The use of near infrared spectroscopy in detecting hemorrhagic shock in swine
Wolfson, Jill Sarah
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A hemorrhagic shock study was performed at the U.S. Army's Institute for Surgical Research in San Antonio, Texas. Surgery was performed on a total of 35 swine averaging between 34-44 kg. The intention of the experimental surgery was to most accurately simulate a human internal uncontrolled hemorrhage. Internal hemorrhages often occur on the battlefield and are challenging to detect because of the compensation mechanisms of basic vital signs including, heart rate and blood pressure. During the surgery, Reflectance Medical Inc.'s Care Guide Oximeter used Near Infrared Spectroscopy to noninvasively measure regional muscle oxygen saturation (SmO2), the amount of oxygen still attached to hemoglobin in the blood after passing through the tissue. We hypothesize that the CareGuide Oximeter will show a significant decrease in SmO2 during and after the hemorrhage. Swine underwent two separate hemorrhages in order to represent a human internal uncontrolled hemorrhage. The swine needed to undergo two separate hemorrhages to compensate for the fact that the swine have a contractile spleen that can store 25% of their total blood volume. Seven of the pigs, the sham group, served as a control and did not undergo either of the hemorrhages. The study began with a controlled hemorrhage during which the swine lost 24 Ml/kg of blood at a rate of 100 Ml/min. After twenty minutes from the start of the controlled hemorrhage, a splenic injury was made in order to cause an uncontrolled hemorrhage. The swine were bled for fifteen minutes after the splenic injury was made at which point resuscitation was started. The swine were split into groups that defmed their resuscitation method: fresh frozen plasma, cryo, hextand, splenectomy, sham and platlets. The pigs that were bled showed a significant decrease in SmO2 during the controlled hemorrhage at 50% Bleed when compared to Baseline (p less than 0.05). The SmO2 also showed a significant decrease between the end of controlled hemorrhage and the end of the uncontrolled hemorrhage. This study compared the SmO2 measurements to other vital signs including systolic blood pressure, cardiac output, stroke volume and heart rate. Evaluation of the data showed that SmO2 had an immediate reaction to blood loss and did not indicate improved tissue perfusion (increased SmO2) when bleeding had stopped, unlike blood pressure which showed that the patient's condition was improving. A continuous SmO2 measurement which showed a significant decrease from baseline would be considered a benefit on the battlefield when trying to quickly diagnose the needs of the patient.
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