Effects of stent stiffness on local haemodynamics with particular reference to wave reflections

Pulmonary artery capacitance and pulmonary vascular resistance as prognostic indicators in acute pulmonary embolism

Aims

The non-invasive calculation of right ventricular (RV) haemodynamics as pulmonary artery (PA) capacitance (PAC) and pulmonary vascular resistance (PVR) have proved to be feasible, easy to perform, and of high prognostic value. We, therefore, evaluated whether baseline PAC and PVR could predict clinical outcomes for patients with acute pulmonary embolism (PE).

Methods and results

We prospectively followed 373 patients [mean (standard deviation) age, 64.1 (14.9) years; 58.4% were men, and 27.9% had cancer] who had acute PE and transthoracic echocardiography within 1 day of diagnosis from 1 March 2013 through 30 June 2020. Pulmonary artery capacitance was calculated as left ventricular stroke volume/(PA systolic pressure − PA diastolic pressure). Pulmonary vascular resistance was calculated as (tricuspid regurgitant velocity/RV outflow tract velocity time integral) × 10 + 0.16. These two variables were calculated retrospectively from the values obtained with transthoracic echocardiography. Pulmonary artery capacitance was acquired in 99 (27%) patients and PVR in 65 (17%) patients. Univariable and bivariable logistic regression analyses, and receiver operating characteristic curves were used to evaluate the ability of these haemodynamic measurements to predict mortality up to 6 months. After using bivariable models to adjust individually for age, cancer, and pulmonary hypertension. Pulmonary vascular resistance was associated with all-cause mortality at 3 months [area under the curve (AUC) 0.75, 95% confidence interval (CI) 0.61–0.86; P = 0.01], and 6 months (AUC 0.81; 95% CI 0.69–0.91; P≤ 0.03). Pulmonary artery capacitance was associated with all-cause mortality at 30 days (AUC 0.95; 95% CI 0.82–0.99; P < 0.001) and 3 months (AUC 0.84; 95% CI 0.65–0.99; P = 0.003).

Conclusion

Non-invasive measurement of RV haemodynamics could provide prognostic information of patients with acute PE. Pulmonary artery capacitance and PVR are potentially important predictors of all-cause mortality in these patients and should be explored in future studies.

Reflected wave intensity increases based on aortic diameter after endovascular aortic therapy in a goat model

Reflected wave increases after endovascular aortic repair (EVAR) in patients with aortic aneurysm. This affects the left ventricular (LV) diastolic function and leads to a poor prognosis. This study aimed to evaluate the relationship between increased reflected wave amplitude and aortic diameter after EVAR. EVAR was performed in seven healthy goats. We assessed wave intensity (WI), aortic diameter, and stiffness parameter β. Moreover, we evaluated the relationship between negative reflected wave (NW, reflected waves toward the heart from the periphery by WI) and other parameters after EVAR. Results showed an increase in stiffness parameter β (3.5 ± 0.3 vs 15.9 ± 4.7, p = 0.018) and a decrease in the change of aortic diameter (6.9 ± 0.7 vs 2.7 ± 0.4%, p = 0.018) after EVAR. The NW was significantly amplified after EVAR from baseline (−589.8 ± 143.4 to  − 1192.3 ± 303.7 mmHg-m/sec³, p = 0.043). The NW showed a significant correlation with maximum aortic diameter (R = 0.707, p = 0.038) and minimum aortic diameter (R = 0.724, p = 0.033). The reflected wave was enhanced after EVAR and was correlated to the aortic diameter at the stent-graft site. It is important to consider that patients with smaller aortic diameters in landing zone who undergo EVAR may develop LV dysfunction.

Left Main Stenting Induced Flow Disturbances on Ascending Aorta and Aortic Arch

Background and Objective

Ostial LM stenting potentially induces turbulence in the aortic wall near the LM ostium, which might be correlated with aorta dilation and dissection. We investigated through a computational fluid dynamic analysis (CFD), the presence and potential consequences of flow turbulences both in the ascending aorta and arch after a stenting left main (LM) mid shaft or distal disease.

Methods

The model of the ascending aorta and left coronary artery was reconstructed reviewing both angiographic and echocardiographic measurements of 80 consecutive patients (43 males, mean age 75.1 ± 6.2 years) with significant LM mid shaft or distal disease treated in our institution. For stent simulation, a third-generation everolimus-eluting stent was reconstructed. Two stenting procedures (lesion 1:1 or ostial coverage) were investigated.

Results

The net area averaged WSS of the model resulted higher when the stent covered the lesion 1:1 compared to the ostial coverage (3.68 vs. 2.06 Pa, P=0.01 and 3.97 vs. 1.98 Pa, P < 0.001, respectively). LM ostial coverage generates more turbulences in the LM itself, in the aortic wall at ostium level, and at the sino-tubular junction compared with the stenting of the lesion 1:1. Conversely, in the ascending aorta, the WSS appears lower when stenting the lesion 1:1.

Conclusion

Extending the stent coverage up to the ostium, when the ostial region is not diseased, might induce unfavorable alterations of flow; not only both at the level of the LM lesion and ostium sites, but also in the ascending aorta and aortic arch, potentially predisposing the aortic wall to long-term damage.

Wave reflection and transmission in multiply stented blood vessels

Closed circulatory systems display an exquisite balance between vascular elasticity and viscous fluid effects, to induce pulse-smoothing and avoid resonance during the cardiac cycle. Stents in the arterial tree alter this balance through stiffening and because a periodic structure is introduced, capable of interacting with the fluid in a complex way. While the former feature has been investigated, the latter received no attention so far. But periodic structures are the building blocks of metamaterials, known for their ‘non-natural’ behaviour. Thus, the investigation of a stent's periodic microstructure dynamical interactions is crucial to assess possible pathological responses. A one-dimensional fluid–structure interaction model, simple enough to allow an analytical solution for situations of interest involving one or two interacting stents, is introduced. It is determined: (i) whether or not frequency bands exist in which reflected blood pulses are highly increased and (ii) if these bands are close to the characteristic frequencies of arteries and finally, (iii) if the internal structure of the stent can sensibly affect arterial blood dynamics. It is shown that, while the periodic structure of an isolated stent can induce anomalous reflection only in pathological conditions, the presence of two interacting stents is more critical, and high reflection can occur at frequencies not far from the physiological values.

Compliance Study of Endovascular Stent Grafts Incorporated with Polyester and Polyurethane Graft Materials in both Stented and Unstented Zones

Compliance mismatch between stent graft and host artery may induce complications and blood flow disorders. However, few studies have been reported on stent graft compliance. This study aims to explore the deformation and compliance of stent graft in stented and unstented zones under three pressure ranges. Compliance of two stent grafts incorporated with polyurethane graft (nitinol-PU) and polyester graft (nitinol-PET) materials respectively were tested; the stents used in the two stent grafts were identical. For the circumferential deformation of the stent grafts under each pressure range, the nitinol-PET stent graft was uniform in both zones. The nitinol-PU stent graft was circumferentially uniform in the stented zone, however, it was nonuniform in the unstented zone. The compliance of the PU graft material was 15 times higher than that of the PET graft. No significant difference in compliance was observed between stented and unstented zones of the nitinol-PET stent graft regardless of the applied pressure range. However, for the nitinol-PU stent graft, compliance of the unstented PU region was approximately twice that of the stented region; thus, compliance along the length of the nitinol-PU stent graft was not constant and different from that of the nitinol-PET stent graft.

Medical Textiles as Vascular Implants and Their Success to Mimic Natural Arteries

Vascular implants belong to a specialised class of medical textiles. The basic purpose of a vascular implant (graft and stent) is to act as an artificial conduit or substitute for a diseased artery. However, the long-term healing function depends on its ability to mimic the mechanical and biological behaviour of the artery. This requires a thorough understanding of the structure and function of an artery, which can then be translated into a synthetic structure based on the capabilities of the manufacturing method utilised. Common textile manufacturing techniques, such as weaving, knitting, braiding, and electrospinning, are frequently used to design vascular implants for research and commercial purposes for the past decades. However, the ability to match attributes of a vascular substitute to those of a native artery still remains a challenge. The synthetic implants have been found to cause disturbance in biological, biomechanical, and hemodynamic parameters at the implant site, which has been widely attributed to their structural design. In this work, we reviewed the design aspect of textile vascular implants and compared them to the structure of a natural artery as a basis for assessing the level of success as an implant. The outcome of this work is expected to encourage future design strategies for developing improved long lasting vascular implants.

Wave reflection at a stent

A simple analytical expression has been derived to calculate the characteristics of a wave that reflects at a stent implanted in a uniform vessel. The stent is characterized by its length and the wave velocity in the stented region. The reflected wave is proportional to the time derivative of the incident wave. The reflection coefficient is a small quantity of the order of the length of the stent divided by the wavelength of the unstented vessel. The results obtained coincide with those obtained numerically by Charonko et al. The main simplifications used are small amplitude of the waves so that equations can be linearized and that the length of the stent is small enough so that the values of the wave functions are nearly uniform along the stent. Both assumptions hold in typical situations.

Stent graft performance in the treatment of abdominal aortic aneurysms: the influence of compliance and geometry

The long-term success of the endovascular procedure for the treatment of Abdominal Aortic Aneurysms (AAAs ) depends on the secure fixation of the proximal end and the geometry of the stent-graft (SG) device. Variations in SG types can affect proximal fixation and SG hemodynamics. Such hemodynamic variations can have a catastrophic effect on the vascular system and may result from a SG/arterial wall compliance mismatch and the sudden decrease in cross-sectional area at the bifurcation, which may result in decreased distal perfusion, increased pressure wave reflection and increased stress at the interface between the stented and non-stented portion of the vessel. To examine this compliance mismatch, a commercial SG device was tested experimentally under a physiological pressure condition in a silicone AAA model based on computed tomography scans. There was a considerable reduction in compliance of 54% and an increase in the pulse wave velocity of 21%, with a significant amount of the forward pressure wave being reflected. To examine the SG geometrical effects, a commercial bifurcated geometry was compared computationally and experimentally with a geometrical taper in the form of a blended section, which provided a smooth transition from the proximal end to both iliac legs. The sudden contraction of commercial SG at the bifurcation region causes flow separation within the iliac legs, which is known to cause SG occlusion and increased proximal pressure. The blended section along the bifurcation region promotes a greater uniformity of the fluid flow field within the distal legs, especially, during the deceleration phase with reduced boundary layer reversal. In order to reduce the foregoing losses, abrupt changes of cross-section should be avoided. Geometrical tapers could lead to improved clinical outcomes for AAA SGs.

Clinical significance and technical assessment of stent cell geometry in carotid artery stenting

Carotid artery stenting has gained popularity due to its minimally invasive approach. However, several design concerns preclude the successful use of carotid stents. Technical issues, such as open versus closed cells, scaffolding, trackability, foreshortening, and changes in local geometry and hemodynamics, affect stent performance. Previous clinical and experimental studies have evaluated current stent models while proposing and testing novel stent designs. This review focuses on the technical aspects of carotid stent design and the clinical significance of key design parameters identified via computational and experimental modeling.

A Scaling Parameter for Predicting Pressure Wave Reflection in Stented Arteries

A one-dimensional finite-difference model was developed to predict pressure wave reflections in stented arteries, and a parametric study of variations in stent and vessel properties was performed, including stent stiffness, length, and compliance transition region, as well as vessel radius and wall thickness. The model was solved using a combination of weighted essentially nonoscillatory and Runge–Kutta methods. Over 100 cases were tested and the magnitudes of the predicted waves were less than 0.5 mm Hg for all cases, less than 1% of the normal pulse pressure of 40 mm Hg. It was also shown that reasonable variations in these parameters could induce changes in reflection magnitude of up to ±50%. The relationship between each of these properties and the resulting wave reflection could be described in a simple manner, and the effect of all of them together could, in fact, be encompassed by a single nondimensional parameter titled “stent authority.” It is believed that stent authority is a novel way of relating the energy imposed upon the arterial wall by the stent to the fraction of the incident pressure energy that is reflected from the stented region. Based on these results, it is believed that stent design can have a significant effect on pressure wave reflections; however, it was concluded that their small magnitudes make clinical relevance of these waves unlikely, regardless of design.

Evidence suggests rigid aortic grafts increase systolic blood pressure: results of a preliminary study

Abdominal aortic aneurysm (AAA) is a serious complication of the aorta and is treated using vascular bypass grafts. Two main classes of graft are available to treat AAA; grafts implanted by open surgery and stent-grafts implanted using minimally invasive endovascular techniques. Both classes of graft consist of an aortic section which bifurcates into two iliac sections. It has been hypothesized that implantation of aortic grafts and stent-grafts serve to significantly increase abdominal aortic pressures. In this study, an open-loop computer-controlled pumping system was built to produce physiologically realistic pressure and flow-rates. Models of a compliant abdominal aortic aneurysm, a compliant walled graft and a tapered graft were manufactured using an injection moulding technique and fused deposition modelling was used to create a rigid walled graft. A specific transient flow-rate waveform was then applied at the inlet of each model and the resulting pressure waveforms 30 mm upstream from the bifurcation was recorded. Peak pressure measurements were recorded over the course of the pulse for each model. The compliant aneurysm model was found to have a systolic pressure of 107 mmHg while the complaint graft model was 153 mmHg. The rigid graft model had a peak systolic pressure of 199 mmHg. In the tapered graft, the peak pressure dropped to 142 mmHg. The data suggests that implanting a graft model in place of an aneurysm model in an in vitro flow circuit can increase the pressures recorded upstream from the iliac bifurcation and that tapered grafts may alleviate this problem.

Endografting of the Descending Thoracic Aorta Increases Ascending Aortic Input Impedance and Attenuates Pressure Transmission in Dogs

OBJECTIVES

Endografting is being used to manage aneurysms, dissections and acute traumatic disruptions of the thoracic aorta. The acute effects of such interventions on ventricular afterload and on pressure wave transmission characteristics are not well known.

METHODS

In five dogs, a 55 mm endograft was introduced into the descending aorta, just distal to the left subclavian artery, with oversizing of 20%. Following formaldehyde induced complete heart block, the hearts were paced (30-120bpm). The ascending aortic pressures and flows were recorded using Millar micro-tip manometers and ultrasonic flowmeters, respectively. Arterial pressures proximal and distal to the stent site were also recorded. For each heart rate, parameters of a modified Windkessel (SVR: systemic vascular resistance, Z0: characteristic impedance, C: total arterial compliance) were estimated. The pulse wave velocity (PWV) and reflection coefficient (Gamma) were calculated from the pressure wave transfer functions.

RESULTS

The Z0 (0.25+/-0.05 vs 0.41+/-0.06 mmHg/ml s(-1), P<.05) was increased and C was decreased (0.45+/-0.07 vs 0.28+/-0.04 ml/mmHg, P<0.001) following endograft placement. SVR tended to increase (P=.06) and ascending aortic Gamma was unchanged. The PWV increased (418+/-67 vs 755+/-135 cm/s, P<.05) and the distal Gamma decreased (0.09+/-0.10 vs -0.49+/-0.07, P<.05).

CONCLUSIONS

Endografting in the proximal descending aorta cause unfavorable changes in the ascending aortic input impedance and an increase in the PWV through the grafted segment, consistent with an increase in the modulus of elasticity. The grafts produce a negative Gamma at the distal end, an uncommon occurrence in the systemic circulation. Whether this change is of sufficient magnitude to result in post-graft dilation is unknown.

The Prognostic Value of Pulmonary Vascular Capacitance Determined by Doppler Echocardiography in Patients with Pulmonary Arterial Hypertension

OBJECTIVES

We sought to determine if a novel measurement of pulmonary vascular (PV) capacitance (PVCAP) by Doppler echocardiography predicts all-cause mortality in patients with primary pulmonary arterial (PA) hypertension (PPAH).

BACKGROUND

The prognosis of patients with PPAH is variable and has been difficult to predict using clinical or hemodynamic parameters. PVCAP is a measure of the workload on the right ventricle (RV) and we recently have shown that PVCAP determined by cardiac catheterization is a strong predictor of survival. This same hemodynamic information to calculate PVCAP can be derived from Doppler echocardiography. Therefore, the purpose of this study was to determine if PVCAP from noninvasive Doppler echocardiography would be a useful measure of survival in patients with PPAH.

METHODS

We analyzed clinical and hemodynamic variables on all patients with PPAH who had a right heart catheterization and echocardiogram from January to December 1999. Because capacitance is directly proportional to stroke volume and inversely proportional to PA pulse pressure, PVCAP was defined as stroke volume/pulse pressure. PVCAP was derived noninvasively from a comprehensive 2-dimensional and Doppler echocardiogram. Using the peak systolic tricuspid regurgitation velocity and the end-diastolic pulmonary regurgitation velocity, the modified Bernoulli equation was used to calculate the PA systolic and diastolic pressures, respectively. Stroke volume was obtained using the volumetric flow through the left ventricular outflow tract. PVCAP was then analyzed as a predictor of mortality, adjusting for other known modifiers of risk.

RESULTS

In all, 54 patients (13 men) were studied with a mean age of 44 +/- 11 years, ejection fraction of 62 +/- 11%, and RV systolic pressure of 90 +/- 21 mm Hg. In all, 24% were in World Health Organization (WHO) class II, 52% in class III, and 24% in class IV. During follow-up of 1498 +/- 108 days, 12 patients died. The strongest noninvasive predictor of mortality was PVCAP (risk ratio 3.0/mL/mm Hg decrease in PVCAP, 95% confidence interval 1.2-8.0, P = .0212). WHO class, RV index of myocardial performance, RV systolic pressure, and RV ejection time were weaker predictors. PVCAP was also a stronger predictor of mortality than invasively determined PV resistance, right atrial pressure, and mean PA pressure. In multivariate analysis, PVCAP was the only noninvasive predictor of mortality. In quartile analysis the lowest PVCAP quartile had a 4-year mortality of 39% whereas the highest PVCAP had a mortality of 7%.

CONCLUSION

The novel measure of PVCAP, as determined by Doppler echocardiography, is a strong noninvasive predictor of mortality in patients with PPAH and adds prognostic value to conventional risk markers.

Relationship of pulmonary arterial capacitance and mortality in idiopathic pulmonary arterial hypertension

OBJECTIVES

The purpose of this study was to determine if pulmonary vascular capacitance predicts survival in patients with idiopathic pulmonary arterial hypertension (IPAH).

BACKGROUND

The prognosis of patients with IPAH is difficult to predict, despite knowledge of clinical and hemodynamic parameters previously identified as predictors.

METHODS

We proposed a capacitance index of stroke volume divided by pulmonary pulse pressure (SV/PP) and prospectively gathered data on IPAH patients who underwent a right heart catheterization. SV/PP was analyzed as a predictor of mortality after adjusting for other modifiers of risk.

RESULTS

During 4-year follow-up of 104 patients, 21 patients died. When compared with conventional markers, SV/PP was the strongest univariate predictor of mortality (hazard ratio 17.0 per ml.mm Hg(-1) decrease, 95% confidence interval 13.0 to 22.0; p < 0.0001). In successive bivariate analysis, SV/PP was the only predictor of mortality. In quartile analysis, the lowest SV/PP quartile had a 4-year mortality of 61%; the highest SV/PP had no deaths.

CONCLUSIONS

The capacitance index (SV/PP) is a strong independent predictor of mortality in patients with IPAH.

Effects of compliance mismatch on blood flow in an artery with endovascular prosthesis

The objective of this paper is to study the mechanical effects caused by the local stiffening of an artery (due to the vascular prosthesis, for instance). At the junction of the host artery and the more rigid implantant, the abrupt change in compliance creates an abnormal stress concentration that initiates an adaptive response in the vascular tissue. The roles of both fluid and solid mechanical phenomena must be considered in the prosthesis design optimization. In this context, even the simple models could provide helpful tools for designing process. We present here a model of blood flow in compliant vessel. The artery is supposed to be an orthotropical thin elastic shell. We obtain the solution by matched asymptotic expansions. The results prove the high flexure concentration close to the compliance jump. It is shown that the use of orthotropical graft may reduce the peak value of these shear forces to a remarkable extent. Waves reflected from the suture and pressure increase in the prosthesis are discussed. Compliance mismatch is shown to reduce the peak value of maximal wall shear stress.