Evaluation of heart rate and blood pressure variability as indicators of physiological compensation to hemorrhage before shock

Individual responses to hemorrhage vary, with varying periods of compensation before the development of shock. We characterized heart rate and blood pressure variability measures during hemorrhage of 25 mL/kgBody Weight for 15 min in conscious sheep (N = 7, 14 total hemorrhages) as markers of the transition from compensated to decompensated shock using the continuous wavelet transform. Heart rateYlow frequency (HR-LF) and systolic blood pressureYlow frequency (SBP-LF) indices were developed to represent the change in spectral power during hemorrhage as low-frequency (0.06 Y 0.15 Hz) power divided by the sum of high (0.15 Y 1.0 Hz)-and very low (0.02 Y 0.06 Hz) frequency power. Heart rate rose from 96.3 (22.2) beats/min (mean [SD] across all trials) to a peak of 176.0 (25.4) beats/min occurring at a minimum time of 5.3 min to a maximum of 22.1 min (11.7 [1.6] min), depending on the trial, after the start of hemorrhage. During the HRcompensated response to hemorrhage, there was elevated HR-LF and SBP-LF in five of the seven animals. In these animals, HR-LF and SBP-LF dropped to below baseline levels around the time of the peak HR. The results from this conscious-animal study suggest that HR and SBP low-frequency power rise during the compensation phase of the response to hemorrhage in conscious sheep. Use of variability monitoring could aid in describing an individual's current response to hemorrhage and anticipation of impending decompensation; however, individual differences in the response limit this potential.