Matthew W. Miller, DVM, MS, DACVIM (Cardiology)
College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX, USA
In its broadest sense, interventional cardiology or interventional catheterization refers to any catheterization, during which some type of therapeutic procedure is undertaken. In human medicine, the most common application of interventional cardiology is dilation of coronary artery stenosis known as percutaneous transluminal coronary angioplasty (PTCA). Interventional catheterization techniques were first applied to congenital heart disease in children in 1966, when Dr. William Rashkind reported the use of balloon catheters to create atrial septal defects in children with transposition of the great vessels. Since that time, guidelines have been put forth for pediatric therapeutic catheterization covering indications and contraindications for balloon and blade atrial septostomies, balloon dilation of all four cardiac valves, balloon dilation of peripheral vessels including the aorta and pulmonary arteries, implantation of occlusion devices, foreign body retrieval, and ablation of abnormal conduction pathways (Allen et al., 1991). A more practical definition for veterinary medicine would include catheterization procedures during which obstructive lesions are dilated (ballooned), abnormal vascular communications are occluded, abnormal conduction pathways are interrupted (catheter ablation), or foreign bodies are retrieved. In this chapter, we will address only balloon dilation and catheter occlusion.
The application of balloon dilation in the therapy of congenital pulmonic, subaortic and tricuspid stenosis has been reported. In 1987, Bright et al. reported the first successful application of balloon dilation in the therapy of pulmonic stenosis in a dog. Since that time, several reports have confirmed the safety and efficacy of this procedure. The long-term efficacy of balloon dilation of subaortic stenosis (SAS) is less universally accepted. Delellis and her colleagues reported the results of balloon dilation in nine dogs with SAS and suggested that balloon dilation was beneficial; however, long-term results have been less encouraging. Preliminary results from Lehmkuhl and Bonagura suggest that despite very favorable early results, in less than half the dogs with SAS undergoing balloon dilation is significant (≥ 50%) gradient reduction at six months maintained. Important questions that remain unanswered regarding balloon dilation of SAS include whether or not balloon dilation improves quality or quantity of life, and whether or not sudden death is significantly altered by the procedure. Ongoing studies using more aggressive dilation techniques (cutting balloons coupled with rigid, high-pressure dilating balloons) warrant attention.
Catheter Occlusion (PDA, ASD, VSD)
Occlusion of abnormal vascular communications (typically congenital) represents an important application for interventional catheterization. The two most common congenital vascular malformations encountered in veterinary medicine include portosystemic shunts (PSS) and patent ductus arteriosus (PDA). There is a single case report of successful transvenous occlusion of an intrahepatic portosystemic shunt in a dog (Partington et al., 1993). In this case report, multiple procedures were utilized to sequentially deploy Gianturco vascular occlusion coils to gradually occlude an intrahepatic shunt. One of the potential complications associated with occlusion of PSS is the development of portal hypertension. This potentially catastrophic complication is one that must be overcome before this procedure will enjoy wide application.
There have been several reports describing the application of vascular occlusion devices in the therapy of left-to-right shunting PDA. These reports have suggested that percutaneous ductal occlusion is a viable alternative to surgical ligation. There are several devices available for percutaneous ductal occlusion, some of which have been developed specifically for veterinary applications.
Success of the procedure is predicated on deployment of an appropriately sized device in the correct position within the ductus. Knowledge of ductal anatomy is paramount, as morphologic variations of the ductus may preclude appropriate engagement of the deployed coil. It is also imperative to accurately determine minimal ductal diameter to decide whether or not the patient is a candidate for percutaneous ductal occlusion. Currently, the only way by which to reliably evaluate ductal morphology is angiography. Although the ductus can easily be identified using transthoracic echocardiography, critical description of the three-dimensional anatomy of the ductus using this imaging modality is limited. Transesophageal echocardiography has been shown to provide excellent anatomic information, sometimes obviating the need for angiography.
Angiography is performed from the femoral artery with a multiple side hole catheter advanced to the level of the descending aorta. Rapid, high-pressure injection of contrast material is essential to adequately outline the ductus (Figure 1A). If ductal anatomy is such that a device can be safely deployed, a catheter is advanced into the pulmonary artery through the ductus and the device deployed. In most patients, occlusion occurs rapidly (less than 15 minutes) with the use of one or two coils (Figure 1B). It has been our experience that once the device has been deployed, dislodgement is very uncommon, but can occur. In some patients, trivial residual flow is present, even after device deployment. This is much less common with the routine use of the Amplatz canine ductal occluder, when compared to the incidence of residual flow reported associated with coil occlusion. This flow is not associated with an auscultable murmur, but is rather detected with color flow Doppler techniques. The importance of this hemodynamically trivial residual flow is the subject of much debate. It has been our experience and that of other investigators that the vast majority of patients with persistent left-to-right shunting have complete resolution of residual flow within 12 months of device deployment.
Complications we have encountered associated with the procedure include local hemorrhage at the arterial access site, trace residual shunting as described above and suboptimal deployment of the device resulting in either pulmonary or systemic embolization. Although coils embolized to the pulmonary circulation have not been retrieved, they have not been associated with any demonstrable clinical symptoms. Assessment of pulmonary vascularity using nuclear profusion scanning suggests minimal long-term alteration in pulmonary perfusion.
Our group has reported our experience with the use of Amplatz atrial septal defect (ASD) occluders in a clinical cohort of dogs made up mostly of standard poodles. The technique of ASD occlusion is much more technically challenging than is that of PDA occlusion. The incidence of early and late dislodgment is substantially higher and, because of the size of the device, is associated with much more dramatic clinical complications. Multimodal imaging, including transesophageal echocardiography, is essential to determine if the lesion lends itself to percutaneous occlusion, and if so, choosing the appropriate device. Because of the anatomy of ASD dogs, hybrid procedures should be considered. We have utilized hybrid techniques, not only for occlusion of atrial septal defects, but most recently in the successful occlusion of a muscular ventricular septal defect.
It is the author's belief that the majority of patients with a left-to-right shunting patent ductus arteriosus are candidates for percutaneous ductal occlusion. We have successfully occluded left-to-right shunting PDAs in dogs, ranging in size from 2.1–50 kg, using Gianturco vascular occlusion coils and the Amplatz canine ductal occluder. Continued development of occlusion devices has certainly made percutaneous ductal occlusion more commonplace. Broader application of balloon dilation procedures is expected. The author has used balloon dilation to successfully relieve the obstruction associated with cor triatriatum dexter on numerous occasions with excellent results. The use of interventional catheterization in dogs and cats with portosystemic vascular anomalies shows promise and will most likely expand. The author reported a transvenous approach to identify and catheterize portosystemic shunts. Improved visualization and assessment of the hemodynamics associated with PSS will most likely increase our ability to occlude both intra-and extra-hepatic malformations percutaneously.
Although the application of interventional catheterization techniques is appealing, it is imperative not to embrace these techniques simply because they are less invasive and potentially less costly. We must always critically evaluate the safety and efficacy of these techniques. This is especially true when interventional catheterization techniques are applied to disease processes for which established and highly successful therapies are available.
VIN editor: Figures 1A and 1B were not provided at time of publication.