In experiments on living cats ocular volume changes were measured at various intraocular pressures after acute common carotid occlusion. A semilogarithmic relationship was found between ocular blood volume changes and the level of intraocular pressure after carotid occlusion. The slope of this relationship, termed the "coefficient of vascular rigidity," was found to correspond to similar measurements made in enucleated cat eyes by direct ophthalmic artery perfusion. The results of this study permitted a quantitative approach to the measurement of vascular rigidity in human eyes by using the carotid compression-tonography test with the 5.5 and 10.0 gm plunger weights. The normal coefficient of vascular rigidity is approximately 0.0238.
Best M, Masket S, Rabinovitz AZ. Measurement of Vascular Rigidity in the Living Eye. Arch Ophthalmol. 1971;86(6):699-705. doi:10.1001/archopht.1971.01000010701016