Surgery (Austin & Northern Health) - Theses

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    The effects of ventilation-perfusion scatter on gas exchange during nitrous oxide anaesthesia
    Peyton, Philip John ( 2011)
    Nitrous oxide (N2O) is the oldest anaesthetic agent still in clinical use. Due to its weak anaesthetic potency, it is customarily administered at concentrations as high as 70%, in combination with the more potent volatile anaesthetic agents. It allows a dose reduction of these agents and, unlike them, is characterised by remarkable cardiovascular and respiratory stability and has analgesic properties. Its low solubility in blood and body tissues produces rapid washin into the body on induction and rapid washout on emergence at the end of surgery. Its rapid early uptake by the lungs is known to produce a concentrating effect on the accompanying alveolar oxygen (O2) and volatile agent, increasing their alveolar concentration and enhancing their uptake, which speeds induction of anaesthesia. This is called the “second gas effect”. The place of N2O in modern anaesthetic practice has increasingly been criticised. It has a number of potential adverse effects, such as immunosuppression with prolonged exposure, and the possibility of cardiovascular complications due to acute elevation of plasma homocysteine levels is currently being investigated. It has been implicated as a cause of post-operative nausea and vomiting (PONV), and is a greenhouse gas pollutant. Its continued use relies largely on its perceived pharmacokinetic advantages, but these have been questioned. The existence of the second gas effect has been denied by recent authors, who have suggested that the real rate of uptake of N2O by pulmonary blood is much lower than assumed, and that a measurable increase in the partial pressure of accompanying volatile agent in arterial blood due to the second gas effect cannot be demonstrated. To resolve these questions and provide a precise picture of the extent of, and mechanisms underlying, the second gas effect, a series of studies was conducted involving both computer modelling of lung gas exchange and clinical measurement in anaesthetised patients. (For complete abstract open document.)