CO2, DEPENDENCE OF RETINAL ARTERIAL AND CAPILLARY BLOOD VELOCITY

Objective: Blood flow to the brain is extremely sensitive to changes i n PCO2, While animal studies show a similar potent PCO2 dependence in retinal and choroidal vessels, the PCO2-retinal blood flow relationshi p has never been adequately studied in humans. Methods: Video scanning laser ophthalmoscopy after fluorescein angiography was used to analyz e retinal arterial and capillary blood velocity under conditions of mi ld hypercapnia and hypocapnia. Control conditions (end-tidal PCO2 = 38 .3 +/- 0.4 mmHg) were contrasted with hyperventilation-induced hypocap nia (PCO2 = 34.0 +/- 0.4 mmHg) and hypercapnia (PCO2 = 42.3 +/- 0.5 mm Hg) created by PCO2 addition to inspired gas. Results: Both larger ves sel and macular capillary blood velocity was dependent upon PCO2: arte riovenous passage time fell as PCO2 rose, and both mean arterial dye v elocity and capillary blood velocity rose as PCO2 rose (all p<0,05). T hese changes in flow velocity occurred despite unchanged heart rate, a rterial systolic and diastolic blood pressure, intraocular pressure, a nd calculated ocular perfusion pressure. Contrast sensitivity was also unchanged by PCO2 variation. Conclusions: The human retinal circulati on, like the whole cerebral circulation, may be strongly dependent upo n PCO2 in a manner that is unrelated to perfusion pressure and apparen tly outside strict autoregulatory controls.