Segmental volume distensibility of the canine thoracic aorta in vivo

Aging impact on thoracic aorta 3D morphometry in intermediate-risk subjects: looking beyond coronary arteries with non-contrast cardiac CT

An increasing number of intermediate risk asymptomatic subjects benefit from measures of atherosclerosis burden like coronary artery calcification studies with non-contrast heart computed tomography (CT). However, additional information can be derived from these studies, looking beyond the coronary arteries and without exposing the patients to further radiation. We report a semi-automatic method that objectively assesses ascending, arch and descending aorta dimension and shape from non-contrast CT datasets to investigate the effect of aging on thoracic aorta geometry. First, the segmentation process identifies the vessel centerline coordinates following a toroidal path for the curvilinear portion and axial planes for descending aorta. Then, reconstructing oblique planes orthogonal to the centerline direction, it iteratively fits circles inside the vessel cross-section. Finally, regional thoracic aorta dimensions (diameter, volume and length) and shape (vessel curvature and tortuosity) are calculated. A population of 200 normotensive men was recruited. Length, mean diameter and volume differed by 1.2 cm, 0.13 cm and 21 cm(3) per decade of life, respectively. Aortic shape uncoiled with aging, reducing its tortuosity and increasing its radius of curvature. The arch was the most affected segment. In conclusion, non-contrast cardiac CT imaging can be successfully employed to assess thoracic aorta 3D morphometry.

Identification and physiological relevance of an exponentially tapered tube model of canine descending aortic circulation

The aim of this study was to evaluate the effect of incorporating aortic tapering in a tube model of descending aortic circulation. We described the descending aorta and its peripheral load by an exponentially tapered transmission tube terminating in a first-order, low-pass filter load. Under the assumption of adaptation between the transmission tube and the terminal load, the input impedance of this model was characterized by five free parameters, the characteristic impedance, Zce(0), at the tube entrance; the product, qde, between the tapering factor q and the tube length, de, the product ce(0)de, between the compliance, ce(0), at the tube entrance and the tube length; the time constant, tau ne, of the load and the peripheral resistance, Rp. We estimated these parameters making use of experimental pressure and flow measurements taken from the high descending aorta of three anaesthetized dogs. We contrasted the behaviour of this model with that of a competing model constituted by a uniform transmission tube also terminating in a first-order low-pass filter load. We compared the data fits and, with the aid of an extra measurement of pressure in the abdominal aorta, we tested the congruence between the estimates of the transmission tubes' parameters and the physical and geometrical properties of descending thoracic aorta. The tapered tube model showed a slightly better ability in fitting to experimental flow and reproducing input impedance data. However, the estimates of the transmission tube parameters failed to assess the physical properties of descending aorta. By contrast, the estimates of tube parameters provided by the uniform model allowed location of the junction between the tube and its terminal load in the abdominal aorta at level of major branches. These estimates were well correlated with the real system's properties. In conclusion, the complexity added to the uniform tube model by accounting for exponential aortic tapering gave rise only to a better curve fitting, but did not show any identifiable benefits regarding physiological interpretation of the physical properties of the descending aorta.

Dimensions and mechanical properties of porcine aortic segments determined by combined impedance planimetry and high-frequency ultrasound

The aim of the study is to devise a method for direct measurements of pressure, cross-sectional area (CSA) and wall thickness of a vessel when pressurised. Segments of porcine descending thoracic aorta from 22 and 35 kg pigs (in groups 1 and 2, respectively) and of abdominal aorta from 35 kg pigs are studied in vitro. Impedance planimetry provides measurements of the luminal pressure and CSA of the aorta. The wall thickness is measured simultaneously by means of 20 MHz A-mode ultrasound. The pressure, CSA and wall thickness are registered at different pressure at steady state. At maximum pressure, the CSAs are 107 +/- 10, 215 +/- 19 and 257 +/- 17 mm2 in the abdominal, group 1 and 2 segments, respectively. There is a difference between the abdominal group and groups 1 and 2 (p < 0.05), but not between group 1 and 2 segments (p > 0.2). At maximum pressure, the wall thickness is 0.52 +/- 0.04, 0.79 +/- 0.07 and 1.07 +/- 0.02 mm, in the abdominal group 1 and 2 segments, respectively (p < 0.05). The circumferential stress-strain relationship accorded well with stress = a x exp(b x strain). The stress-strain curve of the abdominal aortas is shifted to the left, indicating an increase (p < 0.05).

Static elastic wall properties of the abdominal porcine aorta in vitro and in vivo

OBJECTIVES

To characterise possible differences in vascular mechanics between in vitro and in vivo settings.

DESIGN

Experimental animal study.

MATERIAL

Segments of abdominal aorta from five 30kg pigs were studied in vitro and five porcine abdominal aortas were investigated in vivo.

METHODS

Vessel luminal cross-sectional area (CSA) was assessed in vitro and in vivo by measuring the electrical impedance of fluid inside a distensible balloon mounted on a catheter. Wall thickness was measured in vitro by means of 20 MHz A-mode ultrasound. The obtained values in vitro were used to calculate wall thickness in vivo.

RESULTS

The CSAs were larger in vitro than in vivo (p < 0.05). All segments showed stress-strain relations that accorded well with the exponential equation: Stress = a.exp(b. Strain). The y-axis intercepts (a) and the slope of the curves (b) were different in vitro and in vivo (p < 0.05). Differences were found between the aorta in vivo and in vitro regarding the circumferential stress-strain relations, i.e. the stiffest abdominal aorta was found in vivo.

CONCLUSIONS

The differences in dimensions and wall stiffness may be attributed to the tethering to the surrounding tissues in vivo. Impedance planimetry combined with high frequency ultrasound is an accurate way of measuring segmental vascular elasticity in vivo.

Coronary circulation in dogs with an experimental decrease in aortic compliance

OBJECTIVES

This study was designed to investigate the effects of decreased aortic compliance on the coronary circulation.

BACKGROUND

A decrease in aortic compliance due to arteriosclerosis is observed in patients with coronary artery disease. However, the effects of decreased aortic compliance on the coronary circulation have not yet been investigated sufficiently.

METHODS

Hemodynamics, subendocardial electrocardiogram (ECG), myocardial segmental length and myocardial blood flow were investigated in six dogs with aortic bandaging (bandaged group) and five dogs with a sham operation (control group) at rest and during pacing 4 weeks after surgery.

RESULTS

Aortic compliance in the bandaged group was less than that in the control group (0.24 +/- 0.20 vs. 0.50 +/- 0.22 ml/mm Hg, p < 0.05). Pulse pressure and the tension-time index were significantly greater in the bandaged group than in the control group, but systemic vascular resistance was not altered significantly. The subendocardial/subepicardial flow ratio was lower in the bandaged group than in the control group (0.95 +/- 0.31 vs. 1.57 +/- 0.26, p < 0.05). In the region supplied by the left circumflex artery with a stenosis that was adjusted to eliminate reactive hyperemia, rapid atrial pacing (heart rate 200 beats/min) further decreased endocardial flow and the endocardial/epicardial flow ratio in the bandaged group. Moreover, both the reduction of segmental shortening and the ST elevation on the subendocardial ECG in the left circumflex-supplied region during pacing were greater in the bandaged group.

CONCLUSIONS

These results indicate that decreased aortic compliance greatly increases the risk of subendocardial ischemia in the presence of coronary stenosis.

Paradoxically decreased aortic wall stiffness in response to vitamin D3-induced calcinosis. A biphasic analysis of segmental elastic properties in conscious dogs

We studied the aortic elastic behavior in response to vitamin D3-induced accelerated calcinosis in conscious dogs chronically instrumented with a pressure microtransducer and a pair of ultrasonic diameter dimension gauges in the upper descending thoracic aorta. The two functional phases of the elastic segmental properties of the aorta in vivo were discriminated by computation on a beat-by-beat basis from the phasic pressure-diameter (P-D) hysteresis loops in basal conditions and during the transient state of a wide range of pressures obtained mechanically (aortic occlusion) or pharmacologically (angiotensin bolus). The overall P-D curve formed by all P-D hysteresis loops was comprised of two linear relations according to a model that assumes that only elastin is stretched at lower pressures, whereas both elastin and collagen are stretched at higher pressures. The slope of the first linear portion of the P-D curve was considered as the elastin P-D elastic modulus, and the slope of the curve obtained by subtraction between the P-D curve and the extrapolation of the elastin straight line was assumed to be the collagen P-D elastic modulus. After vitamin D3-induced calcinosis, the elastin elastic modulus was unaffected, whereas the collagen elastic modulus decreased significantly during occlusion maneuvers (58.6%, p less than 0.01) and during bolus injections of angiotensin (37.2%, p less than 0.05). The collagen elastic modulus correlated with the serum calcium concentration (r = -0.65, p less than 0.001) and with the aortic pulse pressure (r = 0.51, p less than 0.01), and this relation persisted at constant heart rate. Histopathologic analysis evidenced calcium-depositing elastic lamina, focal disappearance of collagen, and rupture of elastic fibers. The present study shows that accelerated, severe, experimental calcinosis-inducing calcium deposition inside the large artery walls is accompanied by a clear-cut paradoxical reduction in arterial rigidity that is mainly due to functional and structural modification of collagen elasticity.

Haemodynamics of angioaccess venous anastomoses

A large percentage of arteriovenous haemodialysis angioaccess loop grafts (AVLG) fail within the first year after surgery, the occlusive lesions being found predominantly at the venous anastomosis site. This paper presents a detailed flow dynamic study of the AVLG system using three elastic, transparent bench-top flow models, which were based on the geometry of silicone rubber casts obtained at different times from a chronic animal model. Each model thus represented a different stage of the lesion development. Flow visualization and laser Doppler anemometer surveys of the flow field confirmed that the hydrodynamic factors favour lesion development near the stagnation point opposite the anastomotic toe, where the momentum of the impinging jet stream, combined with the oscillating wall shear stress generated in the vicinity of the stagnation point, acts in both directions. The accumulation of tracer particles in the region of flow separation is believed to be a combined contribution from the hydraulic forces and the inward motion of the vessel wall. As these hydrodynamic factors are enhanced upon further development of the occlusive lesion, a vicious cycle may be formed.