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Cerebral effects of nitrous oxide during isoflurane-induced hypotension in the pig.

We have studied the effects of nitrous oxide on cerebral blood flow (CBF), cerebral blood flow velocity (CBFV) and intracranial pressure (ICP) during isoflurane-induced hypotension in 10 pigs. CBF was measured using laser Doppler flowmetry, CBFV in the right middle cerebral artery was calculated using Doppler ultrasound and ICP was measured using an extradural ICP monitor. Each animal was studied under four conditions, examined sequentially: (i) mean intra-arterial pressure (MAP) 85 mm Hg, maintained with isoflurane, (ii) MAP 50-55 mm Hg, induced by isoflurane only, (iii) MAP 85 mm Hg, maintained with isoflurane and 50% nitrous oxide, and (iv) MAP 50-55 mm Hg, induced by isoflurane and 50% nitrous oxide. No significant differences were noted between conditions with respect to ICP. There was a significant difference in CBF during condition (ii) compared with (i) (mean 75(SD 21) vs 100(0)%) and during condition (iv) compared with (iii) (90(26) vs 109(13)%). Animals under condition (iv) exhibited a 20% reduction in CBFV compared with those under condition (iii) (57 vs 69 cm s-1). For animals under normotensive conditions, addition of nitrous oxide to isoflurane resulted in a 16% increase in CBFV (69 vs 60 cm s-1). Comparing isoflurane-induced hypotension ((ii) vs (iv)), there was no statistical difference in either CBF or CBFV on addition of 50% nitrous oxide. The correlation between changes in CBF and CBFV was not significant. We conclude that the use of nitrous oxide during isoflurane-induced hypotension has no significant effect on CBF, CBFV or ICP compared with the use of isoflurane alone.