Abstract

Hypercapnia is known to dilate the pial artery, disrupt cerebral autoregulation and increase pial artery pulsation. The cerebral blood flow velocity (CBFV) response to hypercapnia is biphasic. Below the threshold for the increase of blood pressure (BP) with carbon dioxide partial pressure (pCO2), the CBFV measurement reflects vascular reactivity to pCO2 alone at a constant BP. Above this threshold, the CBFV measurement provides an estimate of the cerebrovascular passive response to BP [4]. As a consequence, the CBFV response to apnea is driven by the interplay between BP and pCO2. In comparison with baseline values, BP drops at the onset of apnea, which is followed by an increase in BP and CBFV in the later part of the apneic episode. In the last decade, a new method based on infrared radiation (IR) called near-infrared transillumination/backscattering sounding (NIR-T/BSS) has been developed. NIR-T/BSS enables the assessment of changes in pial artery pulsation (cc-TQ), which reflect changes in pial artery compliance. In addition, NIR-T/BSS allows for measurement of the subarachnoid space (SAS) width (sas-TQ), which reflects changes in cerebrospinal fluid (CSF) volume and/or intracranial pressure (ICP). Contrary to near-infrared spectroscopy (NIRS), which relies on the absorption of IR by hemoglobin, NIR-T/BSS uses the SAS filled with translucent CSF as a propagation duct for IR. The main advantage of NIR-T/BSS technology is that cc-TQ reflects the functional status of the pial artery, and thus allows for direct assessment of the generalized effect of various factors affecting the pial artery (i.e. increased flow and/or pCO2, elevated ICP, etc.).

Authors (10)