Cerebral oximeters obtain continuous, noninvasive cerebral oxygenation values using near-infrared spectroscopy (NIRS) technology. A cerebral oximeter setup consists of an oximeter probe attached to a monitor cable that is connected to a cerebral oximeter monitor. In general, most cerebral oximeters can support two to four oximeter probes with respective monitor cables. Oximeter probes can be placed anywhere on the head but most commonly on the forehead, where there is the least amount of hair. The oximeter probe includes a fiber optic light source and light detector(s). Depending on the cerebral oximeter, fiber optic strands release light amplification by stimulated emission of radiation or light emitting diodes light. Emitted light wavelengths are sent from the light source penetrating the skull and cerebrum, and the light detector(s) receives the light not absorbed during the light pathway through the skull and cerebrum. The amount of oxygen present in the brain is the difference between the amount of light sent and received by the probe, which is suggested by the percentage of oxygen displayed on the monitor screen. However, each cerebral oximeter measures cerebral oxygenation slightly different because of the number of light wavelengths used and whether the oximeter measures trends or absolute values.
Trend cerebral oxygenation monitoring focuses more on the amount of change from an established baseline cerebral oxygenation value, whereas absolute cerebral oxygenation monitoring focuses more on the meaning of the cerebral oxygenation value. For example, a decrease in cerebral oxygenation from 80% to 70% might be of concern for trend monitoring because there was a drop in cerebral oxygenation by 10%. However, if monitored by an absolute cerebral oximeter, a change in cerebral oxygenation by 10% may not be of concern if an established reference range for this patient is 65% to 80%.
Cerebral oximeters calculate cerebral oxygenation using NIRS technology based on a modified light absorbent theory called the Beer-Lambert law. According to the Beer-Lambert law, an amount of a substance or compound, in this case, oxygen, can be determined by how much light the substance absorbs. In theory, a light source will decrease in intensity when an absorbing substance mediates the light source pathway, and the more light absorbed by a substance, the more a substance is present. The length the light travels from the light source to the light detector(s) determines the light pathway distance. This value is fixed based on the cerebral oximeter probe size and manufacturer.
Oxygen can be bound or unbound to hemoglobin level also known as oxygenated hemoglobin and deoxygenated hemoglobin levels, respectively, with each hemoglobin level type absorbing different light wavelength amounts. Oxygenated and deoxygenated hemoglobin levels found in vessels and tissue oxygen concentrations, reflecting cellular oxygenation, are thought to comprise all sources of oxygen in the brain. These values are unknown at a given time and must be calculated along with weighted algorithm values for arterial, venous, and capillary oxygen[42,43] to acquire cerebral oxygenation values. Light wavelengths within the near-infrared light spectrum (650–900 nm) are the only light wavelengths strong enough to go through the skull bone and capture the presence of cerebral tissue oxygenation. In addition, oxygenated hemoglobin level, deoxygenated hemoglobin level, and tissue oxygenation (cytochrome aa3) are the only substances in the brain with the capacity to change light absorption when oxygenation levels change. Lastly, proprietary formulas calculate the differences between light absorbed by oxygenated hemoglobin level, deoxygenated hemoglobin level, and tissue oxygen to display a percentage of cerebral oxygen present in that particular cerebral light source pathway. This equation divides the amount of oxygenated hemoglobin level by the total hemoglobin level to calculate a percentage of cerebral oxygenation (see Table 1).
There is an increasing number of cerebral oximeters for infants around the world and domestic prototypes for research use only. Currently, there are two cerebral oximeters Food and Drug Administration approved in the United States for use in the infant population: the INVOS System by Somanetics Corporation (Troy, Michigan) and the FORE-SIGHT Cerebral Oximeter by CAS Medical Systems (Branford, Connecticut). Suitable probes sized for infant heads and Food and Drug Administration approval, if used for clinical care, will increase the device options. The INVOS is a trend cerebral oxygenation monitor, whereas the FORE-SIGHT is regarded as measuring absolute measures of cerebral oxygenation. Cerebral oxygenation measured by the FORE-SIGHT Cerebral Oximeter is considered to be composed of 70% venous and 30% arterial blood,[42,46] whereas the INVOS uses a 75% venous and 25% arterial blood ratio. To validate these cerebral oximeters, a comparison of serum blood values was required so it would be known whether cerebral oxygenation reflected similar oxygen quantities in the blood. Furthermore, because of the mixed vascular nature of cerebral oxygenation,[42,43] cerebral oxygenation validation incorporated blood samples from both vessels, which were easiest to access from infants on extracorporeal membrane oxygenation. A small number of studies test validity and reliability in these two cerebral oximeter machines.[48–52] However, these studies clearly demonstrate that these two devices are not sensitive to the same changes in cerebral oxygenation. Therefore, caution is highly advised when comparing cerebral oxygenation values between these and other oximeters because it appears that values are not similar.
NAINR. 2011;11(3):153-159. © 2011 Elsevier Science, Inc.
Cite this: Cerebral Oxygenation Monitoring - Medscape - Sep 01, 2011.