Using near-infrared spectroscopy to measure cerebral metabolic rate of oxygen under multiple levels of arterial oxygenation in piglets

Tichauer KM, Elliott JT, Hadway JA, Lee DS, Lee TY, St. Lawrence K. Using near-infrared spectroscopy to measure cerebral metabolic rate of oxygen under multiple levels of arterial oxygenation in piglets. J Appl Physiol 109: 878-885, 2010. First published July 8, 2010; doi:10.1152/japplphysiol.01432.2009.-Improving neurological care of neonates has been impeded by the absence of suitable techniques for measuring cerebral hemodynamics and energy metabolism at the bedside. Currently, near-infrared spectroscopy (NIRS) appears to be the technology best suited to fill this gap, and techniques have been proposed to measure both cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMR(O2)). We have developed a fast and reliable bolus-tracking method of determining CMR(O2) that combines measurements of CBF and cerebral venous oxygenation [venous oxygen saturation (CSv(O2))]. However, this method has never been validated at different levels of arterial oxygenation [arterial oxygen saturation (Sa(O2))], which can be highly variable in the clinical setting. In this study, NIRS measurements of CBF, CSv(O2), and CMR(O2) were obtained over a range of Sa(O2) in newborn piglets (n = 12); CSv(O2) values measured directly from sagittal sinus blood samples were collected for validation. Two alternative NIRS methods that measure CSv(O2) by manipulating venous oxygenation (i.e., head tilt and partial venous occlusion methods) were also employed for comparison. Statistically significant correlations were found between each NIRS technique and sagittal sinus blood oxygenation (P < 0.05). Correlation slopes were 1.03 (r = 0.91), 0.73 (r = 0.73), and 0.73 (r = 0.81) for the bolus-tracking, head tilt, and partial venous occlusion methods, respectively. The bolus-tracking technique displayed the best correlation under hyperoxic (Sa(O2) = 99.9 +/- 0.03%) and normoxic (Sa(O2) = 86.9 +/- 6.6%) conditions and was comparable to the other techniques under hypoxic conditions (Sa(O2) = 40.7 +/- 9.9%). The reduced precision of the bolus-tracking method under hypoxia was attributed to errors in CSvO2 measurement that were magnified at low Sa(O2) levels. In conclusion, the bolus-tracking technique of measuring CSv(O2), and therefore CMRO2, is accurate and robust for an Sa(O2) > 50% but provides reduced accuracy under more severe hypoxic levels.