The overall goal of this procedure is to simulate central hypovolimia associated with hemorrhage in humans using lower-body negative pressure and to assess the individual's capacity to compensate during this challenge. Lower-body negative pressure is a valid model of human hemorrhage, which has been used to identify early predictors of hemorrhagic shock. The main advantages of this procedure are that all measurements are acquired by non-invasive methods and that tolerance to hemorrhage can be safely assessed in the subjects.
Using lower-body negative pressure, we developed the measurement of compensatory reserve which can provide an early assessment of the progression towards shock during hemorrhage and is more accurate than the current tools to monitor vital signs. Compensatory reserve reflects the integration of all physiological compensatory mechanisms involved in the response to hemorrhage in an individual patient and therefore tracks the progression to shock with a high degree of specificity. Prior to the study visit, instruct the subject to avoid caffeine, alcohol, and strenuous exercise 24 hours prior to testing and to avoid eating at least two hours prior in the even that hemodynamic decompensation induces nausea.
On the study day, begin by turning on all the equipment that requires warm-up and calibration:a data acquisition system, two infrared finger sensors, a capnograph, and a finger pulse oximeter. Synchronize all of the instruments with internal clocks by adjusting the timestamp on each instrument to match a lab master-clock that should be used to mark time during the experiment. Next, escort the subject into an exam room and perform a medical screening exam to ensure that the subject meets minimal health requirements as well as all inclusion criteria.
Then, bring the subject into the testing room, inform them about the experimental procedure, and obtain written consent to participate in the study. Next, place the neoprene lower-body negative pressure, or LBNP, skirt on the subject. Insure that the skirt is snug around the waist and torso in order to create an airtight seal.
Instruct the subject to lay supine on the bed of the LBNP chamber while straddling a stationary post to secure the torso in place. Then, ask the subject to relax their lower body. Slide the bed into the chamber and attach the neoprene skirt to the chamber opening in order to create an airtight seal.
Next, place electrocardiogram, or ECG, electrodes on the right and left humeral clavicular joints and on the right and left lower ribs in a modified lead to configuration for continuous measurement of heart rate. Position the subject's arms on the arm rests, adjusted so that the hands are supported at heart level. Place an infrared finger photoplethysmography device on the left and right middle fingers for continuous non-invasive beat to beat measurement of blood pressure.
Enter subject information to enable the appropriate assumptions for calculation of stroke volume, cardiac output, and peripheral vascular resistance by the model flow algorithm. Then, attach the finger cuffs to the pressure monitors. Calibrate the devices and record blood pressure according to the manufacturer instructions.
Then, place the finger pulse oximeter on the right index finger for continuous measurement of compensatory reserve. Finally, place an nasal cannula on the subject and instruct them to breathe through their nose to assure sensitive reflections in inspiration and expiration. Connect the nasal cannula to the capnograph for the continuous measurement of respiration and end tidal CO2.
Begin data recording by clicking the start button on the data acquisition system and record baseline data for five minutes. Initiate the first level of central hypovolimia by turning on the vacuum motor and setting negative pressure to negative 15 millimeters of mercury and hold this pressure for five minutes. Then, increase the LBNP to negative 30 millimeters of mercury, negative 45 millimeters of mercury, negative 60 millimeters of mercury, and to negative 70 millimeters of mercury and hold each pressure for five minutes.
Continue to increase LBNP levels by negative 10 millimeters of mercury every five minutes until negative 100 millimeters of mercury LBNP or the point of hemodynamic decompensation where the compensatory reserve index, or CRI value, is zero. During the experiment, observe the progressive decrease in the reserve to compensate which is represented by a gradual reduction in the colored bar on the screen. As the bar changes colors from green to yellow to red, notice that the measurement of the CRI moves from a value of one to zero.
After the experiment is complete, terminate the LBNP by pressing the pressure release button on the LBNP chamber. Continue recording data on the data acquisition system for 10 minutes after the cessation of LBNP. Stop recording data at the end of the 10 minute recovery period by clicking the stop button on the data acquisition system.
Detach all instrumentation from the subject and remove the subject from the LBNP chamber. Ask the subject to sit after stepping down from the LBNP platform to ensure they are symptom-free before leaving the laboratory. Finally, download data files from the acquisition system for extraction of the CRI, mean arterial pressure, heart rate, and SpO2 values.
The progressive reduction in central blood volume, or LBNP, produces significant alterations in the features of the arterial wave form. Here, the values for mean arterial pressure, MAP, heart rate, SpO2, CRI, and LBNP are plotted against time. CRI decreases early and progressively throughout the multiple steps of LBNP while changes in heart rate, mean arterial pressure, and SpO2 occur during later phases of hemorrhage.
The LBNP technique will continue to be used as a valid model of human hemorrhage to provide data for creating, testing, and refining future algorithms and devices to measure compensatory reserve. The ability to measure the compensatory changes associated with blood loss is critical to providing acute care in emergency situations in both military and civilian settings.
