Nonsurgical options available for correcting PDA in dogs

Q: Could you provide a brief review on the current nonsurgical procedures for correcting a patent ductus arteriosus (PDA) in dogs?

A: Transarterial and transvenous occlusion of using either theAmplatzer Duct Occluders or Gianturco vascular occlusion coils are now acceptedoptions for management of patent ductus arteriosus (PDA) in dogs. The followingarticles describe their current experiences in the nonsurgical managementof PDAs in dogs. Tobias A., Jacob K., Fine D., Carpenter D.: Patent ductusarteriosus occlusion with Amplatzer® Duct Occluders. Proc 20th AnnualForum ACVIM 20:100-101, 2002 and Miller M. W.: Transarterial coil occlusionof patent ductus arteriosus: outcome in 120 cases. Proc 20th Annual ForumACVIM 20:102, 2002.

The Amplatzer Duct Occluders are manufactured by AGA Medical Corp., GoldenValley, MN (www.amplatzer.com). The Amplatzer Duct Occluder device is madefrom an alloy of nickel and titanium, called nitinol. Nitinol has super-elasticproperties, together with excellent memory and strength, making it particularlyattractive for many medical device applications, especially where compactconfigurations are required for device insertion and placement.

During the last two years, veterinarians at the Cardiology Service, Universityof Minnesota College of Veterinary Medicine, have managed several casesof PDA in dogs using the Amplatzer Duct Occluder device.

This is a transcatheter occlusion device intended for the nonsurgicalclosure of PDA independent of the PDA's shape or size. It is a self-expandingdevice 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 devicein the PDA. Polyester patches, which are sewn into the device, facilitateductal occlusion by inducing thrombosis. The device is intended for usein human patients with a body weight of 5 kg, and the Cardiology Servicehas 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 thatrecommended for humans, which is well described on the AGA Medical Corp.Web site. Various anatomical differences between humans and dogs have necessitatedsome procedural modifications.

With the dog in right lateral recumbency, a cut-down to isolate the rightfemoral artery and vein is performed and introducer sheaths in both femoralvessels are placed. Then, an aortogram is performed with the contrast bolusdelivered close to the PDA in an effort to measure the size of the narrowestsegment of the PDA at the junction with the main pulmonary artery.

Using a graduated guide wire passed via the aorta and across the PDAfacilitates this measurement.

Overall, the Cardiology Service has been unimpressed with the abilityto measure the PDA dimensions accurately by this technique, seemingly dueto superimposition of contrast medium in the PDA and main pulmonary artery.

In human patients, the recommendation is to pass a pigtail catheter withside holes across the PDA via right heart catheterization, and to performa contrast injection within the lumen of PDA itself. While this method allowsvery clear PDA visualization and precise dimension measurements, it is verychallenging to pass any catheter retrograde across the PDA from the pulmonaryartery side in dogs, never mind a pigtail!

The Cardiology Service continues to seek a method to accurately measurePDA size angiographically. However, the Cardiology Service is usually ableto visualize the PDA very well echocardiographically, which enables obtainingan accurate measurement of the narrowest segment of the PDA at the pulmonaryartery insertion prior to the occlusion procedure.

Next challenge

Once the PDA dimensions are measured acceptably well with a combinationof echocardiography and angiography, an end-hole balloon wedge pressurecatheter (Arrow International, Inc., Reading, PA) is used to position anexchange 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 facilitatespulmonary artery catheterization, especially in smaller dogs. To date, theCardiology Service has managed to pass the exchange wire across the PDAwith a combination of good fortune, endurance, and more recently with theuse of snare catheters [Amplatz Goose Neck Snares and Microsnares, MicrovenaCorporation, White Bear Lake, MN (www.microvena.com)].

Occlusion device delivery

Once the exchange wire is across the PDA and in the aorta, the end-holeballoon wedge pressure catheter is removed, and delivery of the occlusiondevice can be done.

The occluder is delivered via a delivery system (Amplatzer Delivery System,AGA Medical Corp., Golden Valley, MN). This delivery system is composedof a dilator within a sheath, which is passed over the exchange wire viathe right heart, pulmonary artery, and PDA and into the aorta. Once thathas been achieved, the dilator and exchange wire are removed, leaving theintroducer sheath in place with its tip positioned within the aorta.

The Amplatzer Duct Occluders are manufactured in a variety of sizes (goto www.amplatzer.com for details). The Cardiology Service usually selectsan occluder with a barrel diameter of about 50 percent greater than thenarrowest segment of the PDA at the junction with the pulmonary artery.

The occluder is screwed onto a delivery cable, immersed in sterile salinesolution, compressed and loaded into a short length of tubing (the loader).The loader is introduced into the delivery sheath, and the device is advancedvia the sheath into the aorta. This part of the procedure has also posedsome challenges. As the occluder is advanced, the sheath frequently kinkswithin the right ventricle, especially in smaller dogs, making it difficultto advance the device. With sufficient manipulation, "getting aroundthe bend" can be managed. Once in the aorta, the occluder is partiallyextruded from the sheath to deploy the retention flange.

The entire assembly is then drawn into the PDA until the retention flangeengages at the pulmonary artery end of the PDA. Once the device is in thecorrect position, the occluder is fully deployed by stabilizing the cableand drawing the sheath off the occluder and into the pulmonary artery. Theoccluder barrel will develop an hourglass shape that indicates the deviceis deployed in the correct location, i.e., with the retention flange withinthe PDA, and with the barrel through the narrowest segment of the PDA andprotruding slightly into the pulmonary artery.

At this stage, the occluder is still attached to the cable, allowingrecapture of the occluder into the sheath, should that become necessary.An aortogram is then performed to ensure complete PDA occlusion. Once thatis confirmed, the cable is unscrewed from the occluder and withdrawn. Theintroducer sheaths are then removed, the femoral vein is ligated, and thefemoral artery is sutured with 6/0 silk material.

Overall, veterinarians in the Cardiology Service have found PDA occlusionwith Amplatzer Duct Occluder to be challenging, but extremely rewardingin 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 CardiologyService believes that PDA occlusion with the Amplatzer Duct Occluder representsan excellent therapeutic option that may be especially appropriate for largerdogs and larger PDAs.

Occlusion coils

Veterinarians at Texas A&M University College of Veterinary Medicinehave reported on the results of transarterial PDA occlusion using both 0.038inch and 0.052-inch diameter Gianturco occlusion coils. Since 1994, thisgroup 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 stenosisand subaortic stenosis. The Gianturco occlusion coil has proven to be verysafe 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 thecoil during implantation, but these "safety nets" increase thetechnical demands of the procedure. New devices with improved control arebeing 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, makesit 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 uncoilingduring implantation. This stronger coil provides improved positioning duringimplantation, and has been used successfully for PDA closure. The 0.052-inchdiameter Gianturco occlusion coil requires a larger catheter for implantation,which complicates its use in small dogs, especially if the coil embolizesand must be retrieved.

The dogs are anesthetized and placed in right lateral recumbency. A short6 French sheath is placed in the right femoral artery. Using a 4 Frenchtransseptal sheath positioned through the short 6 French sheath, an angiogramis performed in the descending thoracic aorta; the minimum and maximum PDAdiameters are measured and the morphology of the PDA is determined. Usinga 0.038-inch diameter Glidewire (Medi-Tech BSC, Watertown, MA), the transseptalsheath and dilator were then positioned either in the lumen of the PDA oradvanced 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 advanced5-8 mm out of the steel delivery tube in which the coil is packaged. Therounded ball on the end of the coil is "stretched out" approximately0.5 mm away from the coil windings using a hemostat allowing the bioptometo firmly hold onto the ball.

The bioptome is then advanced through a section of the housing from theGlidewire. The bioptome jaws are opened and placed around the ball on thecoil tip. The jaws are closed, and then the bioptome-coil is pulled intothe section of Glidewire housing.

This manipulation is necessary because once the bioptome is holding ontothe ball, the bioptome-coil will not fit through the steel coil deliverytube. The portion of Glidewire housing serves as an effective and inexpensivedelivery tube. The Glidewire housing loaded with the coil-bioptome is theninserted into the hemostasis valve of the 4 French transseptal sheath.

The bioptome-coil is advanced through the transseptal sheath. The coilis pushed slowly out of the transseptal sheath such that one loop of coilformed outside is either within the lumen of the PDA or the main pulmonaryartery depending upon initial position of the tip of the sheath. Once appropriateposition is confirmed, the bioptome-coil is advanced further, allowing severalloops of the coil to form in the aortic ampulla.

Coil positioning

If the sheath is within the main pulmonary artery, two-thirds to oneloop of coil is allowed to remain in that location. The bioptome jaws areclearly visible on fluoroscopy. If the positioning of the coils is suboptimal,the coil can be withdrawn into the sheath and the procedure repeated. Followingdeployment of the coil, an angiogram is performed in the descending aortato 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 sheathand dilator can frequently be advanced past the initial coil into the mainpulmonary artery allowing for controlled delivery of additional 0.052-inchdiameter coils. In cases in which the transseptal sheath and dilator willnot traverse the initial coil, a 4 French snare catheter (Microvena Corporation,White Bear Lake, MN) and either a Teflon or hydrophilic Glidewire (Medi-TechBSC, Watertown, MA) is used to traverse the 0.052-inch diameter coil inthe PDA.

A 0.038-inch diameter coil is then implanted, coiling tightly insidethe larger diameter 0.052-inch diameter coil.