We thank Drs. Moerman and De Hert for their interest and remarks on our article.1They provided a possible mechanism accounting for the discrepancy between cerebral oxygen saturation (SctO2) measured by near-infrared spectroscopy and jugular venous bulb oximetry (SjvO2). They also questioned the use of the standard Bland-Altman method to assess the agreement with repeated measures.
It has been recently shown that propofol preserves cerebral oxygen saturation in the cortex through a region-specific alteration of the cerebral blood flow or cerebral metabolic rate of oxygen ratio.2In this context, Moerman and De Hert pointed out that propofol may preserve the cerebral oxygen saturation in the frontal cortex, which is the measurement site of near-infrared spectroscopy, thereby increase SctO2, resulting in comparable near-infrared spectroscopy values with those in the sevoflurane–nitrous oxide group. However, we ascribed the discrepancy between SctO2and SjvO2to the inherent limitations of the near-infrared spectroscopy technology. Moreover, the agreement between SctO2and SjvO2was not acceptable either in the sevoflurane–nitrous oxide or in the propofol–remifentanil group in our study, when assessed separately in each group. The inhomogeneous effect of propofol with an enhanced cerebral oxygenation in the frontal cortex may be responsible for the comparable SctO2in the two groups, but not the lack of agreement between the SctO2and SjvO2, if any.
Moerman and De Hert also doubted whether Bland-Altman and linear regression analyses were applicable for repeated measures. We fully agree with them that standard Bland-Altman method may not be ideal for the repeated data. As such, we reanalyzed the data (SctO2against SjvO2) by using a Bland-Altman plot with multiple measurements per subject.3Nevertheless, we found little change in the 95% limit of agreement (from −37.8% to +23.6% with mean difference −7.2) compared with that (−38.2%, 23.8% with mean difference −7.2) of our previous data.1In fact, we used a Bland-Altman plot with multiple measurements per subject in another study and demonstrated a lack of agreement of SctO2and SjvO2values during the surgery in the beach chair position.4If we had used a modified rather than standard Bland-Altman method also in the current study,1the conclusion that SctO2may not be reliable in detecting a low SjvO2during the surgery in the beach chair position should remain the same.
*Chonnam National University Medical School, Dong-gu, Gwangju, South Korea. email@example.com