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PEDIATRIC ANESTHESIA

The Effect of Nitrous Oxide on Cerebrovascular Reactivity to Carbon Dioxide in Children During Propofol Anesthesia

Department of Anesthesia, The Hospital for Sick Children and the University of Toronto, Ontario, Canada

Address correspondence and reprint requests to Bruno Bissonnette, Department of Anesthesia, The Hospital for Sick Children, 555 University Ave., Toronto, Ontario, M5G 1X8, Canada. Address e-mail to bruno{at}anaes.sickkids.on.ca

Nitrous oxide (N₂O) increases cerebral blood flow when used alone and in combination with propofol. We investigated the effects of N₂O on cerebrovascular CO₂ reactivity (CCO₂R) during propofol anesthesia in 10 healthy children undergoing elective urological surgery. Anesthesia consisted of a steady-state propofol infusion and a continuous caudal epidural block. A transcranial Doppler probe was used to measure middle cerebral artery blood flow velocity. Randomization determined the sequence order of N₂O (N₂O/air or air/N₂O) and end-tidal (ET)CO₂ concentration (25, 35, 45, and 55 mm Hg) using an exogenous source of CO₂. At steady state, three sets of measurements of middle cerebral artery blood flow velocity, mean arterial blood pressure, and heart rate were recorded. A linear preservation of CCO₂R was observed above 35 mm Hg of ETCO₂, irrespective of N₂O. A decrease in CCO₂R to 1.4%–1.9% per millimeters of mercury was seen in the hypocapnic range (ETCO₂ 25–35 mm Hg) with both air and N₂O. We conclude that N₂O does not affect CCO₂R during propofol anesthesia in children. When preservation of CCO₂R is required, the combination of N₂O with propofol anesthesia in children would seem suitable. The cerebral vasoconstriction caused by propofol would imply that hyperventilation to ETCO₂ values less than 35 mm Hg may not be required because no further reduction in cerebral blood flow velocity would be achieved.

IMPLICATIONS: Nitrous oxide, which increases cerebral blood flow, does not affect cerebrovascular reactivity to carbon dioxide (CCO₂R) in children anesthetized with propofol. When preservation of CCO₂R is required, the combination of nitrous oxide with propofol anesthesia in children may be suitable.

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