Peter G. Walker

Left ventricular blood flow patterns in normal subjects: A quantitative analysis by three-dimensional magnetic resonance velocity mapping

OBJECTIVES Magnetic resonance velocity mapping was used to investigate the hypothesis of a vortex motion within the left ventricle interacting with mitral valve motion and inflow velocity. BACKGROUND In vitro flow visualization studies have suggested the presence of a large anterior vortex inside the left ventricle during mitral inflow. However, to our knowledge the occurrence of this phenomenon has not been demonstrated in the human left ventricle. METHODS Magnetic resonance velocity mapping was performed in 26 healthy volunteers using a flow-adjusted gradient sequence for three-dimensional flow velocity acquisition in the long-axis plane of the left ventricle. By computer processing, the flow vectors in the left ventricle were visualized and animated dynamically. RESULTS The early diastolic mitral inflow was apically directed, and a large counterclockwise anterior vortex was created within the left ventricle shortly after the onset of the mid-diastolic semiclosure of the anterior mitral leaflet. During mid-diastolic diastasis, mitral inflow ceased until the flow accelerated again at atrial systole. The final closure of the mitral valve was preceded by a smaller vortex seen at the tips of the mitral leaflets. At systolic ejection, all flow vectors were directed toward the left ventricular outflow tract. The anterior vortex had a radius of 1.62 +/- 0.24 cm (mean +/- SD), and the average angular velocity (i.e., the rotation of an element about the center of the vortex within the central core) was 30.08 +/- 9.98 radians/s. The maximal kinetic energy of the anterior vortex was 4.3 x 10(-4) +/- 7.1 x 10(-5) J. CONCLUSIONS The hypothesis of a diastolic vortex formation in the human left ventricle was confirmed, and its close temporal relation to the motion of the anterior mitral leaflet was demonstrated.

In vitro flow experiments for determination of optimal geometry of total cavopulmonary connection for surgical repair of children with functional single ventricle

OBJECTIVES This study sought to evaluate the effect of offsetting cavopulmonary connections at varying pulmonary flow ratios to determine the optimal geometry of the connection. BACKGROUND Previous investigators have demonstrated energy conservation within the streamlined contours of the total cavopulmonary connection compared with that of the atriopulmonary connection. However, their surgical design of connecting the two cavae directly opposite each other may result in high energy losses. Others have introduced a unidirectional connection with some advantages but with concerns about the formation of arteriovenous malformation in the lung excluded from hepatic venous return. Thus, an optimal surgical design has not been determined. METHODS In the present models, the caval connections were offset through a range of 0.0 to 2.0 diameters by 0.5 superior cava diameter increments. Flow ratios were fixed for superior and inferior cavae and varied for right and left pulmonary arteries as 70:30, 60:40, 50:50, 40:60 and 30:70 to stimulate varying lung resistance. Pressure measurements and flow visualization were done at steady flows of 2, 4 and 6 liters/min to stimulate rest and exercise. RESULTS Our data show that the energy losses at the 0.0-diameter offset were double the losses of the 1.0 and 1.5 diameters, which had minimal energy losses. This result was attributable to chaotic patterns seen on flow visualization in the 0.0-diameters offset. Energy savings were more evident at the 50:50 right/left pulmonary artery ratio. Energy losses increased with increased total flow rates. CONCLUSIONS The results strongly suggest the incorporation of caval offsets in future total cavopulmonary connections.

Clinical significance and origin of artifacts in transesophageal echocardiography of the thoracic aorta.

OBJECTIVES The aim of this study was to identify the mechanism and features of artifacts encountered during transesophageal echocardiography of the aorta. BACKGROUND Artifacts are an important potential limitation of transesophageal echocardiography of the aorta. METHODS The mechanism of the artifacts was examined by in vitro modeling. The frequency and clinical correlates of artifacts were examined by retrospective review of transesophageal echocardiograms in 36 patients with aortic pathologic lesions. RESULTS Two classes of artifact were seen: linear artifacts in the ascending aorta, which may mimic intimal flaps, and mirror image artifacts in the transverse and descending thoracic aorta. Linear artifacts in the ascending aorta, seen in 44% of patients, were shown in vitro to be multiple path artifacts caused by reflection of ultrasound within the left atrium. Linear artifacts in the ascending aorta were associated with dilatation of the ascending aorta and were more frequent when the aortic diameter exceeded the left atrial diameter (p < 0.001). The mirror image artifacts of the transverse and descending thoracic aorta give the appearance of a double-barrel aorta and were shown in vitro to be caused by the aorta-lung interface, which acts as a total reflector of ultrasound. Mirror image artifacts were seen in > 80% of patients. Artifacts were equally frequent with the sagittal and transverse imaging planes when biplane transesophageal echocardiography was used. CONCLUSIONS Artifacts occur frequently during transesophageal echocardiography of the aorta. An understanding of why they occur and the features that distinguish them from true abnormalities should enhance the diagnostic accuracy of transesophageal echocardiography for aortic disease.

Control of ionic polymer metal composites

Robotic devices are traditionally actuated by hydraulic systems or electric motors. However, with the desire to make robotic systems more compact and versatile, new actuator technologies are required. In this paper, the control of ionic polymer metal composite actuators is investigated from a practical perspective. The actuator characteristics are examined through the unblocked maximum displacement and blocked force output. An open-loop position control and closed-loop position proportional-integral-derivative (PID) control are then applied to a strip of actuators. Finally, the performance of the polymer is investigated when implementing an impedance controller (force/position control).

A New Control Volume Method for Calculating Valvular Regurgitation

BACKGROUND The purpose of the present study was to develop a new method of measuring heart valvular regurgitation based on control volume theory and to verify its accuracy in vitro and in vivo. Current methods of quantifying valvular regurgitation rely too much on assumptions about the flow field and therefore are difficult to apply in vivo. In particular, the proximal isovelocity surface area (PISA) method oversimplifies the proximal velocity field by assuming hemispherical isovelocity contours proximal to the orifice. This severely limits the applicability of the PISA method. Use of the basic control volume theory, however, removes the need to assume the manner in which the proximal flow accelerates toward the regurgitant orifice, the shape and size of the orifice, the shape of the orifice plate, and the non-newtonian behavior of the fluid. Apart from a correction that is necessary if the orifice plate is moving, the control volume method assumes only the incompressibility of the fluid and therefore is a potentially more accurate approach. In addition, the use of magnetic resonance imaging (MRI) precludes the need for an acoustic window. METHODS AND RESULTS MRI has been used to measure the three-dimensional velocity field proximal to regurgitant orifices, including single and multiple orifices and a cone-shaped orifice plate. Both steady (0 to 7.5 L/min) and pulsatile (2 and 3 L/min) flows were used. By intergrating this velocity over a control volume surrounding the orifice, we calculated the flow rate through the orifice. As a validation, the cardiac output of a 50-kg pig also was measured and was compared with thermodilution measurements. It was found that MRI could be used to measure the three-dimensional flow proximal to regurgitant orifices. This enabled the calculation of the flow rate through the orifice by integrating the velocity over the surface of a control volume covering the orifice. This flow rate correlated well with the actual rate (0.992; correlation line slope, 1.01). Care had to be taken, however, to exclude from the integration regions of aliased velocity. The cardiac output of the pig measured using MRI was in close agreement with the themodilution measurements. CONCLUSIONS Our new method of measuring valvular regurgitation has been shown to be very accurate in vitro and in vivo and therefore is a potentially accurate way to quantify valvular regurgitation.

Hemodynamics of the Fontan connection: an in-vitro study.

The Fontan operation is one in which the right heart is bypassed leaving the left ventricle to drive the blood through both the capillaries and the lungs, making it important to design an operation which is hemodynamically efficient. The object here was to relate the pressure in Fontan connections to its geometry with the aim of increasing the hemodynamically efficiency. From CT or magnetic resonance images, glass models were made of realistic atrio-pulmonary (AP) and cavo-pulmonary (CP) connections in which the right atrium and/or ventricle are bypassed. The glass models were connected to a steady flow loop and flow visualization, pressure and 3 component LDA measurements made. In the AP model the large atrium and curvature of the conduit created swirling patterns, the magnitude of which was similar to the axial velocity. This led to an inefficient flow and a subsequent large pressure loss (780 Pa). In contrast, the CP connection with a small intra-atrial chamber had reduced swirling and a significantly smaller pressure loss (400 Pa at 8 l.min) and was therefore a more efficient connection. There were, however, still pressure losses and it was found that these occurred where there was a large bending of the flow, such as from the superior vena cava to the MPA and from the MPA to the right pulmonary artery.

Three-dimensional reconstruction of the flow in a human left heart by using magnetic resonance phase velocity encoding

Intraventricular flows have been correlated with disease and are of interest to cardiologists as a possible means of diagnosis. This study extends a method that use magnetic resonance (MR) to measure the three-dimensional nature of these flows. Four coplanar sagittal MR slices were located that spanned the left ventricle of a healthy human. All three velocity components were measured in each slice and 18 phases were obtained per beat. With use of the MR magnitude images, masks were created to isolate the velocity data within the heart. These data were read into the software package. Data Visualizer, and the data from the four slices were aligned so as to reconstruct the three-dimensional volume of the left ventricle and atrium. By representing the velocity in vectorial form, the three-dimensional intraventricular flow field was visualized. This revealed the presence of one large line vortex in the ventricle during late diastole but a more ordered flow during early diastole and systole. In conclusion, the use of MR velocity acquisition is a suitable method to obtain the complex intraventricular flow fields in humans and may lead to a better understanding of the importance of these flows.

A Computational Study of a Thin-Walled Three-Dimensional Left Ventricle During Early Systole

A numerical study was conducted to solve the three-dimensional Navier-Stokes equations for time-dependent flow in a compliant thin-walled, anatomically correct left ventricle during early systole. Model parameters were selected so that the simulation results could be compared to clinical data. The results produced endocardial wall motion which was consistent with human heart data, and velocity fields consistent with those occurring in a normally-contracting left ventricle. During isovolumetric contraction the posterior wall moved basally and posteriorly, while the septal wall moved apically and anteriorly. During ejection, the short axis of the ventricle decreased 1.1 mm and the long axis increased 4.2 mm. At the end of the isovolumetric contraction, most of the flow field was moving form the apex toward the base with recirculation regions at the small pocket formed by the concave anterior leaflet, adjacent to the septal wall and near the left ventricular posterior wall. Fluid velocities in the outflow tract matched NMR data to within 10 percent. The results were also consistent with clinical measurements of mitral valve-papillary muscle apparatus displacement, and changes in the mitral valve annular area. The results of the present study show that the thin-walled, three-dimensional left ventricular model simulates observed normal heart phenomena. Validation of this model permits further studies to be performed which involve altered ventricular function due to a variety of cardiac diseases.

Disturbed flow promotes deposition of leucocytes from flowing whole blood in a model of a damaged vessel wall

Departure from simple laminar flow in arteries may promote the local attachment of leucocytes either to intact endothelium or platelet thrombi. We perfused blood through a chamber with a backward facing step, to observe whether adhesion from whole blood to P‐selectin was indeed localized to a region of recirculating flow, and whether platelets binding to collagen in such a region could capture leucocytes. Blood flowing over the step established a stable vortex, a reattachment point where forward and backward flow separated, and a simple laminar flow with wall shear rate c. 400/s further downstream. Fluorescently labelled leucocytes were observed to attach to P‐selectin immediately upstream or downstream of the reattachment point, and to roll back towards the step or away from it, respectively. There was negligible adhesion further downstream. When a P‐selectin‐Fc chimaera was used to coat the chamber, stable attachment occurred, again preferentially in the disturbed flow region. Numerous platelets adhered to a collagen coating throughout the chamber, although there were local maxima either side of the reattachment point. The adherent platelets captured flowing leucocytes in these regions alone. Leucocytes may adhere from flowing blood in vessels with high shear rate if the flow is disturbed. While platelets can adhere over a wider range of shear rates, their ability to capture leucocytes may be restricted to regions of disturbed flow.

Distribution of hepatic venous blood in the total cavo-pulmonary connection: an in vitro study

OBJECTIVES The objective of this project was to quantify the effects of geometry on the distribution of hepatic blood to the lungs in patients with a total cavo-pulmonary connection. The basis for this work is the supposition that hepatic blood is necessary for proper lung function. METHODS Plastic models of these connections were made with varying degrees of offset between the inferior and superior vena cava and attached to an in vitro flow loop. Dye was injected into the inferior vena cava and its concentration quantified in each pulmonary artery. These data were converted to percentage concentration and distribution of hepatic blood to each lung. RESULTS With no offset between the vena cava, hepatic blood distribution and concentration to each lung was similar to normal. For an offset of one or more diameters, hepatic blood tended to flow preferentially towards the nearest pulmonary artery with the opposite pulmonary artery exhibiting a deficit (<10% of normal). CONCLUSIONS Distribution of hepatic blood to each lung was found to be a function of vena cava offset and pulmonary artery flow split. Under normal conditions, 60% of blood towards the right pulmonary artery, the hepatic blood distribution to both lungs could be maintained above 50% of normal if the inferior vena cava was offset towards the left pulmonary artery. Offsetting the inferior vena cava towards the right pulmonary artery jeopardized the delivery of hepatic blood to one lung.