acquisition of  pre&#45;and post&#45;bolus 3d angiograms and intravascular pressure from a catheterized sheep for measurement of the vessel circumference

3D Angiography for Venous Compliance Assessment in Sheep

Patients with critical cardiac anomalies require reconstructive surgery. Most reconstructive operations require the use of prosthetic materials, including vascular grafts. Potential conduits to bridge this space include synthetic or biologic materials. Initially, homografts were used as the Fontan conduit but have since been abandoned due to a high incidence of calcification and acute phase incidents1. Currently, synthetic vascular grafts derived from inorganic polymers are used. There remains a challenge that these grafts are less compliant than the native vessel into which they are implanted and have long-term complications, such as stenosis, occlusion, and calcification1,2,3,4,5.
The structure of synthetic vascular grafts lends itself to mechanical tensile strength, leading to their invariably lower compliance compared to native tissue2. Vascular compliance, which defines the vessel&#39;s change in volume over a change in pressure, serves as an indicator of a vessel&#39;s responsiveness to mechanical loads. The difference between the graft material and native vessel properties creates a compliance mismatch, which has been demonstrated to disrupt blood flow patterns, resulting in areas of recirculation and flow separation2,6,7,8,9. This phenomenon alters the shear stress on the endothelial wall and induces intimal hyperplasia2,7,8,9. Such responses can lead to graft-related complications, necessitating graft replacement or re-intervention6.
As vascular compliance assumes a key role in determining graft outcomes, the accurate measurement of this property is essential. The current gold standard for measuring vascular compliance is ex vivo tubular biaxial mechanical testing. This method involves excising a graft or vessel of interest, connecting it to latex tubes, and pressurizing it to assess circumferential stress-stretch behavior across various pressures. Compliance is determined by comparing the pressure with a measurement of the inner diameter10. However, ex vivo methods have some disadvantages. When evaluating the functionality of implanted grafts using the ex vivo method, sacrificing the animals and explanting the grafts are necessary, making it impossible to conduct prolonged examinations. Therefore, we have developed an in vivo compliance measurement protocol.
Our group focuses on developing tissue-engineered vascular grafts (TEVGs) for use in the Fontan surgery to ameliorate the congenital heart defect hypoplastic left heart syndrome (HLHS). Recent developments in the field of congenital heart surgery have improved postoperative outcomes, leading to longer life expectancies. This makes the long-term properties and success of the implanted vascular conduit increasingly crucial. Currently, no animal model of HLHS exists so we evaluate our grafts in an accelerated large animal inferior vena cava (IVC) interposition graft model. While this model does not attempt to create the flow of the Fontan circulation, it effectively recapitulates the unique hemodynamic conditions. Our recent use of this in vivo protocol demonstrated significant differences in graft compliance between our TEVG and conventional expanded polytetrafluoroethylene (PTFE) grafts11. As this previous study did not focus on methodology, we have conducted additional experiments detailing this novel in vivo method.
We implanted the synthetic graft currently serving as the standard of care, comprised of expanded polytetrafluoroethylene (PTFE), in Dorset sheep study animals and compared it to the native IVC in surgically na&#239;ve control animals. This protocol was performed on the PTFE group 5-7 years postimplantation of a PTFE conduit and unoperated control animals of varying ages. Thus, in subsequent sections describing the protocol and representative results, we will occasionally refer to the region of interest as, for example, the middle of the graft (midgraft) region of the IVC interposition graft.
This protocol allows us to analyze the in vivo compliance of the PTFE conduit, known to be non-compliant at a long-term time point, with the native vein. We chose to compare the clinical standard material, PTFE, with the native unoperated vein. We selected a long-term time point because the PTFE conduit is known to remain non-compliant and is prone to calcify, further reducing its compliance11. We opted to conduct all comparisons in vivo as systemic hemodynamic changes are accurately reflected in measurements obtained through in vivo methods. From this comparison, we found that this protocol is able to confirm the non-compliance of PTFE and obtain measurements of in vivo venous compliance in a safe and reproducible manner. This method has been successfully implemented in a published study to demonstrate statistically significant differences between PTFE conduits and tissue-engineered vascular grafts (TEVGs) in vivo11.
The overall goal of this protocol is to calculate compliance and distensibility of the thoracic IVC in an ovine large animal model using in vivo measurements from a survival procedure. To this end, we visualized and measured the changes in circumference and cross-sectional area of thoracic IVC to a fluid bolus. We simultaneously measured the intravascular change in pressure and used these measurements to calculate compliance and distensibility. Using 3D angiography imaging allows us multiple advantages, including the ability to adjust the view of the image post capture to ensure our measurements are taken from a cross-section of the vein, as well as allow us to measure multiple locations along the vessel. The three areas of interest in this study were the midgraft region, as well as the two adjacent anastomosis sites of the PTFE graft, and the comparable areas in the native IVC. By conducting experiments in vivo, there are advantages in evaluating the functionality of grafts within the actual flow of blood and surrounded by tissues and organs. The measurements obtained through this method are believed to reflect the actual functionality of the grafts in a living organism.
The protocol is divided into six main sections including preprocedure preparation of the sheep, catheterization, collection of baseline pre bolus data, collection of study data, animal recovery, and data analysis. In the animal preparation section, we discuss sedation, initiating anesthesia, and the placement of monitoring equipment used during the catheterization procedure. In the second section, we explain the process of placing the two catheter sheaths needed for data acquisition. For this protocol, both sheaths are placed in the right internal jugular vein (IJV) to allow two multitrack catheters to be introduced into the vessel. One will be positioned in the region of interest to record the change in pressure, and the other will be placed lower in the vein for contrast injection. Once the catheters are placed, a baseline pre bolus 3D angiograph is taken for comparison. Study data collection begins with preparing the saline bolus in a pressurized bag system for administration, providing the saline bolus with recording intravascular pressures, and taking the post bolus 3D angiograph. We then describe the process to facilitate the recovery of the sheep after the protocol. Lastly, we discuss the method to obtain the proper images and cross-sectional measurements for analysis and statistical comparison.

Author Spotlight: Improved Imaging for Neovascular Development in Congenital Heart Disease Research

Quantifying Inferior Vena Cava Compliance and Distensibility in an In Vivo Ovine Model Using 3D Angiography

We aim to improve long-term outcomes for congenital-heart-disease patients using a regenerative medicine approach. Our tissue-engineered vascular graft develops into a neo vessel comprised of the patient's own cells. Our goal is to make comparisons between our graft, the native vein, and the clinical standard polytetrafluoroethylene or PTFE.We have established that our neo vessel displays growth capacity and that it approaches native vessel functionality. Using this method, we've also recently demonstrated that the neo vessel retains compliance and distensibility at a long-term time point, and is resistant to the formation of dystrophic calcification. Using 3D angiography allows us to image the entire path of the thoracic inferior vena cava in our ovine large-animal model.In addition to allowing us to view the morphology of the vessel, it also allows for post-capture orientation adjustments to ensure we're obtaining measurements from a true cross-section. Our in-vivo method allows us to determine the capacity for vessel compliance and distensibility in its native context. Additionally, it allows us to take longitudinal measurements of the same study animal, which is necessary to track changes in neo-vessel development and remodeling.

Acquisition of  Pre&#45;and Post&#45;Bolus 3D Angiograms and Intravascular Pressure from a Catheterized Sheep for Measurement of the Vessel Circumference

To begin, connect a multi-track angiographic catheter in the region of interest of an anesthetized catheterized sheep to a pressure transducer with a three-way stopcock. Then flush the multi-track with heparinized saline. Flip the off position on the stopcock to the syringe so that the pressure transducer and the multi-track are open to each other.Connect the contrast injector to the multi-track centered in the abdominal IVC. Turn the knob of the contrast injector counterclockwise slowly to pull air bubbles from the multi-track until blood is seen. Then turn the knob clockwise to push contrast forward slowly into the multi-track.Confirm when the contrast has reached the tip of the multi-track with a fluoroscope. Divide the total contrast used for the 3D angiogram by five to obtain the rate of injection in milliliters per second. Then set the rate rise to 0 in 600 pounds per square inch.Next, to set the C-arm to the pre-programmed mode, click the Program button, then press the 3D-110 8"button. Move all objects and people away from the front or sides of the table. Click on button 3 on the control panel to initiate the C-arm.Position the target at the center of the xy plane. Click button 4 on the control panel to move onto the second program. Then adjust the height of the table to center the region of interest.Next, press button 5 to take the test image. click on the Confirm Conditions icon, followed by Start to pretest the C-arm's range of motion. Use the middle acquisition pedal to initiate the program and take a 3D rotational angiogram.Simultaneously, measure and record the mean pressure of the target region. Connect a pressurized saline bag to a 9 French sheath. Allow it to flow until the sheep has received a bolus equivalent to 20 milliliters per kilogram.Record the intravascular pressures of the target region every minute while the bolus is flowing. Perform the second 3D angiography with concurrent intravascular pressure measurement. Once imaging is complete, press the number 77 and hold the Start button to place the C-arm back in its pre-programmed to position.Remove the multi-track angiography catheters and Rosen wires from the sheep. Then remove both sheaths. Apply pressure with a hemostasis patch over the insertion sites for at least seven minutes to control bleeding.For data analysis, export the original raw 3D angiography data from the angiography imaging software in a DICOM file format. Launch the DICOM Viewer software. Then drag and drop the 3D angiography file into the Viewer to open.Select the 3D Multiplanar Reconstruction tool to generate a 3D reconstructed view of the angiography data. Three distinct 2D views from the axial, sagittal, and coronal planes can now be seen. Now adjust the placement and orientation of the target region in the sagittal and coronal planes to place the target region at the center.Use the Hand tool to rotate the direction of the reference lines on each page. With the Pencil tool, outline the vessel wall in the axial view of the target region. The software will automatically calculate and show both the area and perimeter of the region in the middle of the axial view.The intravascular pressures of sheep with and without PTFE grafts were obtained. A change in circumference was seen in the native sheep in response to bolus. The native sheep vessels were more compliant and distensible than the PTFE grafts.

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Research

JoVE Journal

Bioengineering

26 Views. To begin, connect a multi-track angiographic catheter in the region of interest of an anesthetized catheterized sheep to a pressure transducer with a three-way stopcock. Then flush the multi-track with heparinized saline. Flip the off position on the stopcock to the syringe so that the pressure transducer and the multi-track are open to each other.Connect the contrast injector to the multi-track centered in the abdominal IVC. Turn the knob of the contrast injector counterclockwise slow...

Video: Acquisition of  Pre-and Post-Bolus 3D Angiograms and Intravascular Pressure from a Catheterized Sheep for Measurement of the Vessel Circumference