Measurement of cerebral blood flow with near infrared spectroscopy and indocyanine green dye dilution

Early detection and treatment of cerebral ischemia to prevent further neurological damage in patients with severe brain injuries, such as in trauma and stroke patients, is one of the most important issues in Neurocritical Care. Our own clinical experiences in treatment of patients with severe ischemic stroke and subarachnoid hemorrhage have shown that the available methods to monitor cerebral hemodynamics and oxygenation are insufficient with regard to detection of secondary ischemic events. The established methods for bedside monitoring of cerebral blood flow (CBF) and cerebral oxygenation are difficult to perform clinically, involve radioactive radiation, are invasive or require the patient to be transported, thus involving potentially high risks. Transcranial Doppler sonography produces indices that can be related to changes in CBF and cerebral oxygenation but does not measure actual flow rates. During recent years, near infrared spectroscopy (NIRS) was further developed and supplemented with the indocyanine green (ICG) dye dilution mode for bedside monitoring of CBF. The NIRS ICG dye dilution technique is a promising method for serial bedside CBF measurements in the environment of the intensive care unit. The NIRS method with optodes on the skin has the advantage of being non invasive, does not require the patient to be transported and provides data at the bedside within minutes. Preliminary data in a limited number of volunteers indicate that CBF measurements obtained by NIRS ICG dye dilution technique are in agreement with corresponding values obtained by perfusion-weighted MRI. More patient-validated data by correlating the measurement values with clinical events and comparing them with standard methods are needed. For the accuracy of absolute measurements and in order to quantify the individual extracerebral pathlengths as well as to allow appropriate correction to be made for extracerebral dead space tissue further modelling based on patients data is required. New-generation NIRS instrumentation, implementing spatially resolved spectroscopy (SRS), depth-resolved technologies and invasive NIRS probes will give the opportunity to reduce or eliminate extracerebral contamination and will provide some unique physiological information for NIRS technology. To calculate the influence of extracerebral contamination comparative measurements can be performed using noninvasive NIRS probes on the scalp and invasive probes for NIRS and intracranial pressure (ICP) monitoring. Combined monitoring of ICP and NIRS will be of special clinical value in patients with severe stroke, subarachnoid hemorrhage and head trauma, already provided with ICP probes for treatment of intracranial hypertension and being especially at risk for secondary ischemic brain damage.