X-ray investigation of the orientation of collagen fibres in aortic media layer under distending pressure

Preoperative Vascular Medial Fibrosis and Arteriovenous Fistula Development

BACKGROUND AND OBJECTIVES

Arteriovenous fistula maturation requires an increase in the diameter and blood flow of the feeding artery and the draining vein after its creation. The structural properties of the native vessels may affect the magnitude of these changes. We hypothesized that an increase in the collagen content of the vascular media (medial fibrosis) preoperatively would impair vascular dilation and thereby, limit the postoperative increase in arteriovenous fistula diameter and blood flow and clinical arteriovenous fistula maturation.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We enrolled 125 patients undergoing arteriovenous fistula creation between October of 2008 and April of 2012 and followed them prospectively. Any consenting subject was eligible. Arterial and venous specimens were sampled during arteriovenous fistula surgery. Masson's trichrome-stained samples were used to quantify medial fibrosis. Arteriovenous fistula diameter and blood flow were quantified using 6-week postoperative ultrasound. Clinical arteriovenous fistula maturation was assessed using a predefined protocol. The association of preexisting vascular medial fibrosis with arteriovenous fistula outcomes was evaluated after controlling for baseline demographics, comorbidities, and the preoperative venous diameter.

RESULTS

The mean medial fibrosis was 69%±14% in the arteries and 63%±12% in the veins. Arterial medial fibrosis was associated with greater increases in arteriovenous fistula diameter (Δdiameter =0.58 mm; 95% confidence interval [95% CI], 0.27 to 0.89 mm; P<0.001) and arteriovenous fistula blood flow (Δblood flow =85 ml/min; 95% CI, 19 to 150 ml/min; P=0.01) and a lower risk of clinical arteriovenous fistula nonmaturation (odds ratio, 0.71; 95% CI, 0.51 to 0.99; P=0.04), all per 10% absolute difference in medial fibrosis. In contrast, venous medial fibrosis was not associated with the postoperative arteriovenous fistula diameter, blood flow, or clinical maturation.

CONCLUSIONS

Preoperative arterial medial fibrosis was associated with greater arteriovenous fistula diameter and blood flow and a lower risk of clinical arteriovenous fistula nonmaturation. This unexpected observation suggests that medial fibrosis promotes arteriovenous fistula development by yet undefined mechanisms or alternatively, that a third factor promotes both medial fibrosis and arteriovenous fistula maturation.

In vivo estimation of the contribution of elastin and collagen to the mechanical properties in the human abdominal aorta: effect of age and sex

The mechanical properties of the aorta affect cardiac function and are related to cardiovascular morbidity/mortality. This study was designed to evaluate the isotropic (mainly elastin, elastiniso) and anisotropic (mainly collagen, collagenani) material parameters within the human aorta in vivo. Thirty healthy men and women in three different age categories (23–30, 41–54, and 67–72 yr) were included. A novel mechanical model was used to identify the mechanical properties and the strain field with aid of simultaneously recorded pressure and radius in the abdominal aorta. The magnitudes of the material parameters relating to both the stiffness of elastiniso and collagenani were in agreement with earlier in vitro studies. The load-bearing fraction attributed to collagenani oscillated from 10 to 30% between diastolic and systolic pressures during the cardiac cycle. With age, stiffness of elastiniso increased in men, despite the decrease in elastin content that has been found due to elastolysis. Furthermore, an increase in stiffness of collagenani at high physiological pressure was found. This might be due to increased glycation, as well as changed isoforms of collagen in the aortic wall with age. A marked sex difference was observed, with a much less age-related effect, both on elastiniso and collagenani stiffness in women. Possible factors of importance could be the effect of sex hormones, as well as differing collagen isoforms, between the sexes.