Heart rate variability and its impact on pulmonary dysfunction in rats subjected to hemorrhagic shock

Introduction: The activity of autonomic nervous system and its association with organ damage have not been entirely elucidated in hemorrhagic shock. The aim of this study was to investigate heart rate variability (HRV) and pulmonary gas exchange in hemorrhagic shock during unilateral subdiaphragmatic vagotomy. Methods: Male Sprague Dawley rats were randomly assigned into groups of sham, vagotomized (Vag), HS and Vag+HS. HS was induced in conscious animals by blood withdrawal until reaching to mean arterial blood pressure (MAP) of 40±5 mm Hg. Then, it was allowed to MAP returning toward the basal values. MAP and heart rate (HR) were recorded throughout the experiments, HRV components of low (LF, sympathetic index), high (LH, parasympathetic index), and very low (VLF, injury index) frequencies and the LF/HF ratio (sympathetic index) calculated, and the lung histological and blood gas parameters assessed. Results: In the initial phases of HS, the increase in HR with no change in MAP were observed in both HS and Vag+HS groups, while LF increased only in the HS group. In the second phase, HR and MAP decreased sharply in the HS group, whereas, MAP decreased only in the Vag+HS group. Meanwhile, LF and HF increased relative to their baselines in the HS and Vag+HS groups, even though the values were much pronounced in the HS group. In the third phase, HR, MAP, LF, HF, and the LF/HF ratio were returned back to their baselines in both HS and Vag+HS groups. However, in the Vag+HS group, the VLF was lower and HR was higher than those in the other groups. Furthermore, blood gas parameters and lung histology indicated the impairment of gas exchange in the Vag+HS group. Conclusions : The sympathetic activity is predominant in the first phase, whereas the parasympathetic activity is dominant in the second and third phases of hemorrhagic shock. Furthermore, hemorrhagic shock with vagotomy may be linked to lung injury and decreased VLF.