Causality detection in cardio-postural interaction under orthostatic stress induced by quiet standing using transfer entropy

Abstract

Representative signals of the cardiovascular and postural systems are known to interact with each other under orthostatic stress to maintain homeostasis, however, causal interaction between them is still unknown. In this research the mathematical framework of transfer entropy was applied to explore the existence of cause and effect relationship of anterior-posterior (COPy), medio-lateral (COPx) and resultant body sway (COPr) with blood pressure waveform (BPW) and calf muscle electromyogram (EMG). Simultaneous blood pressure, calf EMG, COPx, COPy and COPr data were acquired from 5 healthy young subjects during a 5-minute sit to stand test. Data from the last 4 minutes in the stand phase was analyzed for causality between the signal pairs. A moving time window of 15 second length with a 5 second overlap between successive windows resulted in a total of 23 estimates of causality. The results indicate the existence of a causal interaction between the variables in cardio-postural interactions. The BPW showed dominant causal coupling with COPx, COPy and COPr. Additionally, EMG showed dominant causal coupling with COPy and COPr, with COPx similar amount of bidirectional causality was observed. The results strongly suggest the presence of a directional causal loop from the blood pressure control center playing a central role.