Purpose:
The lower limit of blood flow autoregulation (LLA) refers to a critical ocular perfusion pressure (OPP) below which the autoregulation capacity fails and blood flow (BF) can no longer be maintained at a normal level. The LLA is therefore defined as the minimal OPP that ensures sufficient blood perfusion to a tissue. The purpose of this study was to determine the LLA within the optic nerve head (ONH) of normal monkey eyes.
Methods:
In 12 rhesus monkeys anesthetized with pentobarbital (8-10 mg/kg/h, IV infusion), with mean arterial blood pressure (BP) ranging from 63 to 116 mmHg and intraocular pressure (IOP) set at 10 mmHg manometrically, the BF in the ONH was measured (arbitrary unit) with a laser speckle flowgraph device and plotted against corresponding OPP (calculated as BP - IOP) (Fig A). The OPP was then reduced by increasing IOP from 10 mmHg to 30, 40 or 50 mmHg. Three minutes after IOP elevation, BF was measured again. The percentage BF change in response to each IOP increase was plotted against corresponding OPP after IOP was elevated (Fig B).
Results:
At IOP 10 mmHg and within a range of OPP from 90 to 47 mmHg, ONH BF was essentially constant with no or little association with OPP (Fig A, R2 = 0.01, linear regression). After acute IOP elevation, the relationship between percent BF changes and OPP (from 18 to 76 mmHg) proved to be best fit by a two segment linear model (vs. a single linear function; F [2, 269 df] = 26.2, P<0.0001). The resultant function had a break point at 35.7 mmHg (95% CI: 33-38). BF above 35.7 mmHg remained stable and was independent of OPP (Fig B blue, y=0.0001*OPP-0.0016, P<0.001); however, BF below 35.7 mmHg declined linearly with decreasing OPP (Fig B red, y=0.012*OPP-0.43 , R2=0.31).
Conclusions:
The normal monkey ONH has strong capacity of BF autoregulation. The LLA of BF autoregulation in normal monkey ONH is approximately 36 mmHg (OPP). This value is within the range of most previous studies in both monkeys and humans.
Keywords: optic nerve • blood supply • intraocular pressure