Simulated acute hemorrhage through lower body negative pressure as an activator of the hypothalamic-pituitary-adrenal axis

Physiology of Human Hemorrhage and Compensation

Hemorrhage is a leading cause of death following traumatic injuries in the United States. Much of the previous work in assessing the physiology and pathophysiology underlying blood loss has focused on descriptive measures of hemodynamic responses such as blood pressure, cardiac output, stroke volume, heart rate, and vascular resistance as indicators of changes in organ perfusion. More recent work has shifted the focus toward understanding mechanisms of compensation for reduced systemic delivery and cellular utilization of oxygen as a more comprehensive approach to understanding the complex physiologic changes that occur following and during blood loss. In this article, we begin with applying dimensional analysis for comparison of animal models, and progress to descriptions of various physiological consequences of hemorrhage. We then introduce the complementary side of compensation by detailing the complexity and integration of various compensatory mechanisms that are activated from the initiation of hemorrhage and serve to maintain adequate vital organ perfusion and hemodynamic stability in the scenario of reduced systemic delivery of oxygen until the onset of hemodynamic decompensation. New data are introduced that challenge legacy concepts related to mechanisms that underlie baroreflex functions and provide novel insights into the measurement of the integrated response of compensation to central hypovolemia known as the compensatory reserve. The impact of demographic and environmental factors on tolerance to hemorrhage is also reviewed. Finally, we describe how understanding the physiology of compensation can be translated to applications for early assessment of the clinical status and accurate triage of hypovolemic and hypotensive patients. © 2021 American Physiological Society. Compr Physiol 11:1531-1574, 2021.

Construction of a lower body negative pressure chamber

Lower body negative pressure (LBNP) is an established and important technique used to physiologically stress the human body, particularly the cardiovascular system. LBNP is most often used to simulate gravitational stress, but it has also been used to simulate hemorrhage, alter preload, and manipulate baroreceptors. During experimentation, the consequences of LBNP and the reflex increases in heart rate and blood pressure can be manipulated and observed in a well-controlled manner, thus making LBNP an important research tool. Numerous laboratories have developed LBNP devices for use in research settings, and a few devices are commercially available. However, it is often difficult for new users to find adequately described design plans. Furthermore, many available plans require sophisticated and expensive materials and/or technical support. Therefore, we have created an affordable design plan for a LBNP chamber. The purpose of this article was to share our design template with others. In particular, we hope that this information will be of use in academic and research settings. Our pressure chamber has been stress tested to 100 mmHg below atmospheric pressure and has been used successfully to test orthostatic tolerance and physiological responses to -50 mmHg.

Lower body negative pressure as a model to study progression to acute hemorrhagic shock in humans

Hemorrhage is a leading cause of death in both civilian and battlefield trauma. Survival rates increase when victims requiring immediate intervention are correctly identified in a mass-casualty situation, but methods of prioritizing casualties based on current triage algorithms are severely limited. Development of effective procedures to predict the magnitude of hemorrhage and the likelihood for progression to hemorrhagic shock must necessarily be based on carefully controlled human experimentation, but controlled study of severe hemorrhage in humans is not possible. It may be possible to simulate hemorrhage, as many of the physiological compensations to acute hemorrhage can be mimicked in the laboratory by applying negative pressure to the lower extremities. Lower body negative pressure (LBNP) sequesters blood from the thorax into dependent regions of the pelvis and legs, effectively decreasing central blood volume in a similar fashion as acute hemorrhage. In this review, we compare physiological responses to hemorrhage and LBNP with particular emphasis on cardiovascular compensations that both share in common. Through evaluation of animal and human data, we present evidence that supports the hypothesis that LBNP, and resulting volume sequestration, is an effective technique to study physiological responses and mechanisms associated with acute hemorrhage in humans. Such experiments could lead to clinical algorithms that identify bleeding victims who will likely progress to hemorrhagic shock and require lifesaving intervention(s).