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      Heart Rate Variability during Simulated Hemorrhage with Lower Body Negative Pressure in High and Low Tolerant Subjects


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          Heart rate variability (HRV) decreases during hemorrhage, and has been proposed as a new vital sign to assess cardiovascular stability in trauma patients. The purpose of this study was to determine if any of the HRV metrics could accurately distinguish between individuals with different tolerance to simulated hemorrhage. Specifically, we hypothesized that (1) HRV would be similar in low tolerant (LT) and high tolerant (HT) subjects at presyncope when both groups are on the verge of hemodynamic collapse; and (2) HRV could distinguish LT subjects at presyncope from hemodynamically stable HT subjects (i.e., at a submaximal level of hypovolemia). Lower body negative pressure (LBNP) was used as a model of hemorrhage in healthy human subjects, eliciting central hypovolemia to the point of presyncopal symptoms (onset of hemodynamic collapse). Subjects were classified as LT if presyncopal symptoms occurred during the −15 to −60 mmHg levels of LBNP, and HT if symptoms occurred after LBNP of −60 mmHg. A total of 20 HRV metrics were derived from R–R interval measurements at the time of presyncope, and at one level prior to presyncope (submax) in LT and HT groups. Only four HRV metrics (Long-range Detrended Fluctuation Analysis, Forbidden Words, Poincaré Plot Descriptor Ratio, and Fractal Dimensions by Curve Length) supported both hypotheses. These four HRV metrics were evaluated further for their ability to identify individual LT subjects at presyncope when compared to HT subjects at submax. Variability in individual LT and HT responses was so high that LT responses overlapped with HT responses by 85–97%. The sensitivity of these HRV metrics to distinguish between individual LT from HT subjects was 6–33%, and positive predictive values were 40–73%. These results indicate that while a small number of HRV metrics can accurately distinguish between LT and HT subjects using group mean data, individual HRV values are poor indicators of tolerance to hypovolemia.

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          Heart rate variability: a review.

          Heart rate variability (HRV) is a reliable reflection of the many physiological factors modulating the normal rhythm of the heart. In fact, they provide a powerful means of observing the interplay between the sympathetic and parasympathetic nervous systems. It shows that the structure generating the signal is not only simply linear, but also involves nonlinear contributions. Heart rate (HR) is a nonstationary signal; its variation may contain indicators of current disease, or warnings about impending cardiac diseases. The indicators may be present at all times or may occur at random-during certain intervals of the day. It is strenuous and time consuming to study and pinpoint abnormalities in voluminous data collected over several hours. Hence, HR variation analysis (instantaneous HR against time axis) has become a popular noninvasive tool for assessing the activities of the autonomic nervous system. Computer based analytical tools for in-depth study of data over daylong intervals can be very useful in diagnostics. Therefore, the HRV signal parameters, extracted and analyzed using computers, are highly useful in diagnostics. In this paper, we have discussed the various applications of HRV and different linear, frequency domain, wavelet domain, nonlinear techniques used for the analysis of the HRV.
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            Epidemiology of trauma deaths: a reassessment.

            Recognizing the impact of the 1977 San Francisco study of trauma deaths in trauma care, our purpose was to reassess those findings in a contemporary trauma system. Cross-sectional. All trauma deaths occurring in Denver City and County during 1992 were reviewed; data were obtained by cross-referencing four databases: paramedic trip reports, trauma registries, coroner autopsy reports and police reports. There were 289 postinjury fatalities; mean age was 36.8 +/- 1.2 years and mean Injury Severity Score (ISS) was 35.7 +/- 1.2. Predominant injury mechanisms were gunshot wounds in 121 (42%), motorvehicle accidents in 75 (38%) and falls in 23 (8%) cases. Seven (2%) individuals sustained lethal burns. Ninety eight (34%) deaths occurred in the pre-hospital setting. The remaining 191 (66%) patients were transported to the hospital. Of these, 154 (81%) died in the first 48 hours (acute), 11 (6%) within three to seven days (early) and 26 (14%) after seven days (late). Central nervous system injuries were the most frequent cause of death (42%), followed by exsanguination (39%) and organ failure (7%). While acute and early deaths were mostly due to the first two causes, organ failure was the most common cause of late death (61%). In comparison with the previous report, we observed similar injury mechanisms, demographics and causes of death. However, in our experience, there was an improved access to the medical system, greater proportion of late deaths due to brain injury and lack of the classic trimodal distribution.
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              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).

                Author and article information

                Front Physiol
                Front. Physio.
                Frontiers in Physiology
                Frontiers Research Foundation
                22 August 2011
                21 November 2011
                : 2
                : 85
                [1] 1simpleUS Army Institute of Surgical Research, Fort Sam Houston, TX, USA
                [2] 2simpleDepartment of Health and Kinesiology, University of Texas at San Antonio San Antonio, TX, USA
                Author notes

                Edited by: Heikki Veli Huikuri, University of Oulu, Finland

                Reviewed by: Mikko Paavo Tulppo, Verve, Finland; Phyllis Kravet Stein, Washington University School of Medicine, USA

                *Correspondence: Carmen Hinojosa-Laborde, U.S. Army Institute of Surgical Research, 3698 Chambers Pass, Fort Sam Houston, TX 78234-6315, USA. e-mail: c.hinojosalaborde@ 123456us.army.mil

                This article was submitted to Frontiers in Clinical and Translational Physiology, a specialty of Frontiers in Physiology.

                Copyright © 2011 Hinojosa-Laborde, Rickards, Ryan and Convertino.

                This is an open-access article subject to a non-exclusive license between the authors and Frontiers Media SA, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and other Frontiers conditions are complied with.

                : 25 July 2011
                : 01 November 2011
                Page count
                Figures: 3, Tables: 3, Equations: 0, References: 44, Pages: 9, Words: 7124
                Original Research

                Anatomy & Physiology
                heart rate variability,hypovolemia,hemorrhage,heart period variability,lower body negative pressure


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