Ann E. Ensley

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.

Toward designing the optimal total cavopulmonary connection: an in vitro study

BACKGROUND Understanding the total cavopulmonary connection (TCPC) hemodynamics may lead to improved surgical procedures which result in a more efficient modified circulation. Reduced energy loss will translate to less work for the single ventricle and although univentricular physiology is complex, this improvement could contribute to improved postoperative outcomes. Therefore to conserve energy, one surgical goal is optimization of the TCPC geometry. In line with this goal, this study investigated whether addition of caval curvature or flaring at the connection conserves energy. METHODS TCPC models were made varying the curvature of the caval inlet or by flaring the anastomosis. Steady flow pressure measurements were made to calculate the power loss attributed to each connection design over a range of pulmonary flow splits (70:30 to 30:70). Particle flow visualization was performed for each design and was qualitatively compared to the power losses. RESULTS Results indicate that curving the cavae toward one pulmonary artery is advantageous only when the flow rate from that cavae matches the flow to the pulmonary artery. Under other pulmonary flow split conditions, the losses in the curved models are significant. In contrast, fully flaring the anastomosis reduced losses over the range of pulmonary flow splits. Power losses were 56% greater for the curving as compared to flaring. Fully flaring without caval offset reduced losses 45% when compared to previous models without flaring. If flaring on all sides was implemented with caval offset, power losses reduced 68% compared to the same nonflared model. CONCLUSIONS The results indicate that preferentially curving the cavae is only optimal under specific pulmonary flow conditions and may not be efficient in all clinical cases. Flaring of the anastomosis has great potential to conserve energy and should be considered in future TCPC procedures.

In vivo flow dynamics of the total cavopulmonary connection from three-dimensional multislice magnetic resonance imaging ☆

BACKGROUND The total cavopulmonary connection (TCPC) design continues to be refined on the basis of flow analysis at the connection site. These refinements are of importance for myocardial energy conservation in the univentricular supported circulation. In vivo magnetic resonance phase contrast imaging provides semiquantitative flow visualization information. The purpose of this study was to understand the in vivo TCPC flow characteristics obtained by magnetic resonance phase contrast imaging and compare the results with our previous in vitro TCPC flow experiments in an effort to further refine TCPC surgical design. METHODS Twelve patients with TCPC underwent sedated three-dimensional, multislice magnetic resonance phase contrast imaging. Seven patients had intraatrial lateral tunnel TCPC and 5 had extracardiac TCPC. RESULTS In all patients in both groups a disordered flow pattern was observed in the inferior caval portion of the TCPC. Flow at the TCPC site appeared to be determined by connection geometry, being streamlined at the superior vena cava-pulmonary junction when the superior vena cava was offset and flared toward the left pulmonary artery. Without caval offset, intense swirling and dominance of superior vena caval flow was observed. In TCPC with bilateral superior vena cavae, the flow patterns observed included secondary vortices, a central stagnation point, and influx of the superior vena cava flow into the inferior caval conduit. A comparative analysis of in vivo flow and our previous in vitro flow data from glass model prototypes of TCPC demonstrated significant similarities in flow disturbances. Three-dimensional magnetic resonance phase contrast imaging in multiple coronal planes enabled a comprehensive semiquantitative flow analysis. The data are presented in traditional instantaneous images and in animated format for interactive display of the flow dynamics. CONCLUSIONS Flow in the inferior caval portion of the TCPC is disordered, and the TCPC geometry determines flow characteristics.

Fluid Mechanic Assessment of the Total Cavopulmonary Connection using Magnetic Resonance Phase Velocity Mapping and Digital Particle Image Velocimetry

AbstractThe total cavopulmonary connection (TCPC) is currently the most promising modification of the Fontan surgical repair for single ventricle congenital heart disease. The TCPC involves a surgical connection of the superior and inferior vena cavae directly to the left and right pulmonary arteries, bypassing the right heart. In the univentricular system, the ventricle experiences a workload which may be reduced by optimizing the cavae-to-pulmonary anastomosis. The hypothesis of this study was that the energetic efficiency of the connection is a consequence of the fluid dynamics which develop as a function of connection geometry. Magnetic resonance phase velocity mapping (MRPVM) and digital particle image velocimetry (DPIV) were used to evaluate the flow patterns in vitro in three prototype glass models of the TCPC: flared zero offset, flared 14 mm offset, and straight 21 mm offset. The flow field velocity along the symmetry plane of each model was chosen to elucidate the fluid mechanics of the connection as a function of the connection geometry and pulmonary artery flow split. The steady flow experiments were conducted at a physiologic cardiac output (4 L/min) over three left/right pulmonary flow splits (70/30, 50/50, and 30/70) while keeping the superior/inferior vena cavae flow ratio constant at 40/60. MRPVM, a noninvasive clinical technique for measuring flow field velocities, was compared to DPIV, an established in vitro fluid mechanic technique. A comparison between the results from both techniques showed agreement of large scale flow features, despite some discrepancies in the detailed flow fields. The absence of caval offset in the flared zero offset model resulted in significant caval flow collision at the connection site. In contrast, offsetting the cavae reduced the flow interaction and caused a vortex-like low velocity region between the caval inlets as well as flow disturbance in the pulmonary artery with the least total flow. A positive correlation was also found between the direct caval flow collision and increased power losses. MRPVM was able to elucidate these important fluid flow features, which may be important in future modifications in TCPC surgical designs. Using MRPVM, two- and three-directional velocity fields in the TCPC could be quantified. Because of this, MRPVM has the potential to provide accurate velocity information clinically and, thus, to become the in vivo tool for TCPC patient physiological/functional assessment.

Evaluation of cardiovascular parameters of a selenium-based antihypertensive using pulsed Doppler ultrasound.

&NA; The pharmacology of selenium is of much interest because selenium deficiency has been linked to cardiovascular diseases, cancer, and arthritis, and selenoenzymes are critical cellular antioxidants. We have previously reported that phenyl‐2‐aminoethylselenide (PAESe) and its derivatives represent a novel class of selenium‐based antihypertensive agents that exhibit unique biochemical and pharmacologic properties. We now report on experiments designed to probe the hemodynamic mechanism of action of these compounds in spontaneously hypertensive rats (SHR). A noninvasive pulsed Doppler ultrasound probe was used to measure peak blood flow velocity in the aortic arch from the right second intercostal space. PAESe was found to increase peak aortic blood flow velocity (+44%), heart rate (+16%), and blood flow acceleration (+105%), while decreasing left ventricular ejection time (LVET) (‐37%) concomitant with a decrease in mean arterial pressure (‐54%). These results were compared with the known vasodilator hydralazine, which had similar effects on mean arterial pressure (MAP) and peak velocity but caused an increase in LVET (+42%) and a decrease in heart rate (‐18%). Taken together, our results suggest that PAESe decreases blood pressure via a decrease in peripheral resistance, which overcomes the initial increase in heart rate and acceleration to give a net decrease in MAP.

Evaluation of collagen-elastin hybrid tissue engineered vascular constructs

Collagen-based tissue engineered replacements for small diameter blood vessels have been investigated for many years but typically lack the elasticity and tensile strength necessary for implantation. In this study, we have incorporated elastin with an organized structural architecture into tubular cell-seeded collagen constructs using two different reconstituted collagen sources. We evaluated the mechanical, chemical, and biological properties of these collagen-only and collagen-elastin hybrid grafts. Gel compaction quantification and live/dead staining revealed that cells in all matrix combinations were viable and able to rapidly compact their surrounding matrix. Compared to controls, uniaxial tensile testing revealed an increase in ultimate tensile strength and linear modulus for elastin hybrid constructs and for constructs formed with bovine dermal type I collagen. Histological assessment showed the unique composite structure of the elastin hybrid construct as well as the cell and extracellular matrix organization in all constructs studied. A discussion of these findings and their importance to vascular tissue engineering is discussed.

Numerical simulation of flow in model, total cavopulmonary connections

The total cavopulmonary connection (TCPC) is a surgical procedure employed to treat children with single-ventricle heart defects. Since the end result of the procedure is a complete bypass of the right-heart with the single ventricle driving blood through the entire circulation, minimization of the energy losses associated with the connection may be important for long-term patient success. The current work reports results from numerical simulations complementing existing in vitro experimental data for two TCPC models. Modeling assumptions were validated with supporting numerical experiments. Agreement between numerical and experimental results was favorable.

Is fluid shear stress an indicator of total cavopulmonary connection efficiency

The total cavopulmonary connection (TCPC) is a surgical palliation for single ventricle congenital heart disease which bypasses the right heart. With only one functioning pump for the entire circulatory system, hemodynamic efficiency in the TCPC is an important concern. Energy losses across the connection can be reduced by improving the geometry of the connection and may directly correlate with the fluid shear stresses in the TCPC. Three prototype models of the TCPC were investigated with particle image velocimetry and the resulting fluid shear stresses were compared with previous energy loss data. There was a positive correlation between total shear stress and experimental pressure derived energy loss. This indicated that future assessment of TCPC efficiency may benefit from a complete analysis of fluid shear stresses.