Quantification of Dynamic and Static Cerebral Autoregulation (CA) Under Anaesthesia

Cerebral autoregulation (CA) is the mechanism by which the brain vasculature maintains constancy of cerebral blood flow (CBF). Reliable direct measurements of CBF at different blood pressure levels are difficult because they are invasive and time-consuming. This type of measurement to quantify CA is generally referred to as static cerebral autoregulation (sCA). Alternatively, it is possible to measure CA indirectly from blood pressure oscillations. Dynamic cerebral autoregulation (dCA) measures how quickly the cerebral vessels react to a change in blood pressure to normalize CBF. Since the introduction of transcranial Doppler ultrasound (TCD), it has become possible to estimate CBF velocity relatively easy, which in turn correlates well with CBF changes. This method is widely used to quantify dCA. However, it is not clear how sCA correlates with dCA over a range of physiologic mean blood pressure (MBP). It is important to compare different methods of assessing CA, because impaired CA may result in increased risk of perioperative complications such as stroke. In this study, the investigators were interested in establishing the relationship between sCA and dCA during surgery under general anesthesia. The investigators aim to compare these methods during propofol and sevoflurane anesthesia.