THE EFFECT OF AGE ON THE ELASTICITY OF THE MAJOR BRAIN ARTERIES

Abstract

In previous work on peripheral arteries the law of Laplace has been applied to the pressure-volume data to yield the elastic constants and interrelation of function of elastin and collagenous fibers in the wall, and the changes with age. Similar analysis of major brain arteries has been made on autopsy specimens, aged 2 to 90. Volume (micrometer syringe) and pressure (electromanometer) in arterial segments were accurately measured. Absolute volumes were obtained by collapsing the artery with negative pressure, and the "unstretched circumferences" of the vessel were deduced from the curves. Tension-circumference curves were constructed. These showed the characteristic increasing slope (increased "elastance" or "stiffness") with increasing degree of stretch, interpreted as successive "recruitment" of collagen fibers as they reach their unstretched length. Increases in arterial length with distension were very small and negligible in vessels older than 30 years. Maximum slope (stiffness) was reached at pressures in the physiological range. The maximal stretch was 26 to 38% for vessels aged 30 to 90 years. This is less than for peripheral arteries, except those over 80 years old. Ageing showed itself mainly in reduction of the stretch required to bring 50% of the collagen fibers to their unstretched length, i.e. in the "degree of slackness". This was 30% stretch for younger vessels, 20% for the older. Unlike the peripheral arteries, the brain arteries show no significant increase with age in the maximal stiffness (related to their total collagen content). The mean vlaue of the Young''s modulus of the wall at high pressures was 1.93 [plus or minus] 0.67 107 dynes/sq.cm. No significant change in thickness of the wall of lumen diameter with age was found. It is concluded that the major brain arteries are less distensible than peripheral arteries of comparable diameter, particularly in youth. Distensibility decreases with age, mainly because the "degree of slackness" of the collagen fibers is reduced. At physiological pressures the major resistance to distension is due to the collagen fibers rather than to the elastin fibers, which appear histologically to be less abundant, except in the elastica interna, than in peripheral arteries.