Transfer function analysis of baroreflex function in a rabbit model of endotoxic shock

Sepsis is characterized by profound cardiovascular abnormalities which may result from the derangements in the arterial baroreflex system and other autonomic regulatory functions. In this study, a mechanically ventilated and anesthetized rabbit model of endotoxic shock was utilized to mimic the behaviors of the cardiovascular system in a sepsis patient. 13 adult New Zealand white rabbits were studied, with 8 of them injected with endotoxin and the remaining given saline solution as sham fluid. Measurements of heart period (RRi) and systolic blood pressure (SBP) were obtained pre-and post-intervention after a 90 minute period, which allowed spectral and cross-spectral analysis of heart rate and blood pressure variabilities to be performed. A significant increase of blood lactate level (p < 0.01) in post-intervention stage signified the onset of distributive shock. Based on this model, the novel findings were that in the low frequency (LF) and mid frequency (MF) bands, there was a decrease in coherence (p < 0.01 and p < 0.05 respectively) and loss of phase delay (p < 0.05) between SBP and RRi, along with a depression in transfer function gain in the LF band (p < 0.05), which might indicate an impairment of baroreflex control of heart rate following the administration of endotoxin. None of the above variables changed significantly in the control group. Moreover, endotoxin also led to a decrease in RRi variability in the ventilatory frequency (VF) band, suggesting a suppression of cardiac vagal modulation. These results highlight the potential value of frequency spectrum analysis combined with transfer function analysis of cardiovascular variability in the assessment of autonomic and baroreflex-related changes associated with endotoxic shock.