Objective: To address the ability and reliability of the INVOS 3100A (Somanetics, Troy, MI) cerebral oximeter to detect cerebral desaturation in patients and the interpretation of cerebral oximetry measurements using the INVOS 3100A in stroke and cardiac arrest.
Design: Case reports of two patients.
Setting: Neurologic Intensive Care Unit of a University Hospital.
Patients: Two patients suffering occlusive strokes of the middle cerebral artery. One later suffered a cardiac arrest.
Results: The first case, a patient who suffered cardiac arrest while undergoing continuous cerebral oximetry, clearly demonstrated the ability of the INVOS 3100A to detect rapid tissue vascular oxyhemoglobin desaturation in the brain during circulatory arrest. In the second case, oximetry readings were obtained in a patient with a right internal carotid artery occlusion and an infarct in the middle cerebral artery territory. The circulation of the anterior cerebral artery (ACA) territory was intact. Stable xenon-computed tomography of local cerebral blood flow showed no perfusion in the infarct, and oximetry readings were between 60 and 65. In the border zone between the middle cerebral artery and the ACA, readings of 35 to 40 were obtained, and over the ACA territory, the readings were in the 60s.
Conclusions: Oximetry by near infrared spectroscopy reflects the balance between regional oxygen supply and demand. In dead or infarcted nonmetabolizing brain, saturation may be near normal because of sequestered cerebral venous blood in capillaries and venous capacitance vessels and contribution from overlying tissue. In regionally or globally ischemic, but metabolizing brain, saturation decreases because oxygen supply is insufficient to meet metabolic demand. These observations are supported by previously reported "normal" readings in unperfused or dead brains.