Objective:To compare the characteristics of jugular venous oxygen saturation (SjvO2) and brain tissue PO2(PbtO2) as monitors for cerebral ischemia after severe head injury. SjvO2has been useful as a monitor for cerebral ischemia, but it is limited by its inability to identify regional cerebral ischemia. PbtO2may be superior to SjvO2for this purpose, because oxygenation in localized areas of the brain can be monitored.Design:SjvO2and PbtO2were successfully monitored in 58 patients with severe head injury. The changes in SjvO2and PbtO2were compared during ischemic episodes.Setting:Neurosurgical intensive care unit of a level I trauma center.Measurements and Main Results:During the monitoring period, which averaged 90 hrs/patient, there were 54 episodes during which SjvO2decreased to <50% and/or PbtO2decreased to <8 torr. Two of these episodes were caused by an infarction in the area of the PO2probe, leaving 52 episodes of global hypoxia/ischemia that were identified by one of the two monitors. The sensitivities of the two monitors for detecting ischemia, using the thresholds of 50% and 8 torr for SjvO2and PbtO2, respectively, were similar. The SjvO2catheter detected 69.7% of the episodes and the PbtO2catheter detected 63.5% of the episodes. In most of the remaining episodes, both probes reflected a decrease in oxygenation, but not to levels below the defined thresholds. The major differences in the two measures of oxygenation included the following: a) SjvO2more consistently reflected a reduction in oxygenation during hyperventilation; b) PbtO2was affected more by changes in arterial PO2; and c) during severe global ischemia, PbtO2decreased to 0 and remained at 0, whereas SjvO2initially decreased but then increased again as cerebral blood flow ceased, and the only blood in the jugular bulb was of extracerebral origin.Conclusions:The two monitors provide complimentary information, and neither monitor alone identifies all episodes of ischemia. The best strategy for using these monitors is to take advantage of the unique features of each monitor. SjvO2should be used as a monitor of global oxygenation; but PbtO2should be used as a monitor of local oxygenation, ideally with the catheter placed in an area of the brain that is vulnerable to ischemia but that may be salvageable with appropriate treatment.