Mechanics of porcine coronary arteries ex vivo employing impedance planimetry: a new intravascular technique

Design and Simulation of the Biomechanics of Multi-Layered Composite Poly(Vinyl Alcohol) Coronary Artery Grafts

Coronary artery disease is among the primary causes of death worldwide. While synthetic grafts allow replacement of diseased tissue, mismatched mechanical properties between graft and native tissue remains a major cause of graft failure. Multi-layered grafts could overcome these mechanical incompatibilities by mimicking the structural heterogeneity of the artery wall. However, the layer-specific biomechanics of synthetic grafts under physiological conditions and their impact on endothelial function is often overlooked and/or poorly understood. In this study, the transmural biomechanics of four synthetic graft designs were simulated under physiological pressure, relative to the coronary artery wall, using finite element analysis. Using poly(vinyl alcohol) (PVA)/gelatin cryogel as the representative biomaterial, the following conclusions are drawn: (I) the maximum circumferential stress occurs at the luminal surface of both the grafts and the artery; (II) circumferential stress varies discontinuously across the media and adventitia, and is influenced by the stiffness of the adventitia; (III) unlike native tissue, PVA/gelatin does not exhibit strain stiffening below diastolic pressure; and (IV) for both PVA/gelatin and native tissue, the magnitude of stress and strain distribution is heavily dependent on the constitutive models used to model material hyperelasticity. While these results build on the current literature surrounding PVA-based arterial grafts, the proposed method has exciting potential toward the wider design of multi-layer scaffolds. Such finite element analyses could help guide the future validation of multi-layered grafts for the treatment of coronary artery disease.

Reality based modeling and simulation of gallbladder shape deformation using variational methods

PURPOSE

Accurate soft tissue deformation modeling is important for realistic surgical simulation. The aim of this study is to develop a reality-based gallbladder model and to determine material constants that represent gallbladder wall mechanical properties.

METHODS

Mechanical experiments on porcine gallbladder were performed to investigate tissue deformation, and an exponential strain energy function was used to describe the nonlinear stress-strain behavior of the gallbladder wall. A new volumetric function based upon the exponential strain energy function was proposed to model the gallbladder organ. A genetic algorithm was used to identify the material parameters of the proposed biomechanical model from the experimental data.

RESULTS

The material constants of the exponential strain energy model were determined based on the experimental data. Deformation simulation and haptic rendering using the proposed gallbladder model were presented. Comparison between deformation predicted by the proposed model and that of the experimental data on gallbladder wall and gallbladder organ tissues demonstrates the applicability of this reality-based variational method for deformation simulation.

CONCLUSION

An accurate soft tissue deformation model was developed using material constants identified for gallbladder. The model is suitable for interactive haptic rendering and deformation simulation. This model has potential applications for simulation of other hollow organs.

Mechanical properties of the porcine bile duct wall

Background and Aim

The function of the common bile duct is to transport bile from the liver and the gall bladder to the duodenum. Since the bile duct is a distensible tube consisting mainly of connective tissue, it is important to obtain data on the passive mechanical wall properties. The aims of this study were to study morphometric and biomechanical wall properties during distension of the bile duct.

Methods

Ten normal porcine common bile ducts were examined in vitro. A computer-controlled volume ramp infusion system with concomitant pressure recordings was constructed. A video camera provided simultaneous measurement of outer dimensions of the common bile duct. Wall stresses and strains were computed.

Results

The common bile duct length increased by 25% from 24.4 ± 1.8 mm at zero pressure to 30.5 ± 2.0 mm at 5 kPa (p < 0.01). The diameter increased less than 10% in the same pressure range from 8.6 ± 0.4 mm to 9.3 ± 0.4 mm (p < 0.01). The stress-strain relations showed an exponential behavior with a good fit to the equation: σ = α . (exp^(βε) - 1). The circumferential stress-strain curve was shifted to the left when compared to the longitudinal stress-strain curve, i.e. the linear constants (α values) were different (p < 0.01) whereas the exponential constants (β values) did not differ (p > 0.5).

Conclusion

The porcine bile duct exhibited nonlinear anisotropic mechanical properties.

New Multi-cue Bioreactor for Tissue Engineering of Tubular Cardiovascular Samples under Physiological Conditions

In the present study we have developed a multi-cue bioreactor (MCB) that is capable of delivering a range of stimuli to assist the development of a tissue-engineered construct. The MCB provides an accurate and utilizable computer-controlled pulsatile pump and strain induction mechanism and it has the capability of applying physiological conditions to samples. The device described here emulates the pressure and straining environment found at the aortic root. This function, along with an integral perfusion and sterile containment system, allows for long-term culture and whole-tissue testing capability. Aortic and pulmonary arteries were obtained from freshly isolated porcine hearts and subjected to various loading regimens (Deltapressure/flow/force). Through analyzing data acquired by the MCB transducer array it was possible to differentiate the dynamic mechanical properties of the tissue types tested. In addition, the MCB illustrates a novel concept in cardiovascular tissue engineering: being able to support long-term tissue culture of cell-seeded substrates while they are under the influence of mechanical cues. After 7 days of pulsation in the MCB cell alignment was observed. The MCB represents a versatile model that will enable the development of tissue engineering not only for cardiovascular tissue, but for all tubular tissues such as esophageal, tracheal, and bronchial systems.

Strain measurement in coronary arteries using intravascular ultrasound and deformable images

Atherosclerotic plaque rupture is responsible for the majority of myocardial infarctions and acute coronary syndromes. Rupture is initiated by mechanical failure of the plaque cap, and thus study of the deformation of the plaque in the artery can elucidate the events that lead to myocardial infarction. Intravascular ultrasound (IVUS) provides high resolution in vitro and in vivo cross-sectional images of blood vessels. To extract the deformation field from sequences of IVUS images, a registration process must be performed to correlate material points between image pairs. The objective of this study was to determine the efficacy of an image registration technique termed Warping to determine strains in plaques and coronary arteries from paired IVUS images representing two different states of deformation. The Warping technique uses pointwise differences in pixel intensities between image pairs to generate a distributed body force that acts to deform a finite element model. The strain distribution estimated by image-based Warping showed excellent agreement with a known forward finite element solution, representing the gold standard, from which the displaced image was created. The Warping technique had a low sensitivity to changes in material parameters or material model and had a low dependency on the noise present in the images. The Warping analysis was also able to produce accurate strain distributions when the constitutive model used for the Warping analysis and the forward analysis was different. The results of this study demonstrate that Warping in conjunction with in vivo IVUS imaging will determine the change in the strain distribution resulting from physiological loading and may be useful as a diagnostic tool for predicting the likelihood of plaque rupture through the determination of the relative stiffness of the plaque constituents.

Intraluminal ultrasonic palpation: assessment of local and cross-sectional tissue stiffness

Many intravascular therapeutic techniques for the treatment of significant atherosclerotic lesions are mechanical in nature: examples are angioplasty, stenting and atherectomy. The selection of the most adequate treatment would be advantageously aided by knowledge of the mechanical properties of the lesion and surrounding tissues. Based on the success of intravascular ultrasound (IVUS) in accurately depicting the morphology of atheromatous lesions, ultrasonic tissue characterisation has been proposed as a tool to determine the composition of atheroma. We describe the addition of local compliance information to the IVUS image in the form of a colour-coded line congruent with the lumen perimeter. The technique involves analysis of echo signals obtained at two or more states of incremental intravascular pressure. Using vessel phantoms and specimens, we demonstrate the utility of intravascular compliance imaging. The palpograms are able to identify lesions of different elasticity independently of the echogenicity contrast, because the information provided by the elastograms is generally independent of that obtained from the IVUS image. Thus, the palpogram can complement the characterisation of lesion from the IVUS image. We also describe cross-sectional measures of elasticity that are based on the elastogram. Finally, natural extensions of intravascular palpation to other endoluminal ultrasound applications are proposed.

A new method for combined isometric and isobaric pharmacodynamic studies on porcine coronary arteries

1. The principal aim of the present study was to explore the isometric and isobaric capacity of a new intravascular technique, impedance planimetry, in basic pharmacodynamic investigations on porcine isolated epicardial coronary arteries. 2. The balloon-based catheter technique provides simultaneous measurements of luminal cross-sectional area and pressure. Sources of errors that may influence the accuracy of measurements were evaluated in detail. 3. Under isometric conditions, the stretch ratio-tension diagram showed typical developments of resting and active tensions of the smooth muscle when exposed to alternating maximal K+ depolarization and mechanical stretching. The mean (+/- SEM) maximum active tension was 28.43 +/- 1.72 mN/mm, which was reached at a stretch ratio of 1.26 +/- 0.02, corresponding to a resting tension of 10.50 +/- 0.53 mN/mm (n = 7). The concentration-response relationship to K+ at optimal basal tension was characterized by a mean (+/- SEM) pD2 value of 1.67 +/- 0.01 (n = 7). 4. Under isobaric conditions in the pressure range 40-140 mmHg, the method allowed the investigation of active vascular responses to partial K+ depolarization. The maximal active response to 25 mmol/L K+ was found at the transmural pressure of 60 mmHg (n = 7). To obtain full K+ concentration-response curves, a basal tension corresponding to a transmural pressure of 120 mmHg was required. The mean (+/- SEM) pD2 value for the concentration-response relationship to K+ was 1.53 +/- 0.01 (n = 10). 5. The vascular sensitivities to cumulatively added K+ and various agonists, such as acetylcholine, 5-hydroxytryptamine and noradrenaline, obtained from the same vessel segment at the same initial conditions corresponding to 120 mmHg were significantly higher with the isometric than with the isobaric approach. 6. The results of the present study suggest that impedance planimetry could be a useful tool in pharmacological and physiological investigations of medium-sized arteries, both under isometric and isobaric conditions.

Porcine coronary artery pharmacodynamics in vitro evaluated by a new intravascular technique: relation to axial stretch

A new intravascular balloon catheter-based technique, impedance planimetry and the wire-mounted isometric tension technique commonly employed to study vessel pharmacodynamics in vitro, were compared. Porcine left anterior descendent coronary artery reactivity to nifedipine was assessed and the influence of 20% axial stretch was investigated. There were no histological differences between segments where the impedance planimetry balloon had been inflated and untouched segments. EC50 values differed significantly between the three procedures applied: The isometric method (n = 7): 2.54 +/- 0.44.10(-9) M; nonstretched arteries by the impedance planimetric method (n = 7): 1.99 +/- 0.40.10(-8) M; arteries 20% axially stretched (n = 7): 2.00 +/- 1.36.10(-7)M (isometric and nonstretched: p < 0.05; isometric and stretched: p < 0.001; nonstretched and stretched: p < 0.05). Maximal relaxant responses to nifedipine were 91.8 +/- 2.1% (isometric method), 105.1 +/- 2.3% (nonstretched), and 104.9 +/- 7.7% (stretched) (ANOVA, p = 0.11). In stretched arteries, the initial 12-min response to an increased dose of nifedipine was more rapid than the response of nonstretched arteries at a concentration of 1.10(-7) M (p = 0.038) and had a nonsignificant tendency toward a more rapid response at other concentrations. Resting tone could not be demonstrated and time control experiments showed no change in the maximal vessel response to potassium with any of the three methods. A new method in the evaluation of artery pharmacodynamics in vitro was presented. The study demonstrated that axial stretching of an artery has impact on the pharmacodynamic reactivity to nifedipine in porcine coronary arteries. Further studies are needed to evaluate the impact of the method on endothelial function.