تجزیه و تحلیل سیستم های مکانیزم های تغییرات عملکرد دیاستولیک قلب پس از قرار گرفتن در معرض بی وزنی

Detailed information concerning cardiac function was collected by two-dimensional and M-mode echocardiography at 10 days before flight and View the MathML source3h after landing in astronauts returning from shuttle missions. A comparative analysis of this data suggests that cardiac diastolic function is reduced after microgravity exposure with little or no change in systolic function as measured by ejection fraction However, the mechanisms responsible for these adaptations have not been determined. In this study, an integrative computer model of human physiology that forms the framework for the Digital Astronaut Project (Guyton/Coleman/Summers Model) was used in a systems analysis of the echocardiographic data in the context of general cardiovascular physiologic functioning. The physiologic mechanisms involved in the observed changes were then determined by a dissection of model interrelationships. The systems analysis of possible physiologic mechanisms involved reveals that a loss of fluid from the myocardial interstitial space may lead to a stiffening of the myocardium and could potentially result in some of the cardiac diastolic dysfunction seen postflight. The cardiovascular dynamics may be different during spaceflight.

The potential risks associated with cardiovascular deconditioning during spaceflight have been a central concern in the study of the human physiologic adaptation to the microgravity environment [1]. Changes in cardiac diastolic function have recently been noted after short duration spaceflight in returning astronauts. However, the mechanisms responsible for these adaptations have not been determined. Physiologic adaptation to the microgravity environment is complex and requires an integrative perspective for a more complete understanding. In this study, a systems analysis approach using a detailed computer model of physiologic functioning was employed to examine the possible mechanisms involved in the observed changes [2] and [3].

A detailed understanding of the impact of microgravity exposure on cardiac function is necessary for the risk analysis of a Mars mission and critical to the future of space exploration in general. Previous echocardiographic studies have noted a preservation of cardiac systolic function in returning astronauts after short-duration missions despite some evidence of a loss in left ventricular mass [9]. However, this study presents data suggesting that there may be a detriment in cardiac diastolic function following similar short-duration spaceflights. In order to delineate the complex physiologic mechanisms responsible for this seemingly paradoxical adaptive response, the Digital Astronaut computer model was used for a systems analysis approach to the problem. While the depleted plasma volume commonly seen in returning astronauts could contribute to the increases in the IVRT and decreases in the E/AE/A ratio, the flow propagation velocity (color M-mode) has been determined to be preload independent and for this reason has become a popular measure of diastolic function [10]. While the diastolic function was not convincingly reduced as measured by the reportedly preload independent tissue Doppler imaging technique, there was trending toward a reduction in this ratio. The simulation study of short-duration spaceflight using the computer model appears to comparably predict the microgravity induced physiologic adaptations in cardiac function found experimentally [11]. A model-based systems analysis and dissection of the theoretical physiologic adjustments to the microgravity environment suggests that myocardial interstitial space fluid changes could explain the changes in left ventricular mass [11] (Fig. 2). This lack of decrement in the solid muscle elements of the myocardial tissue might also clarify the observed maintenance of systolic function under the conditions of a loss in overall cardiac mass. However, there is some evidence to support the concept that this relative dehydration of the ventricle could lead to a stiffening of the myocardium and contribute to the cardiac diastolic dysfunction observed postflight. Pogasta et al. have demonstrated a bilinear correlation between cardiac tissue water content and diastolic stiffness [12] and [13]. In this study a “normal” state of myocardial hydration was required to achieve the optimum diastolic function. The literature also supports the concept that either a volume overload or a relative dehydration would result in the some degree of diastolic stiffness [14]. In a series of elegant experiments, Templeton et al. have similarly found that the process of osmotically drawing fluid from the myocardium results in a stiffening of the ventricle [15]. Dialysis patients are also subject to these types of fluid shifts with an acute loss in measured left ventricle mass and limitations in diastolic function [16]. When these established physiologic relationships are incorporated into the model structure of the Digital Astronaut (Fig. 3), there is a shifting of the cardiac diastolic compliance curve when there is a simulation of return from short-duration spaceflight (Fig. 4). This limitation in diastolic function could have important operational consequences in the case where intense exercise and cardiac reserves are required immediately upon landing. Previously published evidence suggests that this myocardial fluid shift adaptation is resolved within days of returning to earth and therefore would not necessarily have a long-term impact [17].It is interesting to note that the model predicts a different cardiovascular dynamics during spaceflight. As opposed to the reductions in diastolic cardiac compliance observed postflight, the model predicts that the upward and outward rotation of the chest wall in microgravity and reductions in intrapleural pressures would counter the dehydrating effect on myocardial stiffness and produce a near normal cardiac compliance curve (Fig. 5). This concept of the impact of microgravity induced chest wall changes on cardiac function has been previously reported and has been postulated as the mechanism responsible for the reduced central venous pressures observed during spaceflight [18] and [19].This study has been limited to short-duration flight and it is quite possible that a more significant impact on cardiac function involving different physiologic mechanisms will be involved during longer duration missions. It is important that similar measurements are procured in astronauts returning from long-duration missions if we are to develop an appropriate risk analysis for interplanetary flights. However, the current analysis is also important in that the systems analysis approach requires that we build upon an understanding of sequential changes due to the time-dependent nature of physiologic adaptive mechanisms. This type of complex interplay of mechanisms can best be understood with the use of integrative models such as the Digital Astronaut that allows us to dissect the systems involved and quantify the relative importance of various physiologic responses. In this way we can begin to consider the risks and consequences of long-duration spaceflight and develop more effective countermeasures.