INTRODUCTION: There is no consensus on whether cardiopulmonary reserve affects the risk of gravity-induced loss of consciousness (G-LOC) or almost loss of consciousness (A-LOC). Few previous studies have used cardiopulmonary exercise testing (CPET) to assess cardiopulmonary reserve function (CPRF) of fighter aviators. We compared CPET-related parameters in G-LOC/A-LOC and non-G-LOC/A-LOC fighter aviators to explore the effect of cardiopulmonary reserve function on G tolerance.
METHODS: A total of 264 male fighter aviators with more than 500 h of flight experience participated in the study, all of whom underwent CPET and human centrifuge testing. We divided the aviators into two groups based on whether they experienced G-LOC/A-LOC during the human centrifuge test and compared the CPET parameters between the two groups.
RESULTS: A total of 37 aviators (14%) experienced G-LOC/A-LOC. There were no significant differences in age (26.65 ± 4.30 vs. 26.01 ± 4.95), height (173.68 ± 4.21 vs. 173.55 ± 3.37), weight (69.51 ± 6.22 vs. 69.63 ± 6.01), or body mass index (23.06 ± 2.11 vs. 23.11 ± 1.82) between the two groups. Forced vital capacity (FVC) (4.95 ± 0.87 vs. 4.65 ± 0.79) and forced expiratory volume in 1 s (FEV1) divided by FVC (FEV1/FVC) (79.88 ± 7.24 vs. 83.72 ± 9.24) of pulmonary function of the G-LOC/A-LOC group was significantly lower than that of the non-G-LOC/A-LOC group. There was no significant difference in CPET-related parameters between the two groups.
DISCUSSION: In conclusion, FEV1/FVC may be a factor affecting aviators’ G-LOC/A-LOC, meaning aviators with slightly lower ventilation are more likely to experience G-LOC/A-LOC. However, oxygen uptake and exercise blood pressure, oxygen pulse, etc., may not be the main factors influencing G-LOC/A-LOC.
Lan X, Zhu W, Du J, Wang J, Yang M, Xu Y, Cao Y. High G tolerance and cardiopulmonary reserve function in healthy air force aviators. Aerosp Med Hum Perform. 2023; 94(12):911–916.
As modern fighter jets become more maneuverable, the inertial force, high load, and duration of high load caused by high-performance fighter jets place high demands on the G tolerance of the aviator. 10 Excellent cardiopulmonary reserve is essential for high-performance fighter aviators to maintain normal functioning of their vital organs under high load, hypoxia, and exposure to low temperatures. Aviators experienced G forces from three directions when maneuvering in the air. The force from the head to the feet can be quantified as the gravitational force (+Gz), which increases the hydrostatic pressure and redistributes the flow of blood through the blood column of the body, putting an enormous pressure on the cardiovascular system. 12 If +Gz causes the cardiovascular system to fail to maintain normal blood flow to the brain, aviators may experience cerebral ischemia, hypoxia, sensory changes, and even G-induced loss of consciousness/almost loss of consciousness (G-LOC/A-LOC). 15 , 20 The British Air Force first reported this phenomenon during World War I. 5 Since then, many similar phenomena have been reported and many measures to solve these problems have been studied worldwide, such as the anti-G straining maneuver (AGSM), anti-G positive pressure breathing and anti-G clothing. 1 , 4 , 9 Related studies have shown that G-LOC/A-LOC is closely associated with the regulation of blood flow and the body’s reserve of oxygen. 19 Previous data have shown that cardiorespiratory parameters such as heart rate, mean arterial pressure, myocardial contractility, cardiac output and total peripheral resistance, and ventilation/blood flow ratio are factors influencing G tolerance. 17 , 18 , 22 As those factors are certainly correlated with each other, it is difficult to consolidate them into a simple one during practical training. Traditional cardiopulmonary function evaluation methods, such as echocardiography, chest computed tomography scan, BNP test, etc., cannot thoroughly evaluate the cardiopulmonary reserve function (CPRF). Cardiopulmonary exercise testing (CPET) can comprehensively and objectively evaluate CPRF and overall body function, 7 but most previous studies on the issue of G tolerance in aviators did not include CPET. This study compares the relevant parameters of CPET between aviators in the poor G-tolerance group and those in the good G-tolerance group to explore the relationship between CPRF and high G tolerance in aviators.
METHODS
Subjects
Male aviators who underwent a regular physical examination in the Air Force Medical Center, Chinese People’s Liberation Army, from December 2020 to December 2022 were included in this study. The number of female aviators was small and none volunteered, so they were not used. The aircraft type was limited to those exposed to high G forces, such as fighters. They were divided into two groups according to the manned centrifuge test results. This study was approved by the Ethics Committee of the Air Force Medical Center, PLA (2021-99-YJ01), and was conducted in accordance with the latest revision of the Declaration of Helsinki. Written informed consent was obtained from all subjects.
Inclusion criteria included: being 25–40 yr of age, with at least 500 flight hours, ability to pass the physical examination according to the “Chinese Air Force Aviator Physical Examination Standards”, experienced in assessing the endpoint of human centrifugation, having normal visual and auditory function, having no lower extremity or cardiovascular/cerebrovascular diseases, having no respiratory disease or dysfunction, having no history of mental illness or epilepsy, and not taking drugs that affect the cardiovascular system. In addition, subjects should not have consumed alcohol, coffee, or other stimulant beverages for 2 wk prior to the study, and should not be taking antidepressants or sedatives, or engage in strenuous exercise or other energy-consuming activities for 3 d prior to the study. Exclusion criteria included: being clinically diagnosed with pulmonary hypertension, high blood pressure, cardiovascular or cerebrovascular diseases, or other diseases; obvious psychological barriers; or unable to complete the operation in the confined space of the human centrifuge.
Procedures
CPET was performed on a bicycle ergometer (Ergoline 200 k; Ergoline GmbH, Bitz, Germany). We used a progressive maximal symptom-limited CPET protocol on the cycle ergometer, tailored explicitly for the athletes. Subjects commenced cycling at 0 W for 3 min; this workload was increased by 25 W every minute, with a 2-min recovery without workload, such that a maximal effort was attained within 10–15 min. It was required that the speed of the power cycle meter during no-load warm-up and load exercise was 70 rpm, the up and down was not to exceed 5 rpm, and the rate of the power cycle meter during no-load recovery was about 40 rpm. The cardiopulmonary exercise tester (model: MasterScreen) is from Vyaire Medical GmbH, Höchberg, Germany. Subjects completed equipment calibration, wore a 12-lead electrocardiogram and sphygmomanometer, and wore a gas collection mask (to ensure sealing). A subject reached maximal effort when three of the following four criteria were met: 1) the test was completed when patients reached 90% of their estimated maximal heart rate; 2) maximum respiratory quotient ≥1.15; 3) there was a leveling off of oxygen uptake (<150 mL · min−1) despite increased power; or 4) the subject experienced exhaustion due to fatigue, with an inability to maintain 60 rpm. The test was terminated immediately if any of the following points occured during the test: 1) electrocardiogram had severe arrhythmia or definite ischemic changes; 2) blood pressure increased to >250/130 mmHg (1 mmHg = 0.133 kPa) or blood oxygen saturation (Spo2) decreased to <90%; or 3) chest pain, dizziness, dyspnea, or other symptoms were experienced.
The relevant parameters of CPET measured in this study are as follows: tidal volume (VT); vital capacity (VC); forced vital capacity (FVC); forced expiratory volume in 1 s (FEV1); FEV1/FVC, maximum volume ventilation; maximum load (maximum work rate); max metabolic equivalent; anaerobic threshold oxygen uptake ( o2 at anaerobic threshold, o2AT); max oxygen uptake ( o2Max); anaerobic threshold kilogram oxygen uptake ( o2/kg); carbon dioxide production ( co2). Anaerobic threshold (AT) was also expressed as values normalized by bodyweight (mL · min−1 · km−1).
The aviators’ +Gz endurances were tested on a manned centrifuge (AMST-HC-4E human centrifuge from AMST Technology Company, Ranshofen, Austria) in the Air Force Medical Center, Chinese People’s Liberation Army. The centrifuge was configured with an upright (17° seat back angle) seat. All subjects were studied during +Gz exposures of rapid onset (3 G · s−1) commencing from +1.4 G (idle level) to +8 G, which was maintained for 10 s and terminated with a decelerating rate of −3 G · s−1 until reaching 1 G. All subjects wore the same standard anti-G clothing and were able to do their AGSM as and when required. During the process, head-up display videotapes were recorded and monitored by a specialist in training. The human centrifuge training report (+8 Gz, 10 s) was confirmed by two senior professionals from the Department of Accelerated Physiology Laboratory of the Air Force Medical Center, PLA. The test outcome of the 8.0-G profile was classified into G-LOC/A-LOC and non-G-LOC/A-LOC categories. One central red light and two peripheral white lights were used to assess loss of vision. If subjects showed myoclonic or seizure-like activity, involuntary discontinuity of AGSM, or similar signs, or loss of ear opacity pulse or severe loss of peripheral vision, it was defined as G-LOC/A-LOC. Subjects were allowed to self-terminate a centrifuge test if they experienced acute lumbar or cervical spine injury, vomiting, or similar. However, such cases were no longer included in this study. Non-G-LOC/A-LOC meant that the subject could tolerate 10 s at the plateau of 8.0 G without losing consciousness.
Statistical Analysis
Statistical analyses were performed with the use of the Statistical Package for the Social Sciences (version 26.0, SPSS Inc., Chicago, IL, United States) and G Power (version 2020.3.1.9.7, Düsseldorf, Germany). The measurement data was expressed as mean ± SD. Two independent sample t-tests were used to compare the two groups of normal distribution data. The Mann-Whitney U-test was used for nonnormal data. Post hoc power analysis showed that a sample size of 227 and 37 subjects in the respective groups would provide 99.8% power to detect a difference in means assuming an effect size of 0.80 and a 2-sided α of 0.05. The level of statistical significance was set at a P-value less than 0.05.
RESULTS
A total of 264 male fighter aviators with more than 500 h of flight experience participated in the study, all of whom underwent CPET and human centrifuge testing. Included in the G-LOC/A-LOC group were 37 aviators (14%) who experienced G-LOC/A-LOC, and 227 aviators were included in the non-G-LOC/A/LOC group. Baseline data including age, height, weight, BMI, etc., had no significant differences between the two groups (P > 0.05,
Table I
).
FVC (4.95 ± 0.87 vs. 4.65 ± 0.79) and FEV1/FVC (79.88 ± 7.24 vs. 83.72 ± 9.24) of pulmonary function of the G-LOC/A-LOC group was significantly lower than that of the non-G-LOC/A-LOC group (P < 0.05). While VT, VT % Predicted, VC, VC % Predicted, FVC % Predicted, FEV1, and other indicators had no statistical differences between the two groups (P > 0.05, Table I). There were no statistically significant differences (P > 0.05) in the parameters of CPET between the G-LOC/A-LOC group and non-G-LOC/A-LOC group (
Table II
).
DISCUSSION
We explored the association between G-LOC/A-LOC and CPRF parameters such as CPET and pulmonary function by investigating a relatively large number of young and relatively experienced aviators in high-performance aircraft. Previous studies 6 showed that the incidence of G-LOC events among Chinese aviators was 8.2%, and 18.7% of them had experienced a brief loss of consciousness. This was consistent with our study. Previous studies 6 , 20 showed that the flight duration of aviators was one of the critical factors affecting the occurrence of G-LOC/A-LOC. Therefore, flying time was limited for all aviators to more than 500 h in our study to reduce its impact on G tolerance.
Pulmonary function tests showed differences in FVC and FEV1/FVC between the G-LOC/A-LOC group and non-G-LOC/A-LOC group in our study. FVC may not fully represent lung ventilatory function as it is easily influenced by height, weight, and age. FEV1/FVC is more suited to describe the impact of aviator G tolerance. Our study found that the G-LOC/A-LOC group had a lower FEV1/FVC ratio than the non-G-LOC/A-LOC group and maximum volume ventilation, a measure of large airway function, did not differ between the two groups. It suggests that aviators in the non-G-LOC/A-LOC group have stronger respiratory muscle rapid recruitment and explosive force, which may be more conducive to promoting pulmonary ventilation and blood flow exchange, thereby increasing blood oxygen content and ultimately reducing the accumulation of blood lactate level in the body. 23 The lower blood lactate levels improve the body’s ability to withstand hypoxia and overall muscle fatigue. Previous studies 13 , 28 have also shown that muscle mass affects G tolerance, such as lower limb and respiratory muscles. In addition, pulmonary function tests can indirectly indicate the level of intrathoracic pressure during AGSM, and a higher FEV1/FVC predicts that intrathoracic pressure can rise rapidly in a shorter period, which can compensate for weak cardiovascular responses and improve cerebral blood flow.
It is well known that a good CPRF is a crucial physiological basis for determining whether an aviator is able to adapt to training and flying. G-LOC/A-LOC is affected by many factors, and previous studies 17 , 18 , 22 have shown that CPRF may be one of the critical influencing factors. CPET parameters are the most objective and comprehensive gold standard for evaluating cardiopulmonary function. 7 o2Max and maximum blood pressure are the best indicators for reflecting cardiorespiratory fitness. 8 , 26 Previous data has shown that anaerobic exercise training can improve the G tolerance of high-performance fighter aviators, 3 with o2Max being a measure of aviators’ exercise endurance. However, in our study, there was no difference in o2Max between G-LOC/A-LOC and non-G-LOC/A-LOC, indicating that o2Max may not be the main factor determining G tolerance. The fundamental mechanism of G-LOC/A-LOC in high-performance fighter aviators is ischemia and hypoxia, whereas there is almost no ischemia in CPET, which may be one of the reasons why there is no significant difference between the two groups.
Normal blood pressure is one of the critical conditions to maintain the body’s operation. Previous studies 2 , 21 showed that blood pressure, especially diastolic blood pressure, was important during aviators’ flight and centrifuge training. Therefore, we measured blood pressure parameters during the CPET test, but there was no significant difference between the G-LOC/A-LOC and non-G-LOC/A-LOC groups. The change in blood pressure during exercise is a manifestation of exercise endurance and a response of the body to stress. It may also be somewhat reflective of changes in blood pressure as the aviator experiences G-LOC/A-LOC. Although maximum blood pressure has no significant statistical difference, the mean blood pressure of the G-LOC/A-LOC group is lower than that of the non-G-LOC/A-LOC group. This is possibly because higher diastolic blood pressure within a certain range helps maintain blood flow to the heart and brain. When aviators perform curve movements such as circling, somersault, and half-roll inversion in the air, blood pressure drops and blood flow transfers to the lower body due to gravity and hydrostatic pressure. When the pressure of the carotid sinus decreases due to +Gz, heart rate increase is one of the first compensatory physiological reactions. It takes 6–12 s for the human body to adjust to changes in blood pressure, 27 during which time a plane may lose control or even crash. In addition, although there were no significant statistical differences between the two groups in systolic blood pressure at anaerobic threshold and at maximum, the average values of both in aviators with good G tolerance were higher. Erickson et al. 11 found that the blood pressure at the aortic root of miniature pigs decreased when exposed to +Gz, and the pigs lost consciousness.
Our study showed no significant difference in the CPET parameters between the G-LOC/A-LOC group and the non-G-LOC/A-LOC group (Table II). Still, aviators with good cardiorespiratory reserve may reduce G-LOC/A-LOC duration. Unfortunately, this aspect was not included in this study, which may be one of the future research directions. Of course, this raises the issue of Type II error, but power analysis reveals that the power in this study exceeded 90%.
Previous studies 24 , 25 showed that some physiological and physical factors of the human body also influenced G tolerance, such as age, height, weight, and BMI. Unfortunately, these findings are still controversial. Cao et al. 6 showed that age was positively correlated with G tolerance of high-performance fighter aviators. However, in our study, all aviators were young and similar in age, and age had little impact on the test results of the manned centrifuge. Some studies 14 , 16 have shown that taller pilots are more likely to suffer from G-LOC/A-LOC because orthostatic stress is more significant in taller people. This also suggested that the aviator’s G tolerance had a complex relationship with physiological indicators such as height, BMI, and CPRF. Therefore, the comprehensive assessments and effects of these parameters on aviators’ G tolerance required further study.
There are some limitations in this study. The study is a single-country, cross-sectional study. Due to differences in race, aircraft type, training methods, and other factors, the applicability of this study to the wider international military aviator community remains to be further confirmed. To the best of our knowledge, this study is the first to use CPET parameters to assess G tolerance in aviators. It can provide a novel idea and method for research in military aeromedicine. Aviators’ G tolerance may be affected by many factors, such as sleep, depression, muscle tension, muscle group, or even relevant biochemical indicators. But these factors were not included in our study. In addition, female military aircrew were excluded from the analysis due to the small sample size.
In conclusion, FEV1/FVC may be a factor affecting aviators’ G-LOC/A-LOC, meaning aviators with slightly lower ventilation are more likely to experience G-LOC/A-LOC. However, o2 and exercise blood pressure, oxygen pulse, etc., may not be the main factors influencing G-LOC/A-LOC.
Contributor Notes
