Non-Invasive Mapping of Cerebral Autoregulation Using Near Infrared Spectroscopy

The transient hyperemic response will be tested via carotid compression method. This test will consist of 5 carotid compressions lasting 5 seconds each with a 60 seconds interval between each compression to allow normalization of the CBF to precompression levels, and the total time for this block would be 6 minutes. The ipsilateral common carotid artery will be compressed for this method, and the response of the middle cerebral artery (MCA) blood flow velocity will be assessed using transcranial doppler (TCD) probe (EMS-9D, Delica). The carotid compressions will only be accepted when there is no further decrease in blood flow velocity can be achieved and stable conditions remained during the whole period of the compression. Otherwise, the compression will be terminated and repeated again after 60 seconds. Satisfactory compression is typically considered to result in a reduction in MCA velocity of 50%, at a minimum.

The orthostatic challenge response will be evaluated by lying-to-sit and sit-to-stand methods 5 times where sitting and standing positions will be held for 3 minutes each for a total block time of 35 minutes. A baseline in lying position will be collected at the start of the block, then the position will be changed from lying to sit for 3 minutes and then position will be changed to standing for another 3 minutes.

vascular chemo-reactivity will be assessed by varying CO2 concentrations through slow and fast breathing exercises with Capnostream 35 Portable Respiratory Monitor (Medtronic Canada) to monitor EtCO2 through nose clip. The breathing trials will occur two times and each of them will have fast and slow trials along with an interval of normal breathing. First time, the normal, fast and slow breathing trials will be 2.5 minutes long while the second time, these breathing trials will be 1.5 minutes long. The normal respiratory rate for adults is 12 breaths/minute, the target slow respiratory rate will be 5 breaths/minute and fast respiratory rate will be 25 breaths/minute as set by metronome. It has been shown that the EtCO2 has a 10% increase during hypoventilation and almost 50% decrease during hyperventilation compared to normal breathing.

The fourth block will evaluate the neurovascular coupling using Automated Neuropsychological Assessment Metrics (ANAM) General Neuropsychological Screening (GNS) Clinical Toolkit which contains 5 tests explained as follows 1) Standard Continuous Performance test will assess sustained attention, concentration and working memory. 2) Manikin test will assess 3D spatial rotation ability, left-right orientation, problem solving and attention. 3) Pursuit Tracking test will measure the visuo-motor control. 4) Switching test will assess divided attention, mental flexibility, and executive function. 5) Stroop test will assess the processing speed, selective attention, interference, and executive functioning with three trial blocks.