We thank Drs. Moerman and De Hert for their interest and remarks on our article.1
They 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.2
In 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 SctO2
to the inherent limitations of the near-infrared spectroscopy technology. Moreover, the agreement between SctO2
was 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 SctO2
in the two groups, but not the lack of agreement between the SctO2
, 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 (SctO2
) by using a Bland-Altman plot with multiple measurements per subject.3
Nevertheless, 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.1
In fact, we used a Bland-Altman plot with multiple measurements per subject in another study and demonstrated a lack of agreement of SctO2
values during the surgery in the beach chair position.4
If we had used a modified rather than standard Bland-Altman method also in the current study,1
the conclusion that SctO2
may not be reliable in detecting a low SjvO2
during the surgery in the beach chair position should remain the same.
Kyung Y. Yoo, M.D., Ph.D.,* Hyejin Jeong, M.D., JongUn Lee, M.D., Ph.D.
*Chonnam National University Medical School, Dong-gu, Gwangju, South Korea. email@example.com
1. Jeong H, Jeong S, Lim HJ, Lee J, Yoo KY. Cerebral oxygen saturation measured by near-infrared spectroscopy and jugular venous bulb oxygen saturation during arthroscopic shoulder surgery in beach chair position under sevoflurane-nitrous oxide or propofol-remifentanil anesthesia. ANESTHESIOLOGY. 2012;117:1047–56
2. Klein KU, Fukui K, Schramm P, Stadie A, Fischer G, Werner C, Oertel J, Engelhard K. Human cerebral microcirculation and oxygen saturation during propofol-induced reduction of bispectral index. Br J Anaesth. 2011;107:735–41
3. Bland JM, Altman DG. Calculating correlation coefficients with repeated observations: Part 1–Correlation within subjects. BMJ. 1995;310:446
4. Jeong H, Lee SH, Jang EA, Chung SS, Lee J, Yoo KY. Haemodynamics and cerebral oxygenation during arthroscopic shoulder surgery in beach chair position under general anaesthesia. Acta Anaesthesiol Scand. 2012;56:872–9
© 2012 American Society of Anesthesiologists, Inc.