title: The effect of prone positioning on intraocular pressure in anesthetized patients.
abstract: BACKGROUND
  Ocular perfusion pressure is commonly defined as mean arterial pressure minus intraocular pressure (IOP). Changes in mean arterial pressure or IOP can affect ocular perfusion pressure. IOP has not been studied in this context in the prone anesthetized patient.
  
  
  METHODS
  After institutional human studies committee approval and informed consent, 20 patients (American Society of Anesthesiologists physical status I-III) without eye disease who were scheduled for spine surgery in the prone position were enrolled. IOP was measured with a Tono-pen XL handheld tonometer at five time points: awake supine (baseline), anesthetized (supine 1), anesthetized prone (prone 1), anesthetized prone at conclusion of case (prone 2), and anesthetized supine before wake-up (supine 2). Anesthetic protocol was standardized. The head was positioned with a pinned head-holder. Data were analyzed with repeated-measures analysis of variance and paired t test.
  
  
  RESULTS
  Supine 1 IOP (13 +/- 1 mmHg) decreased from baseline (19 +/- 1 mmHg) (P < 0.05). Prone 1 IOP (27 +/- 2 mmHg) increased in comparison with baseline (P < 0.05) and supine 1 (P < 0.05). Prone 2 IOP (40 +/- 2 mmHg) was measured after 320 +/- 107 min in the prone position and was significantly increased in comparison with all previous measurements (P < 0.05). Supine 2 IOP (31 +/- 2 mmHg) decreased in comparison with prone 2 IOP (P < 0.05) but was relatively elevated in comparison with supine 1 and baseline (P < 0.05). Hemodynamic and ventilatory parameters remained unchanged during the prone period.
  
  
  CONCLUSIONS
  Prone positioning increases IOP during anesthesia. Ocular perfusion pressure could therefore decrease, despite maintenance of normotension.
authors.affiliation: Department of Anesthesiology, Washington University School of Medicine, St Louis, Missouri, USA. chengm@notes.wustl.edu
authors.forename: M A
authors.initials: MA
authors.lastname: Cheng
authors.forename: A
authors.initials: A
authors.lastname: Todorov
authors.forename: R
authors.initials: R
authors.lastname: Tempelhoff
authors.forename: T
authors.initials: T
authors.lastname: McHugh
authors.forename: C M
authors.initials: CM
authors.lastname: Crowder
authors.forename: C
authors.initials: C
authors.lastname: Lauryssen
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000758:Anesthesia; D005123:Eye / Q000098:blood supply; D005260:Female; D006439:Hemodynamics / Q000502:physiology; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D019635:Neurosurgical Procedures; D016684:Prone Position / Q000502:physiology*; D012039:Regional Blood Flow / Q000502:physiology; D015656:Respiratory Mechanics / Q000502:physiology