have not yet been carried out and quantitative pressure flow relations in human choroidal vessels are as yet unknown. Linear choroidal pressure flow relations have been obtained in animal experiments in different species.5-8 In the rabbit, the choroidal blood flow has been shown to be pressure independent when IOP was less than 20-25 mm Hg.9Blood vessels can be considered as cylinders filled with fluid at a pressure greater than that outside the cylinders. The pressure difference between the inside and the outside of a vessel is called the transmural pressure P. The corresponding tension T in the vessel wall can be calculated by Laplace's law P=T/R, where R is the radius of the cylinder. Any variation of the transmural pressure alters the tension of the vessel wall. Thus, pressure oscillations of cardiac pulse pressure output lead to a pulsation of the vessel wall. As blood is pumped into an artery, the systolic pressure increases and dilates the vessel wall. When cardiac ejection decreases, the pressure falls and the vessel wall returns to its equilibrium position, the diastolic pressure.10 The arterial pressure contour becomes progressively more distorted as the wave is transmitted down the arterial system. The elastic properties of an artery are described by the arterial compliance, the change in diameter over the change in pressure. The non-linear elastic response of arteries implies that their mechanical properties depend on the mean arterial pressure.11In retinal and choroidal vessels the mean transmural pressure at the arteries entering the eye nearly equals the mean perfusion pressure, defined as the pressure in the arteries entering the eye (Pa) minus the pressure in the veins (Pv) leaving the eye. Pf=Pa-Pv. The pressure in the veins, Pv, is significantly higher than the intraocular pressure (IOP) in the retinal vessels'2 whereas the difference in the choroidal vessels is small.'3 Therefore, the transmural pressure in the veins is rather small or even zero. Changes in transmural pressure and the consecutive change in vessel diameter lead to pulsations of the surrounding tissue.Our work is concerned with local tissue pulsations at the foveola. This area of highest visual acuity is approximately 350 ,um in diameter.'4 This is a little smaller than the retinal avascular zone measuring approximately 500-600 ,um in diameter. In our study, where the area involved in the measurement is 20-50 ,um, the effect of tissue pulsations is only influenced by the blood flow in choroidal vessels.
Local fundus pulsations in the macula are reduced in proliferative diabetic retinopathy, which is compatible with previous findings of reduced choroidal blood flow in late stages of the disease. Laser interferometric measurement of fundus pulsations is non-contactile, assures optimal comfort for the patient and could be used for the long-term observation of patients with diabetes mellitus.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.