AUTONOMIC MODULATION OF HEART-RATE-VARIABILITY DURING ENDOTOXIN-SHOCKIN RABBITS

Objective: Gram-negative septic shock is associated with severe hypote nsion and autonomic cardiovascular dysfunction. We hypothesized that i n an anesthetized rabbit model of endotoxin shock, autonomic modulatio n of cardiac activity, as measured by power spectral analysis of heart rate (HR) variability, would be decreased compared with the anestheti zed control rabbits. Design: Experimental, comparative study. Setting: Laboratory of a university hospital. Subjects: Fourteen adult male Ne w Zealand white rabbits (2.7 to 3.1 kg body weight) were studied under anesthesia. Interventions: None. Measurements and Main Results: We st udied the absolute and temporal changes in HR power spectra and plasma catecholamine concentrations in eight experimental and six control Ne w Zealand white rabbits during Escherichia coli endotoxin-induced shoc k. HR, respirations, arterial blood pressure (BP), HR power spectra, a nd plasma catecholamine concentrations were measured at 5- to 10-min i ntervals for 60 mins in control rabbits or until the mean arterial pre ssure (MAP) decreased by greater than or equal to 20 mm Hg in experime ntal rabbits. There were no differences in basal HR, respiratory rate, BP, HR power spectra, or catecholamine concentrations between groups, After endotoxin administration, MAP decreased (82 +/- 7 vs. 62 +/- 5 mm Hg; p < .05) as did log low-frequency HR power (-2.14 +/- 2.46 vs. -2.20 +/- 2.48 beats/min(2); p < .05), Low-frequency HR power and MAP remained unchanged in control animals, Log high-frequency HR power dec reased in control and experimental rabbits (-1.02 +/- 1.34 vs. -1.69 /- 2.12 [control], p < .05; -1.53 +/- 2.19 vs. -2.19 +/- 2.85 beats/mi n(2) [experimental], p < .05), While there was an inverse relationship between low- and high-frequency HR power and MAP, the direction of ch ange was opposite in six of six rabbits in the control group and in si x of eight rabbits in the experimental group. Plasma catecholamine con centrations did not change during the experiment in either group. Conc lusions: Sympathetic modulation of cardiac activity decreased, while t he sympathomedullary response remained unchanged during endotoxin shoc k. We speculate that a concomitant decrease in low-frequency HR power as MAP decreases may prove to be an early marker for impending shock.