Near Infrared Spectroscopy (NIRS)

Continuous-wave spatially resolved near infrared spectrometer (NIRS) is used to monitor changes in oxyhemoglobin and deoxyhemoglobin over a volume of cerebral tissue. The theoretical underpinnings of this method have been well described in the literature and involve complex pathways of reflection and refraction within the cranium. Spatially resolved NIRS can target a specific area of the brain. To do this subjects are  instrumented with an emitter and a  detector pair of NIRS probes typically on the forehead over the frontal cortex to monitor absorption of light across the cerebral frontal area. This area corresponds to the region primarily perfused by the middle cerebral artery . Spacing between optodes needs to be optimized, and headset sizing and placement adjusted to ensure optimal signal fidelity strength for each subject during each clinical encounter. Spatial resolution insures that only infrared signals from the brain were analyzed and cutaneous signals were excluded. The sampling rate of the Oxymon was 50Hz, and an integrated digital to analog convertor provided a reconstructed analog signal. The modified Beer-Lambert Law is used to calculate micromolar (µmolar) changes in tissue oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) across time using optical densities from of near infrared light at 780 and 850 nm. Often  published differential path-length factors for each subject . Although illuminated cerebral volume may differ from subject to subject, the assumption is made that the same volume of the cerebrum is illuminated for a particular subject throughout a protocol performed on a given day. Changes in total Hb (THb) can be  obtained by adding changes in HbO2 to changes in Hb so as to index change in blood volume within the illuminated brain volume. Since only changes in Hb and HBO2 could be measured, the averages of HbO2 and Hb during quiet rest define the resting baseline. With appropriate assumptions the technique can be used to estimate cerebral venous saturation, using methods derived from venous occlusion plethysmography and by compressing the jugular vein, and estimates of the absolute total blood volume containing hemoglobin as well as changes in blood flow entering and leaving the area of the brain that is illuminated. 
The NIRS optodes are here placed on the right  anterior temporal area to approximately illuminate the area of perfusion of the right middle cerebral artery.
The figure shows the response of cerebral oxyhemoglobin (HbO2) and total hemoglobin (THb) to bolus administration of sodium nitroprusside (SNP) followed by phenyleprhine whcih together comprise the modified Oxford maneuver used in tests of baroreflex function. These are representative fits of the NIRS total hemoglobin response (THb in white) to raw THb data and the fit of oxyhemoglobin response (HbO2 in gray) to raw HbO2 data (light gray line). 


Peripheral Vascular Properties Measured by Strain Gauge Plethysmography (SPG)
Laser Doppler Flowmetry (LDF)
Indicator Techniques to Measure Blood Volume and Cardiac Output
Heart Rate and Blood Pressure
Near Infrared Spectroscopy
Microneurography and Muscle Sympathetic Nerve Activity (MSNA)
Transcranial Doppler Ultrasound (TCD)
Impedance Plethysmography (IPG)
Skeletal Muscle Pump