Factors affecting the transient hyperaemic response test of the cerebral autoregulation

Introduction

The transient compression of the internal carotid artery (ICA) results in dilatation of cerebral arterioles by effect of autoregulation. In the THR test, the increase in middle cerebral artery (MCA) blood flow immediately after release of transient compression of the ICA is measured, which is proportional to cerebral autoregulation. The aim of the study was to determine the significance of several factors of the test - blood flow inertia (acceleration), dimensions (length and radius) of ICA, patency of the Circle of Willis (WC), and asynchrony between release of ICA compression and arterial pressure pulsations (phase shift).

Methods

We constructed a mathematical model of the elements included in the THR test and simulated, using various parameters, blood flow in cerebral vessels and the increase in flow velocity in the MCA after release of ICA compression. We simulated two extreme situations - a perfectly patent (all afferent and efferent arteries meet in one point) and a completely isolated Circle of Willis (ICA bifurcates into only the anterior and middle cerebral arteries). The series of ordinary differential equations describing the THR test was solved by MATLAB R2008a. The solution assumed an absence of autoregulation (effect of studied factors is independent of autoregulation) and the rigidity of all vessels.

Results

The effects of acceleration (inertia), dimensions of ICA and patency of WC are negligible. The effect of phase shift (Figure 1) can significantly decrease the immediate post-release MCA flow, which can entirely nullify the increase in post-release MCA flow caused by autoregulation.

Figure 1

Dependence of THRR on phase shift.

Conclusions

The effect of phase shift can significantly alter the results of the THR test. For practical purposes we recommend calculation of THR test parameters from the second peak of post-release MCA flow, or using the highest post-release MCA flow from several THR tests.