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Fentanyl abolishes the hyperdynamic response to isoflurane without affecting the change in bispectral index

Sapporo, Japan

To the Editor:

A sudden increase in the inspired isoflurane concentration activates adrenosympathetic function, resulting in tachycardia and hypertension.^1–5 We studied the effect of fentanyl on the changes in hemodynamics and the hypnotic level, as measured by bispectral index (BIS), after a sudden increase in isoflurane concentration. Forty-two adult patients (ASA I–II) were anesthetized with either 2 mg•kg^-1 thiamylal (control group, n = 22) or 2 µg•kg^-1 fentanyl followed by 2 mg•kg^-1 thiamylal (fentanyl group, n = 20). Mask inhalation of isoflurane at 0.5% in 100% oxygen began after loss of consciousness. Two minutes later, the vapourizer setting for isoflurane was suddenly increased to 3.5% and maintained for five minutes. Heart rate (HR), mean arterial pressure (MAP), and BIS (Model A1050; Aspect Medical Systems, Natick, MA, USA) were measured every minute. Statistical analysis was performed using ANOVA.

Anesthetic induction with thiamylal did not change HR or MAP but significantly reduced BIS in both groups (Figure). The increase in isoflurane concentration significantly increased HR and MAP in the control group. In contrast, in the fentanyl group, HR did not change and MAP decreased significantly. Two minutes after the isoflurane concentration had been increased, BIS significantly and progressively decreased to about 50 in both groups. There were no inter-group differences in the BIS value throughout the study period.

View larger version (19K): [in this window] [in a new window]   FIGURE Changes in heart rate (HR), mean arterial pressure (MAP), and bispectral index (BIS) after anesthetic induction with thiamylal and high concentrations of isoflurane with or without fentanyl pretreatment. Values are expressed as mean ± SD. BL = baseline; LER = loss of eyelash reflex. *P < 0.05 vs the control group; P < 0.05 vs values at LER.

References

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5 Nakayama M, Hayashi M, Ichinose H, Yamamoto S, Kanaya N, Namiki A. Values of the bispectral index do not parallel the hemodynamic response to the rapid increase in isoflurane concentration. Can J Anesth 2001; 48: 958–62.[Abstract/Free Full Text]

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