Assessment of Cerebral Autoregulation Using Ultrasound-Based Intra- and Extracranial Blood Flow Indices in a Swine Model of Brain Injury.

Cerebrovascular autoregulation (CA) is a protective mechanism against brain injury. We present an ultrasound-based volumetric blood flow indices to monitor CA. Swine were instrumented under general anesthesia to monitor mean arterial blood pressure (MAP), intracranial pressure (ICP), and blood flow in the internal carotid artery (ICA) and femoral artery (FA) and flow velocity and volumetric flow in the middle cerebral artery (MCA) using transcranial Doppler. Animals were subjected to several physiological challenges to perturb cerebral pressure and flow hemodynamics as follows: (1) Vasopressor challenge by controlled infusion of norepinephrine to raise MAP to 150-160 mmHg, followed by its down titration. (2) Epidural hematoma by inflating a subdurally-placed balloon to raise ICP to 35-45 mmHg, followed by deflation of the balloon. (3) Hemorrhage and resuscitation by controlled removal of the blood from jugular vein. After a 10-15 min period of hypovolemia, resuscitated started to raise MAP to 65 mmHg. Pressure reactivity index (PRx) was calculated as a moving Pearson correlation coefficient between MAP and ICP. The flow/flow indices [FFx(s)] were calculated as a moving Pearson correlation coefficient between FA blood flow and one of the following (mean flow velocity in the MCA, relative volumetric flow in the MCA and blood flow in the ICA). FFx(s) were compared to PRx using area under the receiver operator characteristic curve (AUROC) and the precision recall curve (AUPRC). FFx(s) demonstrated AUPRC and AUROC ranges (0.88-0.91) and (0.73-0.79). The FFx(s) can act as excellent potential surrogates for the PRx for the assessments of CA.