Weighted Least-Squares Finite Element Method for Cardiac Blood Flow Simulation with Echocardiographic Data

As both fluid flow measurement techniques and computer simulation methods continue to improve, there is a growing need for numerical simulation approaches that can assimilate experimental data into the simulation in a flexible and mathematically consistent manner. The problem of interest here is the simulation of blood flow in the left ventricle with the assimilation of experimental data provided by ultrasound imaging of microbubbles in the blood. The weighted least-squares finite element method is used because it allows data to be assimilated in a very flexible manner so that accurate measurements are more closely matched with the numerical solution than less accurate data. This approach is applied to two different test problems: a flexible flap that is displaced by a jet of fluid and blood flow in the porcine left ventricle. By adjusting how closely the simulation matches the experimental data, one can observe potential inaccuracies in the model because the simulation without experimental data differs significantly from the simulation with the data. Additionally, the assimilation of experimental data can help the simulation capture certain small effects that are present in the experiment, but not modeled directly in the simulation

Intracardiac measurement of pre-ejection myocardial velocities estimates the transmural extent of viable myocardium early after reperfusion in acute myocardial infarction

AbstractOBJECTIVESWe hypothesized that wall motion velocity during pre-ejection is proportional to the regional content of viable myocardium after reperfusion for acute myocardial infarction (AMI).BACKGROUNDPre-ejection wall motion consists of short and fast inward and outward movement towards and away from the center of the left ventricle (LV) and is altered during regional ischemia. This short-lived event can be accurately quantified by Doppler myocardial imaging (DMI).METHODSFourteen open-chest pigs underwent 60 to 120 min of left anterior descending coronary artery occlusion followed by 30 min of reperfusion. The DMI data were collected using a phased-array intracardiac catheter (LV cavity) from ischemic and nonischemic myocardium encompassed within a plane passing through two epicardial bead markers. Peak tissue velocities during isovolumic contraction (IVC) (peak positive and peak negative), ejection (S) and early filling (E) were measured. The cardiac specimen was sliced through the epicardial markers in a plane approximating the ultrasound imaging plane. The transmural extent of necrosis (TEN) (%) was measured by triphenyltetrazolium chloride staining.RESULTSDuring ischemia, positive IVC velocity was zero in ischemic walls with TEN >20%. At reperfusion, positive IVC velocity correlated better with TEN (r = −0.94, p < 0.0001) than it did S (r = −0.70, p < 0.01) and E (r = −0.81, p < 0.01). Differential IVC (the difference between peak positive and peak negative velocity) highly correlated with TEN, during ischemia (r = −0.78, p < 0.001) and during reperfusion (r = −0.93, p < 0.0001).CONCLUSIONSPre-ejection tissue velocity, as measured by intracardiac ultrasound, allows rapid estimation of the transmural extent of viable myocardium after reperfusion for AMI

Higher myocardial strain rates duringisovolumic relaxation phase than duringejection characterize acutely ischemic myocardium

AbstractObjectivesThe aim of this study was to define an index that can differentiate normal from ischemic myocardial segments that exhibit postsystolic shortening (PSS).BackgroundIdentification of ischemia based on the reduction of regional systolic function is sometimes challenging because other factors such as normal nonuniformity in contraction between segments, tethering effect, pharmacologic agents, or alterations in loading conditions can also cause reduction in regional systolic deformation. The PSS (contraction after the end of systole) is a sensitive marker of ischemia; however, inconsistent patterns have also been observed in presumed normal myocardium.MethodsTwenty-eight open-chest pigs underwent echocardiographic study before and during acute myocardial ischemia induced by coronary artery occlusion. Ultrasound-derived myocardial longitudinal strain rates were calculated during systole (SSR), isovolumic relaxation (IVRSR), and rapid filling (ESR) phases in both ischemic and normal myocardium. Systolic strain (ϵsys) and postsystolic strain (ϵps) were calculated by integrating systolic and postsystolic strain rates, respectively.ResultsDuring ischemia, SSR, ESR, and ϵsys in ischemic segments were significantly lower (in magnitude) than in nonischemic segments or at baseline. However, some overlap occurred between ischemic and normal values for all three parameters. At baseline, 18 of 28 animals had negative IVRSR (i.e., PSS) in at least one segment. During coronary artery occlusion, IVRSR became negative and larger in magnitude than SSR in all ischemic segments. The IVRSR/SSR and ϵps best differentiated ischemic from nonischemic segments.ConclusionsIn the presence of reduced regional systolic deformation, a higher rate of PSS than systolic shortening identifies acutely ischemic myocardium

Strain echocardiography tracks dobutamine-induced decrease in regional myocardial perfusion in nonocclusive coronary stenosis

ObjectivesThis study was designed to determine whether strain echocardiography parameters reflect changes in regional myocardial perfusion during dobutamine stress.BackgroundStrain echocardiography depicts regional myocardial mechanical activity. Ischemia has been shown to reduce systolic strain rate (sSR) and prolong the time to regional lengthening (TRL). In an experimental model, we tested whether sSR and TRLtracked dobutamine-induced changes in regional myocardial perfusion (regional myocardial blood flow [RMBF]), as measured by colored microspheres.MethodsWe used a closed-chest pig model of nonocclusive coronary stenosis (n = 14) created by inflating an angioplasty balloon in the proximal left anterior descending artery. Invasive hemodynamics, RMBF, and strain parameters were measured at baseline and peak dobutamine stimulation before and during the coronary stenosis. We compared segments with reduced RMBF versus those with preserved RMBF at peak dobutamine stimulation.ResultsPeak sSR correlated with RMBF (r = 0.70). In the absence of coronary stenosis, dobutamine stimulation caused a significant increase in RMBF and sSR and a decrease in TRL. This response was blunted during coronary stenosis. Using the “best cutoff” method, the sensitivity and specificity for prediction of reduced RMBF (ischemia) was 81% and 91% for sSR and 65% and 91% for TRL, respectively. These changes occurred in the absence of any change in global systolic and diastolic function (dP/dTmax, dP/dTmin, and tau).ConclusionsNovel strain parameters that depict regional myocardial mechanics are able to predict changes in RMBF during dobutamine stress. Quantitative strain parameters may complement current echocardiographic techniques for ischemia detection and potentially improve the accuracy and reproducibility of stress echocardiography

Left Ventricular Structure and Function Basic Science for Cardiac Imaging

The myofiber geometry of the left ventricle (LV) changes gradually from a right-handed helix in the subendocardium to a left-handed helix in the subepicardium. In this review, we associate the LV myofiber architecture with emerging concepts of the electromechanical sequence in a beating heart. We discuss: 1) the morphogenesis and anatomical arrangement of muscle fibers in the adult LV; 2) the sequence of depolarization and repolarization; 3) the physiological inhomogeneity of transmural myocardial mechanics and the apex-to-base sequence of longitudinal and circumferential deformation; 4) the sequence of LV rotation; and 5) the link between LV deformation and the intracavitary flow direction observed during each phase of the cardiac cycle. Integrating the LV structure with electrical activation and motion sequences observed in vivo provides an understanding about the spatiotemporal sequence of regional myocardial performance that is essential for noninvasive cardiac imaging

Ultrasound-Guided Placement of a Renal Artery Stent Using an Intracardiac Probe for Transvascular Imaging

In this set of images obtained during an experimental study using a porcine animal model, we introduce ultrasound guidance of percutaneous transluminal renal angioplasty and renal stenting. A state-of-the-art intracardiac ultrasound catheter is used here for transvascular scanning from within the lumen of the abdominal aorta, thus providing a field of view for navigation of a balloon catheter and a wire coil (“stent”) into each renal artery of a pig. This study is intended as a contribution to the growing field of minimally invasive interventions and their navigation by non-ionizing ultrasound imaging

Impact of pericardial adhesions on diastolic function as assessed by vortex formation time, a parameter of transmitral flow efficiency

<p>Abstract</p> <p>Background</p> <p>Pericardial adhesions are a pathophysiological marker of constrictive pericarditis (CP), which impairs cardiac filling by limiting the total cardiac volume compliance and diastolic filling function. We studied diastolic transmitral flow efficiency as a new parameter of filling function in a pericardial adhesion animal model. We hypothesized that vortex formation time (VFT), an index of optimal efficient diastolic transmitral flow, is altered by patchy pericardial-epicardial adhesions.</p> <p>Methods</p> <p>In 8 open-chest pigs, the heart was exposed while preserving the pericardium. We experimentally simulated early pericardial constriction and patchy adhesions by instilling instant glue into the pericardial space and using pericardial-epicardial stitches. We studied left ventricular (LV) function and characterized intraventricular blood flow with conventional and Doppler echocardiography at baseline and following the experimental intervention.</p> <p>Results</p> <p>Significant decreases in end-diastolic volume, ejection fraction, stroke volume, and late diastolic filling velocity reflected the effects of the pericardial adhesions. The mean VFT value decreased from 3.61 ± 0.47 to 2.26 ± 0.45 (P = 0.0002). Hemodynamic variables indicated the inhibiting effect of pericardial adhesion on both contraction (decrease in systolic blood pressure and +dP/dt decreased) and relaxation (decrease in the magnitude of -dP/dt and prolongation of Tau) function.</p> <p>Conclusion</p> <p>Patchy pericardial adhesions not only negatively impact LV mechanical functioning but the decrease of VFT from normal to suboptimal value suggests impairment of transmitral flow efficiency.</p