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

Importance of experimental evaluation of structural parameters for constitutive modelling of aorta

The study compares stresses and strains in the aortic wall derived using different constitutive models for various stress-strain conditions. Structure-based constitutive models with two fibre families with (GOH) and without (HGO) dispersion of collagen fibres are compared. The constitutive models were fitted to data from equibiaxial tension tests of two separated layers of the porcine aortic wall. The initial fit was evaluated with unrestricted parameters and subsequently, the mean angles of the fibre families and the angular dispersion were fixed to the values obtained from histology. Surprisingly, none of the tested models was capable to provide a good quality fit with histologically obtained structural parameters. Fitting the HGO model to experimental data resulted in two fibre families under angles close to ±45°, while the GOH model resulted in a nearly isotropic fibre distribution. These results indicate that both of these models suffer from the absence of isotropic strain stiffening. After having modified both models with corresponding additional members based on the Yeoh model of matrix, we obtained a perfect fit to the measured data while keeping the structural histology-based parameters. Finally, significant differences in compliance and resulting stresses and strains between different models are shown by means of simulations of uniaxial tension test, equibiaxial tension tests and inflation of the aorta.

Stroke Volume estimation using aortic pressure measurements and aortic cross sectional area: Proof of concept

Accurate Stroke Volume (SV) monitoring is essential for patient with cardiovascular dysfunction patients. However, direct SV measurements are not clinically feasible due to the highly invasive nature of measurement devices. Current devices for indirect monitoring of SV are shown to be inaccurate during sudden hemodynamic changes. This paper presents a novel SV estimation using readily available aortic pressure measurements and aortic cross sectional area, using data from a porcine experiment where medical interventions such as fluid replacement, dobutamine infusions, and recruitment maneuvers induced SV changes in a pig with circulatory shock. Measurement of left ventricular volume, proximal aortic pressure, and descending aortic pressure waveforms were made simultaneously during the experiment. From measured data, proximal aortic pressure was separated into reservoir and excess pressures. Beat-to-beat aortic characteristic impedance values were calculated using both aortic pressure measurements and an estimate of the aortic cross sectional area. SV was estimated using the calculated aortic characteristic impedance and excess component of the proximal aorta. The median difference between directly measured SV and estimated SV was -1.4ml with 95% limit of agreement +/- 6.6ml. This method demonstrates that SV can be accurately captured beat-to-beat during sudden changes in hemodynamic state. This novel SV estimation could enable improved cardiac and circulatory treatment in the critical care environment by titrating treatment to the effect on SV.

Characterization of fracture toughness exhaustion in pig aorta

BACKGROUND

Spontaneous rupture of the aorta (SRA) without aneurysm, dissection, inflammation or infection of the aortic wall can be of two types: traumatic and non-traumatic. SRA is most of the time a fatal event. Consequently, it is important to understand the conditions which lead to the aortic rupture, and, in the case of non-traumatic SRA, to predict the temporal likelihood of rupture.

METHOD OF APPROACH

The present work incorporates the temporal aspect by examining the effects of fatigue on aortic wall properties, and adopts an energy approach, based on fracture toughness, to evaluate the aorta's resistance to rupture. Fracture toughness characterization is a destructive testing process and as a consequence cannot be implemented as a clinical tool. However, using concepts in damage mechanics, in theory, it should be possible to indirectly assess fracture toughness from other mechanical properties, such as aortic wall stiffness. Tissue samples from non-aneurysmal porcine aortas were fatigued and were subjected to both biaxial and guillotine tests to assess wall stiffness variations and fracture toughness exhaustion, respectively.

RESULTS

The experiments reveal that aortic wall stiffness variations and fracture toughness exhaustion decreased as a function of loading cycles and can be modeled with exponential functions. After one million loading cycles, the stiffness ratio between the non-fatigued sample and the fatigued sample, dropped to about 0.85, while the fracture toughness ratio counterpart fell to about 0.80.

CONCLUSION

Consequently, the changes in both stiffness and fracture toughness as a function of applied fatigue cycles can be measured in aortic tissues. Moreover, these results suggest the possibility to use fracture toughness exhaustion curves as a fatigue criterion.

Spontaneous and bolus-induced motility in the chronically obstructed guinea-pig small intestine in vitro

Partial obstruction of the small intestine results in dysmotility and morphometric changes proximal to the site of obstruction. However, our understanding of the relation between the morphometric remodeling and change in the motility pattern during chronic obstruction is sparse. The aim of this study was to investigate the effect of partial chronic intestinal obstruction on motility, morphology, and collagen content proximal and distal to the site of obstruction. Twenty guinea-pigs with partial intestinal obstruction and eight sham-operated controls lived for four weeks. Spontaneous and bolus-induced motility was recorded in isolated intestinal segments proximal and distal to the site of obstruction using a perfused low-compliance pressure-measuring system in vitro. After the motility experiments, the specimens were fixed at 2 kPa luminal distension pressure and sampled for histomorphometric determination of luminal radius, layer thickness, and wall thickness. Total wall collagen was also determined. The area under the curve (AUC) of spontaneous contractions and the amplitude, frequency, and AUC for the bolus-induced motility were higher in the proximal segments of the banded animals compared to distal segments and to the intestinal segments in the control animals (P < 0.05). The radius-to-thickness ratio was lowest in the proximal segments of the obstructed animals (P < 0.01). The collagen content was three times higher proximal to the site of obstruction when compared to distal locations and to the controls (P < 0.01). The AUC at 2 ml bolus injections plotted against the radius-to-thickness ratio showed a strong association (r = 0.97 for control, and r = 0.99 for obstruction, P < 0.01). No correlation was found between the collagen content and AUC. In conclusion, partial intestinal obstruction in guinea pigs caused pronounced changes in morphology and motility. An association was found between the radius-to-thickness ratio and bolus-induced motility.

Biomechanical remodeling of the chronically obstructed Guinea pig small intestine

Small intestinal obstruction is a frequently encountered clinical problem. To understand the mechanisms behind obstruction and the clinical consequences, data are needed on the relation between the morphologic and biomechanical remodeling that takes place in the intestinal wall during chronic obstruction. We sought to determine the effect of partial obstruction on mechanical and morphologic properties of the guinea pig small intestine. Partial obstruction was created surgically in 2 groups of animals living for 2 and 4 weeks. Controls were sham operated and lived for 4 weeks. A combined impedance planimetry-high-frequency ultrasound system was designed to measure the luminal cross-sectional area and wall thickness. These measures were used to compute the circumferential stress and strain of the excised intestinal segments. The incremental elastic modulus was obtained by using nonlinear fitting of the stress-strain curve. Histologic analysis and the measurements of total wall collagen were also performed. The luminal cross-sectional area, wall thickness, and elastic modulus in circumferential direction increased in a time-dependent manner proximal to the obstruction site (P < 0.01), whereas no differences in these parameters were found distal to the obstruction site (P > 0.25). The circumferential stress-strain curves of the proximal segments in 2- and 4-week groups shifted to the left, indicating the intestinal wall became stiffer. Histologic examination revealed a massive increase in the thickness of the muscle layer especially the circular smooth muscle layer (P < 0.05). The collagen content proximal to the obstruction site was significantly larger in the partially obstructed animals compared to controls (P < 0.05). No difference was found distal to the obstruction site. Strong correlation was found between the collagen content and the elastic modulus at stress levels of 70 kPa stress (P < 0.01) and 10 kPa (P < 0.05) proximal to the obstruction site suggesting that the alteration of collagen has great impact on the mechanical remodeling. The morphologic and biomechanical remodeling likely influence the function of the intestine affected by partial obstructed intestine.

A new experimental approach in endothelium-dependent pharmacological investigations on isolated porcine coronary arteries mounted for impedance planimetry

1. The aim of this study was to investigate whether the balloon-based impedance planimetry technique could be a useful tool in endothelium-dependent investigations. 2. Porcine large coronary arteries contracted with prostaglandin F2alpha (PGF2alpha, 10 microM) did not relax to bradykinin (0.1 nM - 0.1 microM), but did relax to sodium nitroprusside (SNP, 10 microM). However, after eversion of the segments, bradykinin induced relaxations with pD2 values and maximal responses of 8.78+/-0.09 and 75+/-2% (n=6), respectively. 3. Incubation with captopril (1 microM) did not reveal a relaxation to bradykinin in the normal vessel configuration and had no influence on the concentration-relaxation relationship in everted segments. 4. Lowering the luminal pressure in contracted segments from 131+/-5 mmHg (isometric, n=5) to 60 mmHg (isobaric, n=5) did not facilitate the action of bradykinin. 5. Eversion of segments did not influence the concentration-response relationship for K+ (4.7 - 125 mM), PGF2alpha (0.3 - 30 microM), and SNP (30 nM - 30 microM), although the time-courses of responses were faster when the agents were added from the intimal compared to the adventitial side of the preparation. 6. In the same everted segment contracted with PGF2alpha, the concentration-response relationship for bradykinin was not different under isometric and isobaric conditions. 7. These results indicate that, (1) reduced endothelium-dependent relaxations to adventitially administered substances can be ascribed to a diffusion barrier in the vessel wall, while enzymatic degradation, luminal pressure and precontractile responses seem not to play a role, (2) impedance planimetry applied to everted cylindrical segments could be a useful experimental approach in pharmacological studies of endothelium-dependent responses under isobaric and isometric conditions.

Application of dynamic computed tomography for measurements of local aortic elastic modulus

A novel computed tomographic (CT) technique used for the instantaneous measurement of the dynamic elastic modulus of intact excised porcine aortic vessels subjected to physiological pressure waveforms is described. This system was comprised of a high resolution X-ray image intensifier based computed tomographic system with limiting spatial resolution of 3.2 mm-1 (for a 40 mm field of view) and a computer-controlled flow simulator. Utilising cardiac gating and computer control, a time-resolved sequence of 1 mm thick axial tomographic slices was obtained for porcine aortic specimens during one simulated cardiac cycle. With an image acquisition sampling interval of 16.5 ms, the time sequences of CT slices were able to quantify the expansion and contraction of the aortic wall during each phase of the cardiac cycle. Through superficial tagging of the adventitial surface of the specimens with wire markers, measurement of wall strain in specific circumferential sectors and subsequent calculations of localised dynamic elastic modulus were possible. The precision of circumferential measurements made from the CT images utilising a cluster-growing segmentation technique was approximately +/- 0.25 mm and allowed determination of the dynamic elastic modulus E(dyn) with a precision of +/- 8 kPa. Dynamic elastic modulus was resolved as a function of the harmonics of the physiological pressure waveform and as a function of the angular position around the vessel circumference. Application of this dynamic CT (DCT) technique to seven porcine thoracic aortic specimens produced a circumferential average (over all frequency components) E(dyn) of 373 +/- 29 kPa. This value was not statistically different (p < 0.05) from the values of 430 +/- 77 and 390 +/- 47 kPa obtained by uniaxial tensile testing and volumetric measurements respectively.