Several factors may affect blood flow to the brain during anaesthesia. During surgery on the internal organs, a hormone may be released that dilates blood vessels and causes a so-called mesenteric traction syndrome characterised by a decrease in blood pressure and flushing. This reaction lasts for approximately thirty minutes and is observed in about half of the patients who undergo surgery on the stomach and intestines. It is unknown whether a mesenteric traction syndrome affects blood flow to the brain. Ventilation is also of importance for blood flow to the brain. Thus, blood flow to the brain is reduced by hyperventilation and increases if breathing is slower. It is unclear whether the relation between blood flow to the brain and ventilation is affected during anaesthesia.