit inflation in the absence of +GZ can also cause significant decreases. We conclude that the reductions in renal blood flow observed in miniature swine, during +GZ stress with anti-G suit support, may be due to a combination of direct anti-G suit interference and +GZ-induced reflex increases in alpha-constrictor sympathetic tone to the renal vascular bed.