Changes in cerebral oxygen saturation following prone positioning for orthopaedic surgery under general anaesthesia: a prospective observational study.

BACKGROUND Prone positioning is often necessary in orthopaedic surgery. The prone position, however, may result in impaired cerebral venous drainage with a subsequent reduction in cerebral perfusion. As a consequence, cerebral hypoxia may occur with the potential for neurological impairment. OBJECTIVE We assessed the changes in cerebral oxygen saturation with near-infrared spectroscopy using two different monitors after positioning the patient from supine to prone. DESIGN Prospective observational study. SETTING Primary Care University Hospital, from May 2010 to February 2011. PARTICIPANTS Forty patients undergoing general anaesthetic procedures, of which 35 completed the investigation. Similar measurements were done in 35 volunteers, who were studied while awake. INTERVENTIONS Near-infrared spectroscopy was measured throughout anaesthesia using INVOS (a trend monitor using two infrared wavelengths) for one hemisphere and FORE-SIGHT (a monitor using four wavelengths of laser light to calculate absolute oxygen saturation) for the other hemisphere in an alternate randomisation pattern. OUTCOME MEASUREMENTS The primary outcome was a change in cerebral oxygen saturation of more than 5% during prone positioning. A comparison with the changes obtained in awake volunteers following similar positioning was also made. RESULTS Cerebral oxygen saturation increased during prone positioning with INVOS 0.032% per minute (P< 0.01) and with FORE-SIGHT 0.032% per minute (P< 0.01) in anaesthetised patients. Awake volunteers showed an increase of 0.171% per minute (INVOS) and 0.082% per minute (FORE-SIGHT) during prone positioning. Comparison of INVOS with FORE-SIGHT showed a good association, with a gradient of 0.80% per 1% change (P< 0.01). CONCLUSION Both monitors detected a small increase in cerebral oxygen saturation of less than 5% in patients undergoing orthopaedic surgery in the prone position and in awake volunteers. This small increase is of limited clinical relevance and prone positioning may be regarded as safe in terms of the maintenance of cerebral oxygen saturation.