Lyapunov Exponent Calculations as Independent Indices of Complexity of Cardiovascular Regulation During Hemorrhage with Autonomic Blockade.

摘要

Objectives: The largest Lyapunov exponent (LLE) is a standard method used to characterize the level of chaos in complex physiologic systems such as the cardiovascular system. Previously, we showed that LLE calculated in beat domain remains positive during lethal hemorrhagic shock (HS). We sought to evaluate the utility of LLE and the asymptote of the divergence curve (AS) for monitoring of subjects in time domain using data from a swine model of severe pump-controlled HS. The AS denotes the average Euclidean divergence distance between adjacent orbits of the attractor. Methods: Ten midazolam-sedated, spontaneously breathing, sexually mature swine underwent a controlled HS protocol with removal of 60% of blood volume in 1 hour. Controls (n = 4) received HS alone. Vagal blockade group (n = 3) received atropine 0.005 mg/kg then 0.5 mg/min continuous rate infusion and HS. Sympathetic blockade (n = 3) group received propranolol 0.5 mg/kg then 1 mg/min and HS. EKG was recorded at 500 Hz and the R-R intervals (RRI) were calculated. The LLE and AS were calculated using Rosenstein's method with an embedding factor of 2 and a delay of 4 over a set of 1024 consecutive RRI samples taken at baseline and HS. The RRI time series was then resampled at 20 Hz to move the LLE calculations from the beat domain to the time domain. Data are medians. Statistics were performed by 1-way analysis of variance and post hoc analysis. Conclusions: The LLE remained positive in every condition, signifying presence of deterministic chaotic trends during HS and HS with branch-specific autonomic blockade. The AS was the lowest after vagal blockade, distinguishing it from sympathetic blockade and baseline. The simplest attractors are associated with vagal blockade and HS and feature low AS, which denotes the degree of constraint of the system in phase space. The AS provides useful additional information to LLE in assessment of complexity of cardiovascular regulation during HS.