Monitoring of Cerebral Oxygenation Metabolism

Jugular Venous Oximetry, Tissue PO2, and Near-infrared Spectroscopy As cerebral ischemia and secondary injury are the common factors leading to deterioration, monitoring of some indices of cerebral oxygenation would provide guide to appropriate therapy. By placing a fiberoptic oximetric catheter into the jugular bulb, cerebral venous oxygenation can be monitored continuously, providing an index of the balance between cerebral blood flow (supply) and cerebral metabolic consumption of oxygen (demand) (CMRO2 = CBF x AVDO2, or AVDO2 = CMRO2/CBF). In essence, it is the arteriovenous oxygen content difference that represents the balance between supply and demand. However, if hemoglobin concentration stays relatively constant, and we ignore the contribution of dissolved oxygen, then the jugular venous saturation (SjvO2) effectively reflects the adequacy of CBF relative to oxygen consumption [AVDO2 = Hgb x 1.39 x (1- SjvO2)]. Thus, a high SjvO2 implies luxury perfusion, and a low value reflects increased extraction, or inadequate delivery relative to the degree of consumption. It has been demonstrated that multiple episodes of desaturation below 50% are associated with poor prognosis in head-injured patients. Paradoxically, a high SjvO2 also indicates a poor prognosis as the brain is no longer extracting oxygen. However, it is a global measurement and does not and cannot reflect regional ischemia. Thus it is a highly specific but very insensitive monitor. Despite these limitations, when used properly it yields information that can help management, and has become a standard monitor in the care of the head-injured patient in many centers.

Combining this with lactate measurement enhances its value as a monitor. The potential complications of this technique include bleeding and thrombosis, none of which has proved to be clinically significant. The most predominant cause of jugular venous desaturation is probably excessive hyperventilation. Treatment of a low SjvO2 should include a careful examination of all systemic and cerebral factors (Fig. 5.5).

Tissue PO2 electrodes are miniature Clark electrodes that can be inserted into brain parenchyma to measure tissue PO2 (PbrO2). The placement of these electrodes necessitates the drilling of burr holes, and is therefore more invasive than jugular oximetry. However, they provide regional measurement and can be inserted into brain tissues considered to be at risk. There are two types of electrodes that are commercially available: the Neurotrend and the Licox. The Neurotrend monitors PCO2 and pH in addition to PO2 (requiring a larger burr hole) whereas the Licox only measures PO2. Both are combined with ICP and temperature monitors. The normal values of PbrO2 are 25-30 mmHg. Values below 15 mmHg are associated with poor prognosis, and values less than 10 mmHg are usually incompatible with survival. It is debatable whether PbrO2 truly reflects tissue oxygenation or a balance between the delivery and consumption of oxygen at the local level. Studies with PbrO2 have repeatedly demonstrated that increase in FiO2 consistently causes an immediate rise in PbrO2. Although this is considered beneficial by some, others consider it may be more related to PaO2 than to brain tissue oxygenation.

Near-infrared spectroscopy measures tissue oxygenation non-invasively using reflectance oximetry. Briefly, a light source is placed on the scalp, and light reflected from the scalp and brain is measured by optodes placed at a distance from the light source. Theoretically this can monitor not only oxygenation saturation, but also the amount of desaturated hemoglobin, regional CBV and the cytochrome redox state. Although attractive in theory, the many drawbacks, including variable optical path, contamination by scalp and bone, interference by ambient light, and the necessary placement on the forehead, have limited its usefulness as a clinical monitor. Extensive development and refinement are required before it can become a functional monitor.

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