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Heart period variability responses during simulated hemorrhage in high and low tolerant subjects

Effective pre‐hospital medical monitoring for early triage of trauma patients should be able to detect those patients that are most susceptible to central hypovolemia. We tested the hypothesis that Heart Period Variability (HPV) responses could differentiate subjects with low (LT) and high (HT) tolerance to simulated hemorrhage via lower body negative pressure (LBNP). Subjects (LT=33; HT=87) were exposed to LBNP (5 min/stage) until onset of presyncope. HPV metrics (n=18) were derived from electrocardiogram R‐R intervals at each stage of LBNP. Lempel‐Ziv Entropy, Fractal Dimensions by Curve Length, Stationarity, Symbol Dynamics Entropy and Poincare Plot SD 1:2 Ratio showed significant group differences between LT and HT (p≤0.05) beginning at the second level of LBNP. Despite these group differences, the variability and overlap between LT and HT subjects was so extensive that 59–100% of LT values were within the HT value range. We conclude that, while population means of HPV metrics can distinguish between LT and HT groups, HPV in individual subjects is a poor indicator of tolerance. This study raises further concern that HPV may not provide reliable or sensitive information to assist in the triage of individual trauma patients.