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Department of Anesthesiology and Pain Medicine, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea, kshwang{at}amc.seoul.kr
To the Editor:
We read with interest the article by Drs. Tanaka and Nishikawa on spontaneous baroreflex indices (SBRI) during general anesthesia (GA) (1). They suggested that SBRI inadequately estimate the pharmacological gain during sevoflurane anesthesia, even though these indices have frequently been used in their own recent articles. We agree, in part, that SBRI may not reflect complex physiology of baroreflex during GA. However, unfortunately, there are significant limitations in the methodology of this study. First, we question why they did not choose high-frequency transfer function gain, which is believed to be of vagal origin and has sufficiently high coherence during positive pressure ventilation (2), despite the authors' claim that "SBRI may be useful simply for quantitative assessments of beat-to-beat vagal modulation of heart rate." Furthermore, we wonder how low-frequency transfer function gain remained unchanged, or increased, during GA when compared with that at conscious baseline. Second, because the SBRI measured at conscious baseline had already shown a large mean difference (Figs. 1 and 2; up to approximately 300 to 500% higher; n = 9), compared with that by Lipman et al. (3), who reported 13 to 46% higher (n = 97), we respectfully question the consistency of SBRI methods used before further assessment of Bland-Altman plot during GA. Moreover, for calculating the 
-index, they incorrectly used the spectral power at 0.25 Hz, although mechanical ventilation was at 0.2 Hz. Taken together, these issues may contribute to the increased bias and broad limit of agreement between indices. Therefore, we believe that this article would be best served by eliminating the issues noted above, and it would be worthwhile to perform additional investigations with a larger sample size using different methodology, such as choosing different SBRI and changing thresholds in the sequence method (4,5).
Footnotes
Dr. Tanaka does not wish to respond.
References
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