Effects of End Tidal Carbon Dioxide Concentration on Depth of Anesthesia in Children Undergoing Total Intravenous Anesthesia

Purpose: Carbon dioxide (CO2) is a major end-product of metabolism and can have marked effects on central nervous system function. It can also be easily manipulated during general anesthesia via controlled ventilation. High levels of carbon dioxide (hypercapnia) are associated with sedation and have been shown to produce selective suppression of thermal and ischaemic pain in animals and humans. This effect was attenuated by dexamethasone and naloxone, indicating that stress pathways and endogenous opioids may be implicated. Hypercapnia during anesthesia may have additional benefits, including reduced levels of wound infection due to improved tissue oxygenation reduced incidence of postoperative nausea and vomiting and reduced recovery time from volatile anesthetic.

It is a known phenomenon for high levels of CO2 to be associated with reduced levels of consciousness in humans, known as CO2 narcosis. A 1927 paper described narcosis of animals when breathing 30-40% CO2 in oxygen, with prompt recovery upon removal. The authors described a 'sharp sour taste' and associated hypertension when the same solution was administered to humans. However, few studies investigate the impact of carbon dioxide on anesthetic requirements. An animal study from 1967 demonstrated that very high levels of CO2 (>95 mmHg) offset halothane requirements in dogs. Most recently, increased carbon dioxide levels during surgery (40 - 45 mmHg) were shown to reduce the Minimal Alveolar Concentration to Blunt Adrenergic Response (skin incision; MAC-BAR) of sevoflurane in adult patients undergoing gastric carcinoma resection.

Total intravenous anesthesia (TIVA), an alternative to inhalational anesthesia, is a commonly used anesthetic technique in the investigator's institution. This is due to its many benefits, including reduced emergence delirium, reduced environmental impact and reduced post-operative nausea and vomiting. Administration can be guided by depth of anesthesia monitoring such as the Bispectral Index (BIS), which measures the patient's level of consciousness derived from electroencephalogram readings. BIS has been shown to help guide propofol dosing in children regardless of whether the TIVA technique was target controlled or a manual infusion regimen, and to correlate well with both modelled and measured propofol levels in children.

The investigator's study aims to determine whether differing levels of CO2 affect the anesthetic depth in anesthetized children, as measured by BIS.

Hypothesis: Hypercarbia is associated with a reduction in BIS readings, in anesthetized children.