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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
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