Perfusion pressure and blood flow: difference and relevance

Purpose Maintenance of appropriate levels of regional blood flow play an essential role in the maintenance of a stable internal environment of an organism. The relative contributions of the different mechanisms vary between and within vascular beds. The ability of vascular beds, especially those not or not particularly responsive to neuronal regulatory mechanisms, to maintain, within certain limits, an independence of regional blood flow from local perfusion pressure is called autoregulation. Methods A proper appraisal of local perfusion pressure is mandatory if one is to understand blood flow and the effects of its perturbation. When speculating about ocular perfusion pressure, it has become customary to estimate mean blood pressure within the ophthalmic artery and to subtract the value of intraocular pressure, assumed to be equal to venous pressure. This concept, likely to be adequate under steady state condition in healthy eyes, may not hold under stress conditions or therapeutic interventions, as well as in disease states. Examples are manifold. Results Simple starvation may put a burden on the circulation to maintain the supply of metabolites, possibly solved by increasing transmural capillary filtration pressure. In glaucoma, an increased venous pressure has been described and, indeed, more force must be applied to the eye to induce retinal venous pulsations. These conditions are simple examples where our current approach in estimating ocular perfusion pressure is probably incorrect. Conclusion The regulatory responses of ocular vascular beds have not yet been elucidated in full detail, and care must be applied when speculating about ocular perfusion pressure, and many of our current assumptions with regards to autoregulation in the ocular circulation may need a very critical scrutiny.