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LETTER TO THE EDITOR

Remifentanil-Induced Cerebral Blood Flow Effects

Department of Anesthesiology; Yale University School of Medicine; New Haven, CT; ramachandran.ramani{at}yale.edu

To the Editor:

Kofke et al.1 addressed activation of the limbic system by remifentanil infusion and its relationship to Apo E genotype. In analgesic doses, opioids activate the cingulate gyrus.2

In the present study, the Paco₂-corrected cerebral blood flow (CBF) values show a decrease in regional CBF (rCBF) in the hippocampus and amygdala and an increase in CBF in the cingulate gyrus with remifentanil infusion. In the subjects with Apo E genotype, the opposite occurred. There was increased rCBF in the hippocampus and amygdala, and decreased rCBF in the cingulate gyrus. If the cause, as theorized by the author, is increased metabolic activity with opioids (reflected by increased rCBF) resulting in neuronal degeneration (as demonstrated in rats), such a mechanism is unlikely based on these data. rCBF not corrected for Paco₂ will not reflect remifentanil-induced change in neuronal activity. The 13 mm Hg increase in Paco₂ (Table 2) with remifentanil 0.2 µg · kg^–1 · min^–1 can, by itself, cause a 26% increase in CBF (1%–3% increase in CBF for 1 mm Hg increase in Paco₂). In future studies, the investigators should measure blood oxygen level dependent (BOLD) contrast (qualitative measure of neuronal metabolism) also3,4 as the change in BOLD is unlikely to be affected by the increase in Paco₂. Because many agents used during anesthesia can independently affect flow and metabolism, many investigators in the field recommend measuring both CBF and BOLD in studies of this nature.5

REFERENCES

1. Kofke AW, Blissit PA, Rao H, Wang J, Addya K, Deter J. Remifentanil-induced CBF effects in normal humans: dose and ApoE genotype. Anesth Analg 2007;105: 167–75[Abstract/Free Full Text]
2. Peyron R, Laurent B, Garcia-Larrea L. Functional imaging of brain responses to pain. A review and meta-analysis. Neurophysiol Clin 2000;30:263–88[Web of Science][Medline]
3. Ogawa S, Lee TM, Kay AR, Tank DW. Brain magnetic resonance imaging with contrast dependent on blood oxygenation. Proc Natl Acad Sci USA 1990;87: 9868–72[Abstract/Free Full Text]
4. Logothetis NK, Pauls J, Augath M, Trinath T, Oelterman A. Neurophysiological investigation of the basis of fMRI signal. Nature 2001;412:150–7[Medline]
5. Qiu M, Ramani R, Swetye M, Constable RT. Spatial nonuniformity of the resting CBF response to sevoflurane: in vivo study of normal humans subjects with magnetic resonance imaging. Hum Brain Mapp 2007 [Epub ahead of print]

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