Ashish Das

Influence of arterial wall-stenosis compliance on the coronary diagnostic parameters

Functional diagnostic parameters such as Fractional Flow Reserve (FFR), which is calculated from pressure measurements across stenosed arteries, are often used to determine the functional severity of coronary artery stenosis. This study evaluated the effect of arterial wall-stenosis compliance, with limiting scenarios of stenosis severity, on the diagnostic parameters. The diagnostic parameters considered in this study include an established index, FFR and two recently developed parameters: Pressure Drop Coefficient (CDP) and Lesion Flow Coefficient (LFC). The parameters were assessed for rigid artery (RR; signifying high plaque elasticity), compliant artery with calcified plaque (CC; intermediate plaque elasticity) and compliant artery with smooth muscle cell proliferation (CS; low plaque elasticity), with varying degrees of epicardial stenosis. A hyperelastic Mooney-Rivlin model was used to model the arterial wall and plaque materials. Blood was modeled as a shear thinning, non-Newtonian fluid using the Carreau model. The arterial wall compliance was evaluated using the finite element method. The present study found that, with an increase in stenosis severity, FFR decreased whereas CDP and LFC increased. The cutoff value of 0.75 for FFR was observed at 78.7% area stenosis for RR, whereas for CC and CS the cutoff values were obtained at higher stenosis severities of 81.3% and 82.7%, respectively. For a fixed stenosis, CDP value decreased and LFC value increased with a decrease in plaque elasticity (RR to CS). We conclude that the differences in diagnostic parameters with compliance at intermediate stenosis (78.7-82.7% area blockage) could lead to misinterpretation of the stenosis severity.

Pulsatile arterial wall-blood flow interaction with wall pre-stress computed using an inverse algorithm

BackgroundThe computation of arterial wall deformation and stresses under physiologic conditions requires a coupled compliant arterial wall-blood flow interaction model. The in-vivo arterial wall motion is constrained by tethering from the surrounding tissues. This tethering, together with the average in-vivo pressure, results in wall pre-stress. For an accurate simulation of the physiologic conditions, it is important to incorporate the wall pre-stress in the computational model. The computation of wall pre-stress is complex, as the un-loaded and un-tethered arterial shape with residual stress is unknown. In this study, the arterial wall deformation and stresses in a canine femoral artery under pulsatile pressure was computed after incorporating the wall pre-stresses. A nonlinear least square optimization based inverse algorithm was developed to compute the in-vivo wall pre-stress.MethodsFirst, the proposed inverse algorithm was used to obtain the un-loaded and un-tethered arterial geometry from the unstressed in-vivo geometry. Then, the un-loaded, and un-tethered arterial geometry was pre-stressed by applying a mean in-vivo pressure of 104.5 mmHg and an axial stretch of 48% from the un-tethered length. Finally, the physiologic pressure pulse was applied at the inlet and the outlet of the pre-stressed configuration to calculate the in-vivo deformation and stresses. The wall material properties were modeled with an incompressible, Mooney-Rivlin model derived from previously published experimental stress-strain data (Attinger et al., 1968).ResultsThe un-loaded and un-tethered artery geometry computed by the inverse algorithm had a length, inner diameter and thickness of 35.14 mm, 3.10 mm and 0.435 mm, respectively. The pre-stressed arterial wall geometry was obtained by applying the in-vivo axial-stretch and average in-vivo pressure to the un-loaded and un-tethered geometry. The length of the pre-stressed artery, 51.99 mm, was within 0.01 mm (0.019%) of the in-vivo length of 52.0 mm; the inner diameter of 3.603 mm was within 0.003 mm (0.08%) of the corresponding in-vivo diameter of 3.6 mm, and the thickness of 0.269 mm was within 0.0015 mm (0.55%) of the in-vivo thickness of 0.27 mm. Under physiologic pulsatile pressure applied to the pre-stressed artery, the time averaged longitudinal stress was found to be 42.5% higher than the circumferential stresses. The results of this study are similar to the results reported by Zhang et al., (2005) for the left anterior descending coronary artery.ConclusionsAn inverse method was adopted to compute physiologic pre-stress in the arterial wall before conducting pulsatile hemodynamic calculations. The wall stresses were higher in magnitude in the longitudinal direction, under physiologic pressure after incorporating the effect of in-vivo axial stretch and pressure loading.

Comparison of stroke work between repaired tetralogy of Fallot and normal right ventricular physiologies

Adult patients who underwent tetralogy of Fallot repair surgery (rTOF) confront life-threatening ailments due to right ventricular (RV) myocardial dysfunction. Pulmonary valve replacement (PVR) needs to be performed to restore the deteriorating RV function. Determination of correct timing to perform PVR in an rTOF patient remains subjective, due to the unavailability of quantifiable clinical diagnostic parameters. The objective of this study is to evaluate the possibility of using RV body surface area (BSA)-indexed stroke work (SWI) to quantify RV inefficiency in TOF patients. We hypothesized that RV SWI required to push blood to the lungs in rTOF patients is significantly higher than that of normal subjects. Seven patients with rTOF pathophysiology and eight controls with normal RV physiology were registered for this study. Right ventricular volume and pressure were measured using cardiac magnetic resonance imaging and catheterization, respectively. Statistical analysis was performed to quantify the difference in SWI between the RV of the rTOF and control groups. Right ventricular SWI in rTOF patients (0.176 ± 0.055 J/m2) was significantly higher by 93.4% (P = 0.0026) than that of controls (0.091 ± 0.030 J/m2). Further, rTOF patients were found to have significantly higher (P < 0.05) BSA normalized RV end-systolic volume, end-systolic pressure, and regurgitation fraction than control subjects. Ejection fraction and peak ejection rate of rTOF patients were significantly lower (P < 0.05) than those of controls. Patients with rTOF pathophysiology had significantly higher RV SWI compared with subjects with normal RV physiology. Therefore, RV SWI may be useful to quantify RV inefficiency in rTOF patients along with currently used clinical end points such as RV volume, pressure, regurgitation fraction, and ejection fraction.

Methodology for implementing patient-specific spatial boundary condition during a cardiac cycle from phase-contrast MRI for hemodynamic assessment

Pulmonary insufficiency (PI) can render the right ventricle dysfunctional due to volume overloading and hypertrophy. The treatment requires a pulmonary valve replacement surgery. However, determining the right time for the valve replacement surgery has been difficult with currently employed clinical techniques such as, echocardiography and cardiac MRI. Therefore, there is a clinical need to improve the diagnosis of PI by using patient-specific (PS) hemodynamic endpoints. While there are many reported studies on the use of PS geometry with time varying boundary conditions (BC) for hemodynamic computation, few use spatially varying PS velocity measurement at each time point of the cardiac cycle. In other words, the gap is that, there are limited number of studies which implement both spatially- and time-varying physiologic BC directly with patient specific geometry. The uniqueness of this research is in the incorporation of spatially varying PS velocity data obtained from phase-contrast MRI (PC-MRI) at each time point of the cardiac cycle with PS geometry obtained from angiographic MRI. This methodology was applied to model the complex developing flow in human pulmonary artery (PA) distal to pulmonary valve, in a normal and a subject with PI. To validate the methodology, the flow rates from the proposed method were compared with those obtained using QFlow software, which is a standard of care clinical technique. For the normal subject, the computed time average flow rates from this study differed from those obtained using the standard of care technique (QFlow) by 0.8 ml/s (0.9%) at the main PA, by 2 ml/s (3.4%) at the left PA and by 1.4 ml/s (3.8%) at the right PA. For the subject with PI, the difference was 7 ml/s (12.4%) at the main PA, 5.5 ml/s (22.6%) at the left PA and 4.9 ml/s (18.0%) at the right PA. The higher percentage differences for the subject with PI, was the result of overall lower values of the forward mean flow rate caused by excessive flow regurgitation. This methodology is expected to provide improved computational results when PS geometry from angiographic MRI is used in conjunction with PS PC-MRI data for solving the flow field.

Energy Transfer Ratio as a Metric of Right Ventricular Efficiency in Repaired Congenital Heart Disease

OBJECTIVE With the success of early repair, continued functional assessment of repaired congenital heart disease is critical for improved long-term outcome. Pulmonary regurgitation, which is one of the main postoperative sequelae of congenital heart disease involved with the right ventricle (RV) such as tetralogy of Fallot and transposition of the great arteries, results in progressive RV dilatation coupled with pulmonary artery (PA) obstruction causing elevated RV pressures. The appropriate timing of intervention to correct these postoperative lesions remains largely subjective. In the present study, we evaluated an energy-based end point, namely energy transfer ratio (eMPA ), to assess the degree of RV and PA inefficiency in a group of congenital heart disease patients with abnormal RV-PA physiology. METHODS Eight patients with abnormal RV-PA physiology and six controls with normal RV-PA physiology were investigated using a previously validated technique that couples cardiac magnetic resonance imaging and invasive pressure measurements. RESULTS The mean eMPA of the patient group (0.56 ± 0.33) was significantly lower (P <.04) than that of the control group (1.56 ± 0.85), despite the fact that the patient group had a significantly higher RV stroke work indexed to body surface area (RV SWI ) than the control group (0.205 ± 0.095 J/m(2) vs. 0.090 ± 0.038 J/m(2) ; P <.02). CONCLUSION We determined that the patients had inefficient RV-PA physiology due to a combination of RV dilatation with pulmonary regurgitation and RV outflow obstruction leading to an elevated end-systolic pressure. Using coupled magnetic resonance imaging and invasive pressure measurements, eMPA is determined to be a sensitive energy-based end point for measuring RV-PA efficiency. It may serve as a diagnostic end point to optimize timing of intervention.

Evaluation of Hemodynamics in a Prestressed and Compliant Tapered Femoral Artery Using an Optimization-Based Inverse Algorithm.

The important factors that affect the arterial wall compliance are the tissue properties of the arterial wall, the in vivo pulsatile pressure, and the prestressed condition of the artery. It is necessary to obtain the load-free geometry for determining the physiological level of prestress in the arterial wall. The previously developed optimization-based inverse algorithm was improved to obtain the load-free geometry and the wall prestress of an idealized tapered femoral artery of a dog under varying arterial wall properties. The compliance of the artery was also evaluated over a range of systemic pressures (72.5-140.7 mmHg), associated blood flows, and artery wall properties using the prestressed arterial geometry. The results showed that the computed load-free outer diameter at the inlet of the tapered artery was 6.7%, 9.0%, and 12% smaller than the corresponding in vivo diameter for the 25% softer, baseline, and 25% stiffer arterial wall properties, respectively. In contrast, the variations in the prestressed geometry and circumferential wall prestress were less than 2% for variable arterial wall properties. The computed compliance at the inlet of the prestressed artery for the baseline arterial wall property was 0.34%, 0.19%, and 0.13% diameter change/mmHg for time-averaged pressures of 72.5, 104.1, and 140.7 mmHg, respectively. However, the variation in compliance due to the change in arterial wall property was less than 6%. The load-free and prestressed geometries of the idealized tapered femoral artery were accurately (error within 1.2% of the in vivo geometry) computed under variable arterial wall properties using the modified inverse algorithm. Based on the blood-arterial wall interaction results, the arterial wall compliance was influenced significantly by the change in average pressure. In contrast, the change in arterial wall property did not influence the arterial wall compliance.

Assessment of Right Ventricular Inefficiency Using Energy Transfer Ratio in Repaired Tetralogy of Fallot

Pulmonary insufficiency (PI) induces pulmonary regurgitation and often leads to right ventricular (RV) enlargement and RV pressure overloading in repaired Tetralogy of Fallot (rTOF) patients. The appropriate timing of surgical treatments to renormalize RV function remains uncertain due to lack of suitable clinical diagnostic parameters. An energy transfer ratio (eMPA) between the net energy (Enet) transferred at main pulmonary artery (MPA) from RV and stroke work (SW) by RV was calculated using RV volume and pressure data for subjects in two study groups: the rTOF patient group (n = 7) and the control group (n = 7). Statistical analysis was performed to determine the difference of eMPA between the two groups. The mean eMPA for rTOF patients (0.64) was significantly lower (60.2%, p<0.05) than that of controls (1.61).Copyright

Development of a Methodology for Direct Utilization of Phase-Contrast MRI in Hemodynamic Computations

Development of non-invasive diagnostic indices often requires accurate blood-flow calculation using physiologically realistic velocity profiles as boundary conditions. In this research, a methodology is being developed and validated that can directly use phase-contrast MR imaging (PC-MRI) based velocity measurement to perform blood-flow computation with patient-specific geometry. Using this methodology, the pressure drop can also be calculated non-invasively. Although the main focus of our research has been pulmonary insufficiency (PI) in tetralogy patients, our method can be employed in many other pathophysiologies. As a pilot study, the methodology is tested using a simple model of blood-flow through a straight artery of uniform cross-section.Copyright

Assessment of Energy Loss due to Pulmonary Valve Insufficiency in Tetralogy of Fallot Physiology Using Patient Specific Geometry

Right ventricular (RV) enlargement and pulmonary valve insufficiency (PI) are well-known, unavoidable long term sequelae encountered by patients who undergo tetralogy of Fallot (TOF) surgery. Energy losses in the pulmonary artery (PA) of TOF and normal subjects was calculated using patient specific geometry and pressure and flow data to develop quantifiable clinical diagnostic parameters. The TOF subject shows 35% higher energy losses than our normal subject.Copyright

Misinterpretation of the Functional Severity of Coronary Stenosis Due To Variability in Arterial Wall Compliance

Effect of arterial wall compliance on the invasive coronary diagnostic parameters for various severities of coronary stenoses was assessed. The Mooney-Rivlin model was used to define the non-linear properties of the arterial wall and the plaque regions. The nonNewtonian viscosity of blood was modeled using the Carreau model. A finite element method was employed to solve the pulsatile fluid (blood)-structure (arterial wall) interaction (FSI) equations. Variability in the diagnostic parameter values can occur near the cut-off value due to change in compliance of stenotic arteries between the range of 84% and 89% area stenosis. This may lead to misdiagnosis and might wrongly lead to postponement of coronary intervention.