The Effect of Blood Pressure on Cerebral Blood Flow During Propofol Anesthesia

Background Propofol anesthesia reduces cerebral blood flow (CBF) and mean arterial pressure (MAP) but it is unclear whether the decrease in CBF is accentuated by hypotension. Cerebral autoregulation is generally considered to maintain CBF when MAP is between 60-150 mmHg. Hence, vasoactive medication is administered if MAP decreases to below approximately 60 mmHg.

It is controversial whether there is a plateau for cerebral autoregulation. Thus, middle cerebral artery blood velocity (MCA Vmean), as an index of CBF, is associated to MAP during pharmacological changes in MAP between approximately 40-125 mmHg. Similarly, CBF is affected by pharmacological changes in MAP between 40-80 mmHg during hypothermic cardiopulmonary bypass surgery in propofol anesthesia. Arterial hypertension may increase the lower limit of cerebral autoregulation which can be mitigated by antihypertensive treatment.

Cognitive dysfunction and delirium are common following major surgery, particularly in the elderly, and may be related to hypotension and cerebral hypoperfusion. Further, hypotensive anesthesia is associated with an increase in markers of neuronal damage, but studies have been too small to detect any difference in incidence of cognitive dysfunction following hypotensive as compared to normotensive anesthesia.

In young healthy adults, propofol anesthesia causes limited reduction in blood pressure and decreases CBF by approximately 50% by a reduction in neuronal activity. An increase in MAP from approximately 80 to 100 mmHg during propofol anesthesia does not affect CBF but it is unknown whether CBF can be increased by an increase in MAP from approximately 60 to 80 mmHg. Propofol appears to maintain cerebral autoregulation but it is unknown whether the lower level of cerebral autoregulation is affected. The internal carotid artery supplies most of CBF and dilates during moderate hypotension with maintained blood flow which indicates that the vessel contributes to cerebral autoregulation. Further, central blood volume and cardiac output may be important factors for maintaining CBF.

The study will include thirty patients planned for major abdominal surgery in propofol-remifentanil anesthesia combined with epidural analgesia. Internal carotid artery blood flow will be evaluated on the neck using duplex ultrasound. The study will be conducted one hour after the start of surgery and lasts for approximately half an hour. In the study, MAP is set pharmacologically at 80-85, 70-75 and 60-65 mmHg for a short time in random order. The level of 60-65 mmHg corresponds to the level at which MAP is maintained in clinical practice. Control of MAP is by intravenous infusion of noradrenaline, an α- and β-adrenergic agonist.