Heart rate variability during massive hemorrhage and progressive hemorrhagic shock in dogs

Purpose: To investigate the sequential changes in heart rate (HR), autonomi c nervous activity presented by the spectral analysis of heart rate variabi lity (HRV), hemodynamics and metabolism during massive hemorrhage and progr essive hemorrhagic shock in dogs. Methods: Twelve dogs were subjected to acute massive hemorrhage until mean arterial pressure (MAP) reached 50 mmHg. Then bleeding was stopped and they were allowed to reach a plateau phase. They were divided, port hoc, into b radycardic or tachycardic groups according to their HR response to the acut e massive hemorrhage. After reaching a plateau phase, the dogs were further bled to keep their MAP around 50 mmHg (progressive hemorrhagic shock). The ir heart rate power spectra were quantified into low-frequency (LF) (0.04-0 .15 Hz) and high-frequency (HF) (0.15-0.4 Hz) components. Results: In the bradycardic group, both LF and HF increased after massive h emorrhage, but during progressive hemorrhagic shock these components decrea sed while HR increased. In the tachycardic group, LF increased after massiv e hemorrhage, but during progressive hemorrhagic shock LF decreased with co ntinuous suppression of HF: Conclusion: Massive hemorrhage caused two types of HR response: bradycardia and tachycardia. The HRV profile showed differential autonomic characteris tics, and could be a valuable tool in assessing various degrees of hemorrha gic shock.