Non-Invasive and Minimally-Invasive Cerebral Autoregulation Assessment: A Narrative Review of Techniques and Implications for Clinical Research

Abstract

The process of cerebral vessels regulating constant cerebral blood flow over a wide range of systemic arterial pressures is termed cerebral autoregulation (CA). Static and dynamic autoregulation are two types of CA measurement techniques, with the main difference between these measures relating to the time scale used. Static autoregulation looks at the long-term change in blood pressures, while dynamic autoregulation looks at the immediate change. Techniques that provide regularly updating measures are referred to as continuous, whereas intermittent techniques take a single at point in time. However, a technique being continuous or intermittent is not implied by if the technique measures autoregulation statically or dynamically. This narrative review outlines technical aspects of non-invasive and minimally-invasive modalities along with providing details on the non-invasive and minimally-invasive measurement techniques used for CA assessment. These non-invasive techniques include neuroimaging methods, transcranial Doppler, and near-infrared spectroscopy while the minimally-invasive techniques include positron emission tomography along with magnetic resonance imaging and radiography methods. Further, the advantages and limitations are discussed along with how these methods are used to assess CA. At the end, the clinical considerations regarding these various techniques are highlighted.

Keywords: Transcranial Doppler; cerebrovascular autoregulation; computed tomography; dynamic autoregulation; magnetic resonance imaging; near-infrared spectroscopy; positron emission tomography; static autoregulation.

Figure 1

Lassen autoregulatory curve. The curve represents the Lassen autoregulatory curve (3) where the lower limit of autoregulation (LLA) and upper limit of autoregulation (ULA) are appropriately labeled. The part of the curve in the green area represents intact autoregulation and the part in the red area signifies impaired autoregulation. CBF, cerebral blood flow; g, grams; LLA, lower limit of autoregulation; MAP, mean arterial pressure; min, minute; ml, milliliters; mmHg, millimeter of mercury; ULA, upper limit of autoregulation.

Figure 2

Static vs. dynamic autoregulation. (A) Static autoregulation is shown using Lassen autoregulatory curve with orange circles representing blood vessels state of vasodilation near the LLA and vasoconstriction near the ULA. (B) Dynamic autoregulation is shown by the MAP and CBF curves with orange circles representing the vasodilation and vasoconstriction of blood vessels with time. CBF, cerebral blood flow; LLA, lower limit of autoregulation; MAP, mean arterial pressure; ULA, upper limit of autoregulation.