Simultaneous Measurements of Arterial Diameter and Blood Pressure to Determine the Arterial Compliance, Wall Mechanics and Stresses In vivo

Heart failure with preserved ejection fraction in pigs causes shifts in posttranscriptional checkpoints

Left ventricular pressure overload (LVPO) can lead to heart failure with a preserved ejection fraction (HFpEF) and LV chamber stiffness (LV Kc) is a hallmark. This project tested the hypothesis that the development of HFpEF due to an LVPO stimulus will alter posttranscriptional regulation, specifically microRNAs (miRs). LVPO was induced in pigs (n = 9) by sequential ascending aortic cuff and age- and weight-matched pigs (n = 6) served as controls. LV function was measured by echocardiography and LV Kc by speckle tracking. LV myocardial miRs were quantified using an 84-miR array. Treadmill testing and natriuretic peptide-A (NPPA) mRNA levels in controls and LVPO were performed (n = 10, n = 9, respectively). LV samples from LVPO and controls (n = 6, respectively) were subjected to RNA sequencing. LV mass and Kc increased by over 40% with LVPO (P < 0.05). A total of 30 miRs shifted with LVPO of which 11 miRs correlated to LV Kc (P < 0.05) that mapped to functional domains relevant to Kc such as fibrosis and calcium handling. LVPO resulted in reduced exercise tolerance (oxygen saturation, respiratory effort) and NPPA mRNA levels increased by fourfold (P < 0.05). RNA analysis identified several genes that mapped to specific miRs that were altered with LVPO. In conclusion, a specific set of miRs are changed in a large animal model consistent with the HFpEF phenotype, were related to LV stiffness properties, and several miRs mapped to molecular pathways that may hold relevance in terms of prognosis and therapeutic targets.NEW & NOTEWORTHY Heart failure with preserved ejection fraction (HFpEF) is an ever-growing cause for the HF burden. HFpEF is particularly difficult to treat as the mechanisms responsible for this specific form of HF are poorly understood. Using a relevant large animal model, this study uncovered a unique molecular signature with the development of HFpEF that regulates specific biological pathways relevant to the progression of this ever-growing cause of HF.

Probing the possibility of lesion formation/progression in vicinity of a primary atherosclerotic plaque: A fluid-solid interaction study and angiographic evidences

It is shown that certain locations in the arterial tree, such as coronary and cerebral arteries, are more prevalent to plaque formation. Endothelial activation and consequent plaque development are attributed to local hemodynamic parameters such as wall shear stress (WSS), oscillatory shear index (OSI), relative residence time (RRT), and stress phase angle. After a certain level of plaque progression, these hemodynamic parameters are disturbed before and after the plaque. In the current study, it is hypothesized that the vicinity of a primary lesion is susceptible for further degeneration and second plaque formation. A fluid-solid interaction (FSI) model of the coronary artery with different levels of asymmetric constriction, is simulated and the trend of hemodynamic parameters were studied in both of the plaque side (PS) and the opposite wall (facing the plaque [PF]). Also, a novel factor is introduced that can identify the high-risk regions associated with WSS oscillations to negative values. Our results indicate that when more than half of the artery is constricted, the downstream of the plaque is highly exposed to endothelial pathogenesis the PS, such that negative WSS, and as well, critical values of OSI and RRT, that is, -1.2 Pa, 0.42 and 6.5 s, respectively arise in this region. PS endothelial cells in this region exposed to the highest risk of atherosclerosis based on the proposed index (3 out of 3). As well, three cases of angiographic images are provided that confirms existence of secondary lesion close to the primary one as predicted by our computational simulations.

Omental Flow-Through Flap: Experimental Hemodynamic Study

BACKGROUND

The omental flow-through flap (OFTF) is based on the use of an anatomic unit composed of the right gastro-omental artery (bypass) with its omental branch or branches supplying the greater omentum (flap). The greater omentum flap is known for its capacity of resistance to infection, for its use in the treatment of ischemic lesions and as a high-flow tissue. Several hypotheses regarding the hemodynamic behavior of a distal bypass with a flap were discussed in the literature. We made the assumption that the OFTF was a low peripheral resistance flap and that the greater omentum did not induce a steal phenomenon. We demonstrated the anatomical feasibility of the experimental model with a morphologic study in the pig. The mail objective of this study was to measure the blood flow to evaluate the hemodynamic effects of the OFTF.

METHODS

Twelve domestic pigs were used for this study. Four cadavers of pigs were dissected for the anatomic study of the OFTF, and 8 live pigs were used for the experimental surgery and hemodynamic measurements. Hemodynamic measurements were taken before transplantation on in situ arteries using periarterial ultrasonic flow transducers. After transplantation of the OFTF, flows were measured before, then during clamping and unclamping of the flap.

RESULTS

OFTF was feasible in the porcine model. With the experimental model, the flow increased by 56.15% in the distal part of the bypass after the implantation of the flap with decrease of peripheral resistances.

CONCLUSIONS

Our results suggest that the OFTF is a low resistance flap and that the greater omentum does not induce a steal phenomenon. This anatomic unit could be used to carry out simultaneously limb revascularization and cover a tissue loss.

Noninvasive in vivo determination of residual strains and stresses

Vascular growth and remodeling during embryonic development are associated with blood flow and pressure induced stress distribution, in which residual strains and stresses play a central role. Residual strains are typically measured by performing in vitro tests on the excised vascular tissue. In this paper, we investigated the possibility of estimating residual strains and stresses using physiological pressure-radius data obtained through in vivo noninvasive measurement techniques, such as optical coherence tomography or ultrasound modalities. This analytical approach first tested with in vitro results using experimental data sets for three different arteries such as rabbit carotid artery, rabbit thoracic artery, and human carotid artery based on Fung's pseudostrain energy function and Delfino's exponential strain energy function (SEF). We also examined residual strains and stresses in the human swine iliac artery using the in vivo experimental ultrasound data sets corresponding to the systolic-to-diastolic region only. This allowed computation of the in vivo residual stress information for loading and unloading states separately. Residual strain parameters as well as the material parameters were successfully computed with high accuracy, where the relative errors are introduced in the range of 0-7.5%. Corresponding residual stress distributions demonstrated global errors all in acceptable ranges. A slight discrepancy was observed in the computed reduced axial force. Results of computations performed based on in vivo experimental data obtained from loading and unloading states of the artery exhibited alterations in material properties and residual strain parameters as well. Emerging noninvasive measurement techniques combined with the present analytical approach can be used to estimate residual strains and stresses in vascular tissues as a precursor for growth estimates. This approach is also validated with a finite element model of a general two-layered artery, where the material remodeling states and residual strain generation are investigated.

Hemodynamic Shear Stress and Endothelial Dysfunction in Hemodialysis Access

Surgically-created blood conduits used for chronic hemodialysis, including native arteriovenous fistulas (AVFs) and synthetic AV grafts (AVGs), are the lifeline for kidney failure patients. Unfortunately, each has its own limitations: AVFs often fail to mature to become useful for dialysis and AVGs often fail due to stenosis as a result of neointimal hyperplasia, which preferentially forms at the graft-venous anastomosis. No clinical therapies are currently available to significantly promote AVF maturation or prevent neointimal hyperplasia in AVGs. Central to devising strategies to solve these problems is a complete mechanistic understanding of the pathophysiological processes. The pathology of arteriovenous access problems is likely multi-factorial. This review focuses on the roles of fluid-wall shear stress (WSS) and endothelial cells (ECs). In arteriovenous access, shunting of arterial blood flow directly into the vein drastically alters the hemodynamics in the vein. These hemodynamic changes are likely major contributors to non-maturation of an AVF vein and/or formation of neointimal hyperplasia at the venous anastomosis of an AVG. ECs separate blood from other vascular wall cells and also influence the phenotype of these other cells. In arteriovenous access, the responses of ECs to aberrant WSS may subsequently lead to AVF non-maturation and/or AVG stenosis. This review provides an overview of the methods for characterizing blood flow and calculating WSS in arteriovenous access and discusses EC responses to arteriovenous hemodynamics. This review also discusses the role of WSS in the pathology of arteriovenous access, as well as confounding factors that modulate the impact of WSS.

A STRUCTURE-MOTIVATED MODEL OF THE PASSIVE MECHANICAL RESPONSE OF THE PRIMARY PORCINE RENAL ARTERY

The primary renal arteries transport up to one fourth of cardiac output to the kidneys for blood plasma ultrafiltration, with a functional dependence on the vessel geometry, composition and mechanical properties. Despite the critical physiological function of the renal artery, the few biomechanical studies that have focused on this vessel are either uniaxial or only partially describe its bi-axial mechanical behavior. In this study, we quantify the passive mechanical response of the primary porcine renal artery through bi-axial mechanical testing that probes the pressure-deformed diameter and pressure-axial force relationships at various longitudinal extensions, including the in-vivo axial stretch ratio. Mechanical data are used to parameterize and validate a structure-motivated constitutive model of the arterial wall. Together, experimental data and theoretical predictions of the stress distribution within the arterial wall provide a comprehensive description of the passive mechanical response of the porcine renal artery.

Atherosclerotic plaques: is endothelial shear stress the only factor?

Initiation and development of atherosclerosis has largely been attributed to irregular shear stress patterns and values, in the current literature. Abnormalities such as low shear stress, reversing and oscillatory shear force patterns, as well as temporal variations of shear stress are the most cited factors. However, clinical findings have further indicated that plaques have still been formed and developed in arterial sites that possess relatively more steady and higher shear stresses than those observed in studies correlating low or oscillatory shear stresses with atherosclerosis. These data imply that deviations in shear stress from its normal physiological pattern alone may not be the only factor inducing atherosclerosis, and additional haemodynamics parameter other then shear stress may also contribute to the initiation and development of plaques. In this paper, we hypothesise that the combined effect of wall shear stress and circumferential stress waves, in the form of angular phase difference between the two waves at each cardiac cycle, may be a more accurate determinant of plaque formation and growth. Furthermore, arterial sites that possess more positive values of this angular phase difference may be more prone to plaque formation or development. If proved correct, this theory can transform our understanding of endothelial cell mechanotransduction and mechanobiology, and may lead to design and utilisation of new diagnostic procedures and equipment as predictive and preventive clinical tools for patients with abnormal arterial blood pressure.

The haemodynamics of the distal arterial Y-shaped autograft bypass-flap in a porcine experimental model

BACKGROUND

The haemodynamic effects of revascularisation with combined bypass and free-muscle flap remain controversial. In a porcine experimental model, we investigated the transplantation-induced changes in the haemodynamics of a Y-shaped combined arterial autograft bypass-muscle flap (AABF).

METHODS

Anatomy of AABF was identified in eight dissections in four porcine cadavers. In five animals, AABF served as a superficial femoral artery (SFA) defect replacement. Modelled, triggered pulsatile pressure (P) and flow (Q) waves delivered mean haemodynamics and PQ hysteresis loops before and after transplantation at days 0 and 10.

RESULTS

Anatomically, AABF combined subscapular and circumflex-scapular arteries, and thoracodorsal artery as latissimus dorsi flap pedicle. Surgical feasibility and AABF patency were confirmed in each case. At day 0, the proximal flow was increased in the grafted Y-shaped AABF, which also adopted the specific SFA pulsatile haemodynamics. Regulatory mechanisms of AABF vasomotricity were preserved and AABF-flow-dependence amplified the flow in the distal segment, which otherwise preserved its own flow dependence. At 10 days, the AABF flow was unchanged in the distal segment, and remained elevated in the proximal and pedicle segments.

CONCLUSIONS

Combined AABF, as a single one-piece arterial autograft, was shown highly adaptive to the receiving arteries. The transplantation-induced changes in AABF pulsatile flow profile and vascular reactivity improve the overall graft flow, and strongly advocate for beneficial effects on the blood propelling capacity of the grafted circulation.

Phasic hemodynamics and reverse blood flows in the aortic isthmus and pulmonary arteries of preterm lambs with pulmonary vascular dysfunction

Time-domain representations of the fetal aortopulmonary circulation were carried out in lamb fetuses to study hemodynamic consequences of congenital diaphragmatic hernia (CDH) and the effects of endothelin-receptor antagonist tezosentan (3 mg/45 min). From the isthmic aortic and left pulmonary artery (PA) flows (Q) and isthmic aortic, PA, and left auricle pressures (P) on day 135 in 10 controls and 7 CDH fetuses (28 ewes), discrete-triggered P and Q waveforms were modelized as Pt and Qt functions to obtain basic hemodynamic profiles, pulsatile waves [P, Q, and entry impedance (Ze)], and P and Q hysteresis loops. In the controls, blood propelling energy was accounted for by biventricular ejection flow waves (kinetic energy) with low Ze and by flow-driven pressure waves (potential energy) with low Ze. Weak fetal pulmonary perfusion was ensured by reflux (reverse flows) from PA branches to the ductus anteriosus and aortic isthmus as reverse flows. Endothelin-receptor antagonist blockade using tezosentan slightly increased the forward flow but largely increased diastolic backward flow with a diminished left auricle pre- and postloading. In CHD fetuses, the static component overrode phasic flows that were detrimental to reverse flows and the direction of the diastolic isthmic flow changed to forward during the diastole period. Decreased cardiac output, flattened pressure waves, and increased forward Ze promoted backward flow to the detriment of forward flow (especially during diastole). Additionally, the intrapulmonary arteriovenous shunting was ineffective. The slowing of cardiac output, the dampening of energetic pressure waves and pulsatility, and the heightening of phasic impedances contributed to the lowering of aortopulmonary blood flows. We speculate that reverse pulmonary flow is a physiological requirement to protect the fetal pulmonary circulation from the prominent right ventricular stream and to enhance blood flow to the fetal heart and brain.

Vibration mode imaging

A new method for imaging the vibration mode of an object is investigated. The radiation force of ultrasound is used to scan the object at a resonant frequency of the object. The vibration of the object is measured by laser and the resulting acoustic emission from the object is measured by a hydrophone. It is shown that the measured signal is proportional to the value of the mode shape at the focal point of the ultrasound beam. Experimental studies are carried out on a mechanical heart valve and arterial phantoms. The mode images on the valve are made by the hydrophone measurement and confirmed by finite-element method simulations. Compared with conventional B-scan imaging on arterial phantoms, the mode imaging can show not only the interface of the artery and the gelatin, but also the vibration modes of the artery. The images taken on the phantom surface suggest that an image of an interior artery can be made by vibration measurements on the surface of the body. However, the image of the artery can be improved if the vibration of the artery is measured directly. Imaging of the structure in the gelatin or tissue can be enhanced by small bubbles and contrast agents.

Compliance matching stent placement in the carotid artery of the swine promotes optimal blood flow and attenuates restenosis

OBJECTIVES

We assessed the value of a gradient-compliant stent in an animal model.

METHODS

Bilateral carotid arteries were stented with nitinol stents having variable-oversizing, variable-stiffness, and with (CMS, 10 animals) and without (SMART, four animals) compliance-matching endings. Angiography, hemodynamic, scanning-electron-microscopic and histological analyses were performed at 3-month. The protocol was completed in 14 among 19 swines.

RESULTS

Transient (1-month) exaggerated recoil, attributable to stress-induced phasic inhibition of vasorelaxation, developed at CMS endings. At mid-term, all stents were endothelialized; CMS-stents, but not SMART-stents, were incorporated into walls (one-strut-thickness). Restenosis developed outside SMART-stents (cell migration+wall-compensatory enlargement) whereas CMS-stents elicited no or focalized cell-accumulations at endings that bulged vascular walls radially outward. SMART-stents were blood-flow neutral, whereas CMS-stents favored (higher-stiffness, higher-oversizing) or opposed (lower-stiffness, less-oversizing) carotid blood flow.

CONCLUSIONS

Direct carotid stenting with stents having compliance-matched endings and specific requirements of stiffness and oversizing can optimize blood flow to the brain and restrict local restenosis.

Pressure-flow loops and instantaneous input impedance in the thoracic aorta: another way to assess the effect of aortic bypass graft implantation on myocardial, brain, and subdiaphragmatic perfusion

BACKGROUND

The serious disturbances in ventriculoarterial coupling after thoracic aorta bypass grafting are addressed through aortic entry impedance in the frequency domain from flow-pressure waves. We designed a method for synthesizing pressure and flow waves to evaluate opposal to aortic flow along the cardiac cycle, addressing myocardial, brain, and visceral tissue perfusions from pressure-flow hysteresis loops and forward-backward aortic entry impedance in the ascending aorta, transverse aortic arch, and distal descending aorta, respectively, before and after extra-anatomic grafting of the descending aorta in the swine.

METHODS

Twelve pigs underwent extra-anatomic grafting (woven double-velour prosthesis, 18-mm diameter), bypassing the descending aorta. Periarterial flow and endovascular pressure signals were mathematically synthesized (error minimization) to yield continuous functions of flow, pressure along the cardiac cycle before treatment for mean hemodynamics, pressure-flow hysteresis loops, and aortic entry impedance.

RESULTS

Grafting of the descending aorta overshadowed pressure-flow hysteresis loops in the ascending aorta by shortening maximum pressure delay on maximum flow and diastolic flow reversal. Clamping of the descending aorta substantially restored hemodynamics in the ascending aorta, although the diastolic flow decrease was accelerated. Identical processes developed in the transverse aorta. Subdiaphragmatic descending aortic flow was flattened after grafting and restored, although thickened, after clamping of the descending aorta. Flow wave peak was framed by a diastolic aortic entry impedance peak, which was damped along the transverse aortic arch (aortic entry impedance peak in the ascending aorta, 1700 +/- 102 kN x s x m(-5); aortic entry impedance peak in the descending aorta, 292 +/- 45 kN x s x m(-5); P <.05). After grafting, the aortic entry impedance peak was transferred to early systole (aortic entry impedance peak in the transverse aortic arch, 2104 +/- 94 kN x s x m(-5); aortic entry impedance peak in the descending aorta, 450 +/- 75 kN x s x m(-5); P <.05). Clamping of the descending aorta attenuated the early systolic aortic entry impedance peak (aortic entry impedance peak in the transverse aortic arch, 1269 +/- 104 kN x s x m(-5); aortic entry impedance peak in the descending aorta, 491 +/- 75 kN x s x m(-5); P <.05), although aortic entry impedance in the descending aorta remained higher than before grafting (P <.05). Specifically, the backward flow ascending aorta to coronary trunks generated a backward aortic entry impedance peak (2234 +/- 350 kN x s x m(-5)) superimposed onto the forward aortic entry impedance peak with asymptotic boundaries that diminished after grafting and further enlarged after clamping of the descending aorta.

CONCLUSIONS

Hemodynamic opposition of grafting of the descending aorta are specific to the aortic site and cardiac cycle and are dependent on clamping of the descending aorta. Our approach to thoracic aorta hemodynamics could enable optimization of bypass grafting.

Local delivery of NO-donor molsidomine post-PTA improves haemodynamics, wall mechanics and histomorphometry in atherosclerotic porcine SFA

OBJECTIVES

we investigated the therapeutic effect of angioplasty with local drug delivery (LDD) of the wall-accumulating NO-donor molsidomine (M) in the superficial femoral arteries (SFA) of atherosclerotic swine.

MATERIALS AND METHODS

atherosclerotic Pietrin swines (n=14) underwent PTA-LDD-M (4 mg/2 ml) vs contralateral PTA-LDD-Placebo in the SFA using a channelled balloon angioplasty catheter. Invasive and colour Doppler energy (CDE) assessments of haemodynamics and wall mechanics were performed at 24 h (n=4) and 5 months (n=10). Immuno-histolabelling of cell proliferation and histomorphometry were serially performed in perfusion fixed SFA samples.

RESULTS

at 24 h, PCNA-positive nuclei revealed 33+/-14 and 12+/-3 proliferating cells/mm2 at placebo and molsidomine PTA-LDD sites, respectively (p<0.001). At 5 months, PTA-LDD-M vessels, compared with PTA-LDD-P, had increased compliance (66+/-9 vs 11+/-4 ml/mmHg) and lowered impedance (0.11+/-0.05 vs 0.45+/-0.14 mmHg/ml x min(-1)) (p<0.05). CDE revealed low, middle and high velocity peaks at 7.5, 20 and 35, and 8, 15 and 22 cm x s(-1) in systolic and diastolic flows, respectively; and PTA-LDD-M prevented emergence of restenosis-associated increases in low blood velocities (p<0.01). PTA-LDD-M inhibited restenotic intimal thickening and medial thinning which decreased mean lumenal diameter in placebo-treated (2.6+/-0.3) as compared to molsidomine-treated (3.4+/-0.3 mm) vessels (p<0.05).

CONCLUSIONS

in the atherosclerotic porcine SFA model, PTA-LDD with molsidomine consistently improved haemodynamic wall mechanics, lowered cell proliferation and prevented late lumen loss observed with PTA-LDD with placebo.