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.

Inclusion Criteria:

• ASA-I or ASA-II, willing and able to give written informed consent, scheduled for elective, non-cardiothoracic surgery under general anaesthesia and age 18 years and above.

Exclusion Criteria:

• Patient related

  • Unable/ unwilling to participate
  • ASA-III or higher
  • Age < 18 years
  • History of: uncontrolled hypertension, diabetes, Parkinson's disease, uncontrolled cardiac arrhythmia, Pure autonomic failure (formerly called idiopathic orthostatic hypotension), Multiple system atrophy with autonomic failure (formerly called Shy-Drager syndrome), Addison's disease and hypopituitarism, pheochromocytoma, peripheral autonomic neuropathy (e.g., amyloid neuropathy, idiopathic autonomic neuropathy), known cardiomyopathy, extreme left ventricle hypertrophy or ejection fraction < 30%, proven or suspected allergy for any of the medication used during induction of anaesthesia, malignant hyperthermia.
  • Unability to record transcranial Doppler ultrasound due to anatomical variance (~5% of population)
  • Contra-indications for intravenous or inhalational anaesthesia.
  • Contra-indications for phenylephrine: severe hypertension, peripheral vascular illness, severe hyperthyroidism
  • Simultaneous use of MAO-inhibitors, dopaminergic or vasoconstrictor ergot alkaloids (bromocriptine, cabergoline, pergolide, ergotamine, methylergometrine, methysergide), linezolid, tricyclic antidepressants, serotonin-norepinephrine reuptake inhibitors, digoxin, quinidine, oxytocin.

• Surgery related

  • Day case surgery
  • Laparoscopy with CO2 insufflation
  • Extreme positioning during surgery (head-down/up tilt, lateral decubitus position, prone)
  • Surgery < 60 minutes.