Integrated Compensatory Responses in a Human Model of Hemorrhage

Summary

The purpose of this protocol is to demonstrate the techniques for measuring compensatory responses to reduced central blood volume using lower body negative pressure as a noninvasive experimental model of human hemorrhage which can be used to quantify the total integration of compensatory mechanisms to blood volume deficit in humans.

Abstract

Hemorrhage is the leading cause of trauma-related deaths, partly because early diagnosis of the severity of blood loss is difficult. Assessment of hemorrhaging patients is difficult because current clinical tools provide measures of vital signs that remain stable during the early stages of bleeding due to compensatory mechanisms. Consequently, there is a need to understand and measure the total integration of mechanisms that compensate for reduced circulating blood volume and how they change during ongoing progressive hemorrhage. The body's reserve to compensate for reduced circulating blood volume is called the 'compensatory reserve'. The compensatory reserve can be accurately evaluated with real-time measurements of changes in the features of the arterial waveform measured with the use of a high-powered computer. Lower Body Negative Pressure (LBNP) has been shown to simulate many of the physiological responses in humans associated with hemorrhage, and is used to study the compensatory response to hemorrhage. The purpose of this study is to demonstrate how compensatory reserve is assessed during progressive reductions in central blood volume with LBNP as a simulation of hemorrhage.

Introduction

The most important function of the cardiovascular system is the control of adequate perfusion (blood flow and oxygen delivery) to all tissues of the body through homeostatic regulation of arterial blood pressure. Various mechanisms of compensation (e.g., autonomic nervous system activity, cardiac rate and contractility, venous return, vasoconstriction, respiration) contribute to maintain normal physiological levels of oxygen in the tissues.¹ Reductions in circulating blood volume such as those caused by hemorrhage can compromise the ability of cardiovascular compensatory mechanisms and ultimately lead to low arterial blood pressure, serious tissue ....

Protocol

Prior to any human procedure, the institutional review board (IRB) must approve the protocol. The protocol used in this study was approved by the US Army Medical Research and Materiel Command IRB. The protocol is designed to demonstrate the physiological responses of compensation to a progressive reduction in central blood volume similar to that experienced by individuals during ongoing hemorrhage in a controlled and reproducible laboratory setting. The laboratory room temperature is controlled at 23 - 25 ˚C.

1. Equipment Preparation

1. Turn on equipment and devices requiring warm-up and calibration.
   NOTE: Equipment and devices include....

Results

The LBNP procedure causes a reduction in air pressure around the lower torso and legs. As this vacuum is progressively increased, blood volume shifts from the head and upper torso to the lower body to create a state of central hypovolemia. The progressive reduction in central blood volume (i.e., LBNP) produces significant alterations in the features of the arterial waveform measured with the infrared finger photoplethysmograph (Figure 5). The Compensatory Reserve.......

Discussion

Using LBNP to cause progressive and continuous reductions in central blood volume, we were able to induce a typical response of hemodynamic decompensation in the subject, characterized by a sudden onset of hypotension and bradycardia (Figure 7). It is important to understand that the integrated compensatory response to hemorrhage is very complex,¹⁹ resulting in significant individual variability in the tolerance to blood loss.¹ As such, some individuals have relatively responsive co.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Dynamic Research Evaluation Workstation (DREW) data acquisition syetem	NA	NA	Custom Built by ISR personnel. The DREW allows for time synchronization of both digital and analog signal data collection from up to 16 independent instruments with a sampling rate of 1,000 Hz.
Finometer	Finapress Medical Systems (FMS)	Model 1	Device that provides noninvasive, continuous measurements of brachial artery blood pressure and arterial oxygen saturation (SpO2) using two separate infrared finger photophlethymography cuff sensors.
BCI Capnocheck Plus	Smith Medical PM Inc.	9004	Capnograph used to measure end tidal CO2 and respiration rate
CipherOX	Flashback Technologies Inc.	R200	Investigational device used to calculate Compensatory Reserve Index (CRI)
Nonin 9560 Pulse Oximeter	Nonin	9560	finger pulse oximeter
Lower Body Negative Pressure Chamber (LBNP)	NASA	79K32632-1	Custom Chamber built by NASA
ECG Biotach	Gould	13-6615-65	Electrocardiograph for measuring ECG
Nasal CO2 Sample Line	Salter Labs	REF 4000	Latex free nasal cannula for sampling expired air