Department of Integrative Physiology and Anatomy, University of North Texas Health Science Center, Fort Worth, Texas, USA.
Compr Physiol. 2015 Sep 20;5(4):1585-621. doi: 10.1002/cphy.c140058.
Massive uncontrolled blood loss can occur under a variety of conditions including trauma, as a complication of childbirth or surgery, ruptured ulcers, clotting disorders, and hemorrhagic fevers. Across the continuum of hemorrhage, loss of blood volume is a significant challenge to the maintenance of cerebral perfusion. During the initial stages of hemorrhage, reflex mechanisms are activated to protect cerebral perfusion, but persistent blood loss will eventually reduce global cerebral blood flow and the delivery of metabolic substrates, leading to generalized cerebral ischemia, hypoxia, and ultimately, neuronal cell death. Cerebral blood flow is controlled by various regulatory mechanisms, including prevailing arterial pressure, intracranial pressure, arterial blood gases, neural activity, and metabolic demand. Hemorrhage represents a unique physiological stress to the brain, as it influences each of these regulatory mechanisms, resulting in complex interplay that ultimately challenges the ability of the brain to maintain adequate perfusion. Early studies of actual hemorrhage in humans employed blood loss protocols up to 1000 mL, but did not include any measurements of cerebral blood flow. As ethical considerations necessarily constrain the use of human volunteers for massive blood loss studies that induce irreversible shock, most of what is known about cerebral blood-flow responses to hemorrhage has been determined from animal models. Limitations of species differences regarding regulatory mechanisms, anatomy, and the effect of anesthesia, however, must be considered. Advances in monitoring technologies, and a recent renewed interest in understanding cerebral blood-flow regulation in humans, however, is rapidly accelerating knowledge in this field.
大量的失控性失血可能发生于多种情况下，包括创伤、分娩或手术并发症、溃疡破裂、凝血障碍和出血性发热。在出血的整个过程中，血容量的丢失是维持脑灌注的重大挑战。在出血的初始阶段，反射机制被激活以保护脑灌注，但持续的失血最终将导致全脑血流和代谢底物的输送减少，从而导致广泛的脑缺血、缺氧，并最终导致神经元细胞死亡。脑血流由多种调节机制控制，包括动脉压、颅内压、动脉血气、神经活动和代谢需求。出血对大脑来说是一种独特的生理应激，因为它影响到所有这些调节机制，导致复杂的相互作用，最终挑战大脑维持足够灌注的能力。人类实际出血的早期研究采用了高达 1000 毫升的失血方案，但没有包括任何脑血流测量。由于伦理考虑必然限制了使用人类志愿者进行导致不可逆休克的大量失血研究，因此，关于出血后脑血流反应的大部分知识是从动物模型中确定的。然而，必须考虑到物种差异对调节机制、解剖结构和麻醉效果的影响。监测技术的进步，以及最近对理解人类脑血流调节的重新关注，正在迅速加速这一领域的知识发展。
