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With a split image focusing technique, it has been demonstrated by angiography that the human cerebral arteries dilate rapidly under the influence of Urografin 60%. The effect is most pronounced on the smallest measurable arteries, which have diameters from 0.5 to 1.0 mm, whereas the larger arteries of the more elastic type do not change their diameters significantly. The dilatation of the smallest arteries averages approximately 25%. It is assumed that this dilatation is due to the hypertonicity of the contrast material since a similar dilatation was observed following the injection of a hypertonic solution of glucose (50%). A similar degree of dilatation occurred following CO2 inhalation which elevated the arterial pCO2 to levels around 58 mm Hg and following injections of papaverine (30mg) into the carotid artery. This dilatation of 25% of the arteries with a diameter between 0.5 and 1.0 mm does not represent the maximum possible dilatation since Urografin 60% causes a further small increase in diameter of these pre-dilated arteries. On the other hand, Urografin 60% has nearly its full dilatory effect on arteries which are pre-dilated in a lesser degree with, for instance, a small dose of intravenous papaverine (40 mg).