Heart rate and blood pressure variability differentiate hemorrhagic shock severity in rats

Hemorrhagic shock remains a leading cause of preventable trauma mortality, yet early identification of physiological decompensation remains challenging because conventional vital signs often remain preserved during compensatory phases. Heart rate variability (HRV) and blood pressure variability (BPV) derived from continuous arterial waveforms reflect dynamic autonomic and hemodynamic regulation and may provide earlier indicators of cardiovascular instability. We investigated whether variability-based physiological markers could stratify hemorrhagic shock severity and whether vagal integrity influences these autonomic signatures. Male Sprague-Dawley rats underwent graded hemorrhagic shock using a delayed fluid resuscitation paradigm and were classified as moderate or severe shock. Animals were assigned to non-vagotomized and subdiaphragmatic vagotomized groups. Heart rate variability and blood pressure variability metrics, respiratory sinus arrhythmia, and indices of vagal and baroreflex regulation were derived from arterial pressure recordings obtained during steady state and the compensatory phase preceding cardiovascular decompensation. Classification performance was evaluated using discriminant and receiver operating characteristic analyses.Progressive hemorrhage was associated with reduced blood pressure variability and increased heart rate variability during compensation. In non-vagotomized animals, high-frequency diastolic blood pressure variability differentiated shock severity. In vagotomized animals, classification depended on a broader combination of autonomic and hemodynamic features. Vagotomy also altered autonomic variability responses and disrupted relationships between baseline heart rate variability and post-resuscitation vascular variability. These findings support further investigation of variability-based physiological monitoring for early detection of hemorrhagic decompensation.