Approximately 1/3 of the calculated AV-ratios were above 1 or below 0. The data were retained in our analysis to avoid statistical bias. In our calculation, the AV-ratio is determined by , , and [Eq. (3)]. The estimate of NIRO-300 has been shown to estimate lower baseline values compared to other devices in neonates34 and on the adult forearm.35 Also, a study in a blood–lipid phantom revealed significant difference in estimates between three commercial devices, including the NIRO-300.20,21 Errors in the measurement of would equally be able to estimate AV-ratios below 0 if the venous samples were not taken strictly anaerobically. Air in the syringe was carefully avoided and, if present, quickly removed when blood samples were drawn and the blood samples were analyzed immediately after sampling. The procedure did not differ from that in clinical practice. Although air-contamination cannot be ruled out in some samples, it is unlikely to be the sole cause of the large number of data points outside the physiological range. Wong et al.11 excluded eight nonphysiological values out of 49 during the analysis of the AV-ratio and . Finally, imprecision of NIRS oximetry may explain some of the nonphysiological data. It has been well-documented that irrespective of the NIRS instrument the precision of the estimate is about 5% to 6%.36,37 This means that a single estimate could be from the “true” oxygenation, thus NIRS values below could simply be due to random noise in the NIRS signal. Another limitation of our study is the fact that it was performed in newborn piglets. There are obvious differences between piglets and neonates. Newborn piglet brains average weight is 30 g, whereas extremely preterm infant brains weigh at least 100 g. The NIRS sensor was designed for human use and a source–detector distance of 4 cm might probe beyond the brain when used in a piglet including extracerebral tissue, such as tongue and basis cranii. Finally, the study was part of a larger setup where the effect of dopamine-infusion on cerebral autoregulation was studied. The piglets received dopamine when half of the blood samples were drawn. Dopamine is an inotrope affecting vascular resistance and cardiac output through dopaminergic, - and -adrenoceptors in arteries and the heart.38 Thus, dopamine has the potential to change cerebral blood flow. Importantly, dopamine did not affect the relationship between and , between and MABP, nor between the AV-ratio and MABP. Dopamine did not affect autoregulation (manuscript submitted). We are, therefore, confident that our results are relevant in a wider context.