title: Provocative intraocular pressure challenge preferentially decreases venous oxygen saturation despite no reduction in blood flow.
abstract: OBJECTIVE
  Ocular disease can both alter the retina's oxygen requirements, and decrease its ability to cope with changes in metabolic demand. We examined the influence of a moderate intraocular pressure (IOP) elevation on three outcome measures: arterial and venous oxygen saturation, blood flow, and the pattern electroretinogram (PERG).
  
  
  METHODS
  We increased IOP to ˜30 mmHg in 23 healthy participants (22-39 years) using a mechanical probe applied to the eyelid, thereby lowering ocular perfusion pressure (OPP) by ~30%. The Oxymap retinal oximeter was used to measure oxygen saturation for arteries and veins. Blood flow, volume and velocity were measured using the Heidelberg retinal flowmeter and steady-state PERG waveforms (8.34 Hz) were recorded bilaterally (200 sweeps). For each outcome measure, data was obtained three times: at baseline, 1 min into sustained IOP elevation, and 1 min after the probe was removed.
  
  
  RESULTS
  During IOP elevation, changes in oxygen saturation of retinal arteries failed to reach statistical significance [F(1,30) = 3.69, p = 0.05], whereas venous oxygen saturation was significantly reduced [F(1,21) = 27.43, p < 0.01]. Blood flow increased slightly [F(2,40) = 6.28, p < 0.0001], PERG amplitude significantly reduced [F(2,44) = 24.24, p < 0.0001] and PERG phase was significantly delayed [F(2,44) = 17.00, p < 0.0001]. Contralateral eyes were unchanged. OPP reduction correlated little with PERG amplitude, PERG phase or venous oxygen saturation.
  
  
  CONCLUSIONS
  Mild, acute IOP elevation increases arterio-venous oxygen saturation differences primarily through lowering venous oxygen saturation, suggesting increased oxygen consumption by healthy neurons when physiologically stressed.
authors.affiliation: Department of Optometry and Vision Sciences, The University of Melbourne, Melbourne, Australia; Department of Ophthalmology, Countess of Chester Hospital NHS Foundation Trust, Chester, UK.
authors.forename: Rachael A
authors.initials: RA
authors.lastname: O'Connell
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Anderson
authors.forename: Sarah L
authors.initials: SL
authors.lastname: Hosking
authors.forename: Bang V
authors.initials: BV
authors.lastname: Bui
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity / Q000502:physiology; D004596:Electroretinography; D005260:Female; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D009798:Ocular Hypertension / Q000503:physiopathology; D010100:Oxygen / Q000097:blood*; D012039:Regional Blood Flow / Q000502:physiology*; D012160:Retina / Q000502:physiology*; D012171:Retinal Vessels / Q000502:physiology*; D014065:Tonometry, Ocular; D055815:Young Adult
chemical_list: D010100:Oxygen
keywords: blood flow; intraocular pressure elevation; oxygen saturation; pattern electroretinogram