Causal coherence analysis of heart rate variability and systolic blood pressure variability under mental arithmetic task load.

Causal coherence analysis based on a closed-loop bivariate autoregressive model was applied to heart rate variability and systolic blood pressure (SBP) variability during mental arithmetic tasks to clarify how mental task load affects the linear closed loop interaction between cardiac and vascular systems. Thirteen normal male subjects performed a mental arithmetic task, button press task, and rest task while measuring their RR interval (RRI) and SBP. The mean value in the low frequency (LF) band (0.04-0.15Hz) of the squared causal coherence function from SBP to RRI during the mental arithmetic task was significantly higher than during the other two control tasks. Conversely, the LF band of the squared causal coherence function from RRI to SBP during the mental arithmetic task tended to be lower than during the rest task. These results suggest that mental arithmetic tasks enhance linear causal coupling from the vascular to cardiac system, and conversely weaken that from the cardiac to vascular system.