Nonsurgical options available for correcting PDA in dogs
A: Transarterial and transvenous occlusion of using either the Amplatzer Duct Occluders or Gianturco vascular occlusion coils are now accepted options for management of patent ductus arteriosus (PDA) in dogs. The following articles describe their current experiences in the nonsurgical management of PDAs in dogs. Tobias A., Jacob K., Fine D., Carpenter D.: Patent ductus arteriosus occlusion with Amplatzer® Duct Occluders. Proc 20th Annual Forum ACVIM 20:100-101, 2002 and Miller M. W.: Transarterial coil occlusion of patent ductus arteriosus: outcome in 120 cases. Proc 20th Annual Forum ACVIM 20:102, 2002.
The Amplatzer Duct Occluders are manufactured by AGA Medical Corp., Golden Valley, MN (www.amplatzer.com). The Amplatzer Duct Occluder device is made from an alloy of nickel and titanium, called nitinol. Nitinol has super-elastic properties, together with excellent memory and strength, making it particularly attractive for many medical device applications, especially where compact configurations are required for device insertion and placement.
During the last two years, veterinarians at the Cardiology Service, University of Minnesota College of Veterinary Medicine, have managed several cases of PDA in dogs using the Amplatzer Duct Occluder device.
This is a transcatheter occlusion device intended for the nonsurgical closure of PDA independent of the PDA's shape or size. It is a self-expanding device made from a nitinol wire mesh. Depending upon the size of the device, a 2-3 mm wide retention flange assures secure positioning of the device in the PDA. Polyester patches, which are sewn into the device, facilitate ductal occlusion by inducing thrombosis. The device is intended for use in human patients with a body weight of 5 kg, and the Cardiology Service has limited their patient selection to dogs of that size.
Procedure for occluder placement
The procedure used for PDA occluder placement in dogs is similar to that recommended for humans, which is well described on the AGA Medical Corp. Web site. Various anatomical differences between humans and dogs have necessitated some procedural modifications.
With the dog in right lateral recumbency, a cut-down to isolate the right femoral artery and vein is performed and introducer sheaths in both femoral vessels are placed. Then, an aortogram is performed with the contrast bolus delivered close to the PDA in an effort to measure the size of the narrowest segment of the PDA at the junction with the main pulmonary artery.
Using a graduated guide wire passed via the aorta and across the PDA facilitates this measurement.
Overall, the Cardiology Service has been unimpressed with the ability to measure the PDA dimensions accurately by this technique, seemingly due to superimposition of contrast medium in the PDA and main pulmonary artery.
In human patients, the recommendation is to pass a pigtail catheter with side holes across the PDA via right heart catheterization, and to perform a contrast injection within the lumen of PDA itself. While this method allows very clear PDA visualization and precise dimension measurements, it is very challenging to pass any catheter retrograde across the PDA from the pulmonary artery side in dogs, never mind a pigtail!
The Cardiology Service continues to seek a method to accurately measure PDA size angiographically. However, the Cardiology Service is usually able to visualize the PDA very well echocardiographically, which enables obtaining an accurate measurement of the narrowest segment of the PDA at the pulmonary artery insertion prior to the occlusion procedure.
Once the PDA dimensions are measured acceptably well with a combination of echocardiography and angiography, an end-hole balloon wedge pressure catheter (Arrow International, Inc., Reading, PA) is used to position an exchange wire within the pulmonary artery. Then, is the next challenge to pass the exchange wire retrograde through the PDA into the aorta.
Using an end-hole balloon wedge pressure catheter greatly facilitates pulmonary artery catheterization, especially in smaller dogs. To date, the Cardiology Service has managed to pass the exchange wire across the PDA with a combination of good fortune, endurance, and more recently with the use of snare catheters [Amplatz Goose Neck Snares and Microsnares, Microvena Corporation, White Bear Lake, MN (www.microvena.com)].
Occlusion device delivery
Once the exchange wire is across the PDA and in the aorta, the end-hole balloon wedge pressure catheter is removed, and delivery of the occlusion device can be done.
The occluder is delivered via a delivery system (Amplatzer Delivery System, AGA Medical Corp., Golden Valley, MN). This delivery system is composed of a dilator within a sheath, which is passed over the exchange wire via the right heart, pulmonary artery, and PDA and into the aorta. Once that has been achieved, the dilator and exchange wire are removed, leaving the introducer sheath in place with its tip positioned within the aorta.
The Amplatzer Duct Occluders are manufactured in a variety of sizes (go to www.amplatzer.com for details). The Cardiology Service usually selects an occluder with a barrel diameter of about 50 percent greater than the narrowest segment of the PDA at the junction with the pulmonary artery.
The occluder is screwed onto a delivery cable, immersed in sterile saline solution, compressed and loaded into a short length of tubing (the loader). The loader is introduced into the delivery sheath, and the device is advanced via the sheath into the aorta. This part of the procedure has also posed some challenges. As the occluder is advanced, the sheath frequently kinks within the right ventricle, especially in smaller dogs, making it difficult to advance the device. With sufficient manipulation, "getting around the bend" can be managed. Once in the aorta, the occluder is partially extruded from the sheath to deploy the retention flange.
The entire assembly is then drawn into the PDA until the retention flange engages at the pulmonary artery end of the PDA. Once the device is in the correct position, the occluder is fully deployed by stabilizing the cable and drawing the sheath off the occluder and into the pulmonary artery. The occluder barrel will develop an hourglass shape that indicates the device is deployed in the correct location, i.e., with the retention flange within the PDA, and with the barrel through the narrowest segment of the PDA and protruding slightly into the pulmonary artery.
At this stage, the occluder is still attached to the cable, allowing recapture of the occluder into the sheath, should that become necessary. An aortogram is then performed to ensure complete PDA occlusion. Once that is confirmed, the cable is unscrewed from the occluder and withdrawn. The introducer sheaths are then removed, the femoral vein is ligated, and the femoral artery is sutured with 6/0 silk material.
Overall, veterinarians in the Cardiology Service have found PDA occlusion with Amplatzer Duct Occluder to be challenging, but extremely rewarding in appropriate cases.
The Cardiology Service has had its share of frustrations and complications, and the learning curve has been (and remains) steep and long. The Cardiology Service believes that PDA occlusion with the Amplatzer Duct Occluder represents an excellent therapeutic option that may be especially appropriate for larger dogs and larger PDAs.
Veterinarians at Texas A&M University College of Veterinary Medicine have reported on the results of transarterial PDA occlusion using both 0.038 inch and 0.052-inch diameter Gianturco occlusion coils. Since 1994, this group has attempted occlusion in 120 canine cases.
Most of these dogs have had PDA as an isolated congenital cardiac malformation. The most common concurrent congenital anomalies have been pulmonic stenosis and subaortic stenosis. The Gianturco occlusion coil has proven to be very safe and effective for occluding small to moderate diameter PDAs. However, there is limited control of the Gianturco occlusion coil during implantation. This lack of control complicates closure of larger diameter PDAs.
Various mechanisms have been developed to provide more control of the coil during implantation, but these "safety nets" increase the technical demands of the procedure. New devices with improved control are being developed for occluding large PDAs and other vascular structures.
However, the tremendous effectiveness of the Gianturco coil for PDA occlusion, along with the low cost and widespread availability of this coil, makes it the preferred device if applicable.
The 0.052-inch diameter Gianturco occlusion coils (Cook Inc., Bloomington, IN) are constructed from a heavier wire, thus are more resilient to uncoiling during implantation. This stronger coil provides improved positioning during implantation, and has been used successfully for PDA closure. The 0.052-inch diameter Gianturco occlusion coil requires a larger catheter for implantation, which complicates its use in small dogs, especially if the coil embolizes and must be retrieved.
The dogs are anesthetized and placed in right lateral recumbency. A short 6 French sheath is placed in the right femoral artery. Using a 4 French transseptal sheath positioned through the short 6 French sheath, an angiogram is performed in the descending thoracic aorta; the minimum and maximum PDA diameters are measured and the morphology of the PDA is determined. Using a 0.038-inch diameter Glidewire (Medi-Tech BSC, Watertown, MA), the transseptal sheath and dilator were then positioned either in the lumen of the PDA or advanced into the main pulmonary artery.
Preparing the coil
The 0.052-inch diameter coil is prepared as follows.
Using a 0.038-inch Teflon wire, the proximal end of the coil is advanced 5-8 mm out of the steel delivery tube in which the coil is packaged. The rounded ball on the end of the coil is "stretched out" approximately 0.5 mm away from the coil windings using a hemostat allowing the bioptome to firmly hold onto the ball.
The bioptome is then advanced through a section of the housing from the Glidewire. The bioptome jaws are opened and placed around the ball on the coil tip. The jaws are closed, and then the bioptome-coil is pulled into the section of Glidewire housing.
This manipulation is necessary because once the bioptome is holding onto the ball, the bioptome-coil will not fit through the steel coil delivery tube. The portion of Glidewire housing serves as an effective and inexpensive delivery tube. The Glidewire housing loaded with the coil-bioptome is then inserted into the hemostasis valve of the 4 French transseptal sheath.
The bioptome-coil is advanced through the transseptal sheath. The coil is pushed slowly out of the transseptal sheath such that one loop of coil formed outside is either within the lumen of the PDA or the main pulmonary artery depending upon initial position of the tip of the sheath. Once appropriate position is confirmed, the bioptome-coil is advanced further, allowing several loops of the coil to form in the aortic ampulla.
If the sheath is within the main pulmonary artery, two-thirds to one loop of coil is allowed to remain in that location. The bioptome jaws are clearly visible on fluoroscopy. If the positioning of the coils is suboptimal, the coil can be withdrawn into the sheath and the procedure repeated. Following deployment of the coil, an angiogram is performed in the descending aorta to assess coil position and occlusion.
In cases where a residual shunt is present, using the retrograde approach, additional coils can be deployed as needed. The 4 French transseptal sheath and dilator can frequently be advanced past the initial coil into the main pulmonary artery allowing for controlled delivery of additional 0.052-inch diameter coils. In cases in which the transseptal sheath and dilator will not traverse the initial coil, a 4 French snare catheter (Microvena Corporation, White Bear Lake, MN) and either a Teflon or hydrophilic Glidewire (Medi-Tech BSC, Watertown, MA) is used to traverse the 0.052-inch diameter coil in the PDA.
A 0.038-inch diameter coil is then implanted, coiling tightly inside the larger diameter 0.052-inch diameter coil.