Abstract
Arterial rupture is a well-recognized cause of sudden death in horses, which mainly affects older horses. The arterial wall is known to stiffen with age, although the underlying age-related histological and biomechanical changes remain unclear. The purpose of this study was to investigate the effect of aging by histological analysis of the arterial wall and examination of the arterial wall biomechanical properties using an inflation-extension test. Entire circular samples of the proximal and distal aorta, cranial and caudal common carotid, external iliac, femoral and median artery were collected from 6 young (6 years) and 14 old horses (≥15 years). Samples of all arteries were histologically examined and intima media thickness as well as area % of elastin, smooth muscle actin and collagen type I and III were determined. Older horses had a significantly larger intima media thickness and a significantly higher area % of smooth muscle actin compared to young horses. Samples of the proximal and distal aorta, the caudal common carotid and the external iliac artery were mechanically assessed using an in-house developed inflation-extension device with ultrasound analysis. Rupture occurred in a minority of arteries (8/78) at high pressures (between 250–300 mmHg), and mostly occurred in older horses (7/8). Pressure-area, pressure-compliance and pressure-distensibility curves were constructed. A significant difference in the pressure-area curves of the distal aorta, common carotid artery and external iliac artery, the pressure-compliance curves of the proximal aorta and carotid artery and the pressure-distensibility curve of the proximal aorta was observed between young and old horses. Results demonstrate an effect of age on the histological and biomechanical properties of the arterial wall, which might explain why arterial rupture occurs more often in older horses.
Highlights
Conduit arteries in mammals are compliant by nature and provide a low resistance path for the blood supply to the visceral organs and the limbs
At the level of the proximal aorta, the tunica media consisted of one layer with an almost uniform distribution of fibres and smooth muscle cells
At the level of the distal aorta, two different layers within the tunica media could be distinguished in 40% of the horses (5 old and 3 young horses)
Summary
Conduit arteries in mammals are compliant by nature and provide a low resistance path for the blood supply to the visceral organs and the limbs. The key structural components contributing to the compliance of the arterial wall are elastin fibres, collagen fibres, smooth muscle cells and cross-linking matrix constituents [1, 2]. The arterial compliance at physiological pressures and the pressure at which compliance is maximal decrease with age, indicating a shift of the pressure-compliance curve to lower pressures [5]. These mechanical alterations are due to major structural changes. Aging causes thickening of the tunica intima and increases tunica media thickness, while its cellularity decreases due to vascular smooth muscle cell hypertrophy and the build-up of extracellular matrix [12]
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