The effect of experimental hypertension on retinal vascular autoregulation in humans: a mechanism for the progression of diabetic retinopathy

SUMMARY Since theretinal vessels havenosympathetic innervation, blood flowinresponse toraised blood pressure isdependent onautoregulation. Todetermine theeffect ofhypertension onretinal haemodynamics andtheautoregulatory capacity oftheretinal circulation underconditions of normoglycaemia andhyperglycaemia, retinal bloodflowwasmeasured before andafter raising thesystemic bloodpressure intennormal control subjects, tendiabetic subjects withblood glucose 15mmol1-'. A controlled nseinsystemic blood pressure wasachieved using anintravenous infusion oftyramine. Retinal volumeflowwasdetermined fromredcell velocity using laser Doppler velocimetry andfrom retinal vessel diameter measurements using digital image analysis offundus photographs. Witha 40% increase inmeanarterial blood pressure (MAP), there wasasignificant increase inretinal blood flow of 32.9 +7-1% innon-diabetic controls. Indiabetics atthelowblood glucose level, theincrease inretinal bloodflowwassignificant at30% increase inMAP (23.6 +87%, P = 0-032) andat40% increase (49-9 + 12-03 %,P= 0-004). Diabetics withhigh blood glucose failed toautoregulate atanyoftheincreased levels ofMAP (15% increase, 270+ 111%;30% increase, 66.9 + 19.8 %;and40% increase, 1019+214%;P<0.022). Thecoefficients of autoregulation showed that innon-diabetic controls, retinal vascular autoregulation broke down withincreases inMAP ofbetween 30and40 %.Indiabetic subjects, itbroke downbetween 15 and30% innormoglycaemia andatless than15% inhyperglycaemia. Thisstudy demonstrates animpairment inretinal vascular autoregulation inresponse toraised systemic blood pressure in diabetic subjects, moresoatanelevated blood glucose level, thusproviding amechanism forthe detrimental effect ofhypertension ondiabetic retinopathy.