Computer Simulation Of Changed Gravity On Cardiovascular Responses And Potential Countermeasures

The mechanism and countermeasures of G-induced loss of consciousness (G-LOC) induced by sustained +Gz are crucial problems in aviation medicine. A rapid transition from hypogravity (fractional or zero Gz) or–Gz to hypergravity (+Gz) would result in a reduction in the +Gz tolerance, compared with high +Gz following a baseline of +1 Gz. This phenomenon is termed"push-pull effect"(PPE) and the maneuver producing PPE is called"push-pull maneuver"(PPM).PPM is employed by tactical pilots for strategic advantages, such as dive, disperse, mission escape, et al. However, it may result in a reduction of +Gz tolerance and cause unexpected accidents involving G-LOC. Due to the demands of attacking maneuver and the development of high performance aircrafts, alternations of±Gz have more chances to appear in real flight and the occurring frequency of PPM is increasing which bring serious threats to the flight safety. Therefore, study on PPE became a new research focus in the field of aviation medicine. Traditional view to the mechanism of PPE was that the arterial blood pressures above heart-level and afferent impulses of carotid baroreflex increase during the push phase. The immediate consequence were reduced heart rate, decreased cardiac output and total peripheral resistance, and reduction of arterial blood pressure at heart-level. The delay time of carotid baroreflex is about 2~5 s, therefore, the above-mentioned effect can not restore to normal immediately. When transition to high-sustained +Gz levels, arterial blood pressures above heart-level would reduce more seriously than that without preceded–Gz. Several recent studies suggested that autonomic reflexes and essentially myogenic peripheral vascular response may contribute to the push-pull effect.Most research concerning PPE has been performed with ground-based simulations, including lower body positive pressure, tilt table, and centrifuge experiments. However, these simulation methods have inherent limitations more or less. Now in the Air Force of different countries, besides increasing awareness of this potential hazard among pilots, doing pilot's best to avoid PPM, and doing anti-G straining maneuver in advance, there were no special equipments and techniques to counteract the PPE. And traditional anti-G techniques have not yet been examined to show their effectiveness in counteracting PPE.In this study, a structurally-based mechanistic model that incorporates real-time carotid baroreflex and detailed modeling of vessel segments for different anatomic regions was established, and a new research method for PPE was provided. With this model, computer simulations predicting the effect of the PPM on cardiovascular responses, the protection afforded by possible protective measures, such as extended coverage anti-G suits (ECGS) and neck pressure (NP) were performed, and the mechanism of PPM and possible countermeasures, and the effect of carotid baroreflex on PPM were explained. And the model was extended to investigate the microgravity.The main results and findings of the present work are as follows:1. Influence of PPM acceleration profile on PPETo validate the extended model for simulation study of the PPE, model output of +Gz tolerance and other physiological variables during PPM with or without NP were generated and compared with experimental data. To investigate how a PPM acceleration profile would affect +Gz tolerance, a simulation study was performed by changing the parameters among five fundamental factors of the PPM acceleration profile in the model. Model predictions have shown that the effects of different factors on the +Gz tolerance are quite different. For example, among the five factors, +Gz tolerance is reduced when the higher the +Gz-level of the baseline, and the more negative the–Gz-level during push-phase and the longer the push-phase duration. Thus, it suggests that these three factors may have a higher impact on +Gz tolerance. A baseline level of +0.5 Gz, a–Gz exposure of–0.4 Gz, and a 8-s duration of the push phase seem to be the turning points where the decreasing trend of +Gz tolerance become slower, suggesting that the effect of the carotid baroreflex may achieve its upper limit at these points due to fast rise of carotid pressure. And based on the simulation study, the onset rate has less effect on the +Gz tolerance, suggesting that the tilt table would be appropriate for PPE studies.2. Potential countermeasures for PPETo investigate the benefits of ECGS as a countermeasure for PPE, computer simulation studies were performed to predict +Gz tolerance afforded by different versions of ECGS under PPM runs. Simulation studies have indicated that a two-level pressure schedule of the ECGS (bladder inflation rate on the lower limbs was set to 95 mmHg/Gz, while that of abdomen bladder remained unchanged with 75 mmHg/Gz), would restrain the effect of PPE on +Gz tolerance almostly. And multi-level pressure schedule might improve +Gz tolerance by means of preventing blood pooling in abdomen and lower extremities. To elucidate whether NP could ba a potential countermeasure, we proposed the concept of saturation percentage index and provided an explanation of NP.3. Potential mechanisms for PPEWith the model, we investigated the delay time and action intensity of carotid baroreflex, suggesting that the delay time is not the major factor causing reduction of +Gz tolerance induced by PPM. And the relationship between gain and +Gz tolerance is provided, suggesting that training can improve +Gz tolerance, but with only small effect, and training has no special effect on PPE.4. Simulation of vascular and cardiac pressure in human during tiltingWith the simulation study, we simulated to locate the hydrostatic indifference point and predicte the profiles of changes in vascular and cardiac pressure during tilting with different angles in humans. The simulation output compare well with the published data. Therefore, based on the basis of these findings, we would like to further improve the model performance in future microgravity studies.In conclusion, this work investigated five factors of PPM acceleration profile, explored the efficacy of potential countermeasures for PPE, analyzed the effect of carotid baroreflex on PPE, and preliminarily studied the application of the model in the study of microgravity. In addition, the results of this work may contribute to the studies on the mechanism of PPE and taking effective countermeasures.