The influence of changes in end‐tidal carbon dioxide upon the Bispectral Index *

Carbon dioxide is known to affect consciousness in animals and humans. We surmised that changes in end‐tidal carbon dioxide during anaesthesia might affect the Bispectral Index. Twenty‐four patients due to undergo surgery were anaesthetised with fentanyl and a propofol infusion. The Bispectral Index, pulse rate and blood pressure were recorded while end‐tidal carbon dioxide levels were changed. The patients acted as their own controls as they were subjected to high, normal and low levels of end‐tidal carbon dioxide (3–12 kPa) according to a randomised sequence. There were no changes in the Bispectral Index or haemodynamic variables resulting from manipulation of the end‐tidal carbon dioxide. At the level of hypnosis involved in this study, changes in end‐tidal carbon dioxide, within the range tested, do not result in changes in the Bispectral Index.

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