Cerebrovascular variability interactions after acute ischemic stroke: insights from directionality analysis based on transfer entropy

Abstract

Dynamic cerebral autoregulation (CA) limits fluctuations of mean cerebral blood flow, approximated as mean cerebral blood velocity (MCBv) measured via transcranial Doppler ultrasound, in the presence of variations of mean arterial pressure (MAP). This mechanism is impaired after acute ischemic stroke (AIS). CA impairment is usually assessed by hypothesizing that MAP variations are completely responsible for MCBv changes, although disregarding the MCBv contributions to MAP variability. We exploited transfer entropy (TE) and conditional TE (CTE) to assess the strength of the directional interactions from MAP to MCBv and vice versa accounting for partial pressure of end-tidal carbon dioxide. Traditional markers were computed for comparison. We analyzed recordings from 34 control individuals (CTRL, age: 66 ± 7 yr) and 48 patients with AIS (age: 66 ± 13 yr) acquired within 48 h of stroke symptom onset. MCBv was recorded in both hemispheres including affected and unaffected hemispheres in patients with AIS. Patients with AIS exhibited hypertension and hypocapnia. After AIS MCBv diminished, especially in the affected hemisphere. TE and CTE decreased along the pressure-to-flow pathway as well. Both directional markers tend to increase along the flow-to-pressure arm irrespective of the hemisphere. Traditional indexes could not detect any difference. Our analysis suggests that the CA impairment was characterized by an imbalance of information transfer within the MCBv-MAP closed loop with a reduced importance of the pressure-to-flow and increased relevance of the flow-to-pressure arm. The study stresses the relevance of assessing MCBv-MAP relationship in closed loop, especially when variability of MAP and MCBv are considered.NEW & NOTEWORTHY The study emphasizes the importance of understanding the closed-loop relationship between mean arterial pressure and mean cerebral blood velocity when characterizing cerebrovascular control. Directional analysis is particularly insightful in acute ischemic stroke because modifications occurring along the flow-to-pressure link complement the ones derived from the more traditionally examined pressure-to-flow pathway.

Keywords: blood pressure variability; causality; cerebral autoregulation; cerebral blood flow; cerebrovascular control.