Date Published: September 5, 2012
Publisher: Hindawi Publishing Corporation
Author(s): Jakob Højlund, Marie Sandmand, Morten Sonne, Teit Mantoni, Henrik L. Jørgensen, Bo Belhage, Johannes J. van Lieshout, Frank C. Pott.
Background. The prone position is applied to facilitate surgery of the back and to improve oxygenation in the respirator-treated patient. In particular, with positive pressure ventilation the prone position reduces venous return to the heart and in turn cardiac output (CO) with consequences for cerebral blood flow. We tested in healthy subjects the hypothesis that rotating the head in the prone position reduces cerebral blood flow. Methods. Mean arterial blood pressure (MAP), stroke volume (SV), and CO were determined, together with the middle cerebral artery mean blood velocity (MCA Vmean) and jugular vein diameters bilaterally in 22 healthy subjects in the prone position with the head centered, respectively, rotated sideways, with and without positive pressure breathing (10 cmH2O). Results. The prone position reduced SV (by 5.4 ± 1.5%; P < 0.05) and CO (by 2.3 ± 1.9 %), and slightly increased MAP (from 78 ± 3 to 80 ± 2 mmHg) as well as bilateral jugular vein diameters, leaving MCA Vmean unchanged. Positive pressure breathing in the prone position increased MAP (by 3.6 ± 0.8 mmHg) but further reduced SV and CO (by 9.3 ± 1.3 % and 7.2 ± 2.4 % below baseline) while MCA Vmean was maintained. The head-rotated prone position with positive pressure breathing augmented MAP further (87 ± 2 mmHg) but not CO, narrowed both jugular vein diameters, and reduced MCA Vmean (by 8.6 ± 3.2 %). Conclusion. During positive pressure breathing the prone position with sideways rotated head reduces MCA Vmean ~10% in spite of an elevated MAP. Prone positioning with rotated head affects both CBF and cerebrovenous drainage indicating that optimal brain perfusion requires head centering.
Prone positioning of patients during anesthesia is required for a variety of surgical procedures . Furthermore, the prone versus supine position promotes ventilation/perfusion matching in critically ill patients with acute respiratory distress syndrome . When moving a patient into the prone position, arterial pressure usually remains stable whereas a reduction in cardiac output (CO)  is attributed to a decrease in venous return  by abdominal compression with partial inferior caval vein obstruction . Many anesthetists place the head centered in a headrest during prone positioning especially during more extended procedures, for example, spine surgery. However, since it is less cumbersome and/or the head position may not be considered important many patients are placed with the head positioned to the side. This is especially the case when laryngeal mask airways are used for surgery in the prone position, which is an increasingly used practice [5–9]. In the anesthetized patient, the positioning-related reduction in CO is amplified by positive pressure ventilation. Complications associated with prone positioning often with the head in a sideward-rotated position include occlusion of cervical arteries or veins, and injuries of the cervical spine and peripheral nerves [1, 6, 10]. Particularly, carotid and vertebral artery occlusion and dissection and middle cerebral artery (MCA) infarction have been linked to head rotation or extension . In a recent review, postoperative visual loss as a recognized complication of prone positioning has been linked to hemodynamic alterations . Positive pressure breathing in the supine position reduces cerebral blood flow . Also, prone positioning raises intracranial pressure with potentially adverse consequences for cerebral perfusion and/or cerebral venous drainage . Additional sideward rotation of the head in that position leads to venous compression with potential cerebral venous outflow impairment. In infants, prone positioning with the head rotated versus the supine position with the head centered reduces transcranial Doppler determined MCA blood velocity (MCA Vmean) . Also, rotating of the head increases cerebral blood volume attributed to impairment of venous drainage  whereas in adults the effects of head direction during prone positioning have not been studied. We hypothesized that during positive pressure breathing in the prone position, head rotation would reduce MCA Vmean. We set out to investigate systemic hemodynamics and brain arterial and venous characteristics by following MCA Vmean as an index of cerebral blood flow and jugular vein diameter in the prone position with and without positive pressure breathing and/or head rotation.
22 healthy volunteers (7 women), aged 24 ± 4 years (mean ± standard deviation), height 182 ± 10 cm, weight 78 ± 12 kg participated in this investigation. All gave their informed consent prior to inclusion in the study. The study was approved by the Ethics Committee for Copenhagen and Frederiksberg (KF 01 287338) and was performed in accordance with the Helsinki Declaration.
The results of this study provide insight into the effects of prone positioning, head rotation and positive pressure breathing on brain perfusion and brain venous drainage. The prone position with sideways rotated head and positive breathing narrowed the ipsilateral internal jugular vein together with a ~10% reduction in MCA Vmean in spite of an elevated MAP. These results suggest that for this commonly applied anesthetic approach both cerebral blood flow and cerebrovenous drainage are optimal only with the head centered.