Effect of intraoperative position on the zero flow pressure of the cerebral circulation under propofol anesthesia: a prospective observational study.

Purpose: The effect of intraoperative position on cerebral perfusion remains unclear. We examined changes in zero flow pressure (ZFP) relative to the supine position in the beach-chair, Trendelenburg, and prone positions under propofol anesthesia. ZFP is defined as the arterial pressure at which cerebral blood flow theoretically ceases, and it can represent effective downstream pressure, including cerebrovascular tone and intracranial pressure.

Methods: This prospective, observational study conducted at two institutions included patients undergoing shoulder surgery, robotic-assisted laparoscopic prostatectomy, or lumbar spine surgery (n = 15 for each position) under propofol anesthesia. The flow velocity of the middle cerebral artery was recorded using transcranial Doppler. ZFP was determined by linear regression analysis of the pressure-flow velocity plot, with a pressure transducer calibrated at the heart level regardless of patient posture.

Results: Head height relative to the heart increased by 25 cm in the beach-chair position, decreased by 13 cm in the Trendelenburg position, and was unchanged in the prone position. ZFP significantly increased by 12 and 5 mmHg in the beach-chair and Trendelenburg positions, respectively, whereas no significant change was observed in the prone position.

Conclusions: The beach-chair and Trendelenburg positions increased ZFP. In both positions, it might be preferable to maintain mean arterial pressure higher than in the supine position to preserve cerebral perfusion.