Interventional procedures for treatment of congenital cardiac disease in small animals

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Mavropoulou A. DVM, Ms, PhD, MRCVS, Diplomate of the European College of Internal Medicine (Specialty of Cardiology), RCVS Recognized specialist in Cardiology, Plakentia Veterinary Clinic

MeSH keywords: Patent ductus arteriosus, dog, pulmonic stenosis

Abstract

Percutaneous cardiac interventions were introduced in veterinary medicine in the 1970s and the field has had a remarkable growth since then. Nowadays a wide variety of procedures are being performed including balloon dilation and stent implantation for stenotic valves or vessels, closure of abnormal vessels with devices or coils, closure of septal defects with devices, intracardiac pacing for symptomatic bradyarrhythmias, biopsy of intracardiac masses and extraction of parassites. The present article reviews the most common procedures that are performed in congenital cardiac diseases.

Introduction

Interventional cardiology is an advanced field of cardiology that uses minimally invasive catheter- based techniques to diagnose and treat several structural heart diseases. During the procedures, imaging (usually fluoroscopy alone or together with echocardiography) provides guidance and allows catheterization of the heart from peripheral blood vessels so that diagnostic tests (e.g. angiography) and therapeutic management of the disease can occur.

  Percutaneous cardiac interventions were introduced in the 1960s in human medicine (Faxon and Williams 2016) and have been described since the 1970s in veterinary medicine (Musselman et al 1976, Bright et al 1987). Since the first reports this field of veterinary cardiology has expanded and nowadays includes a wide variety of procedures like balloon dilation and stent implantation for stenotic valves or vessels (Scansen 2015, Kleman 2015), closure of abnormal vessels with devices or coils (Gordon and Miller 2005, Gordon et al 2010, Leach et al 2010), closure of septal defects with devices (Bussadori et al 2007, Gordon et al 2009, Saunders et al 2013), intracardiac pacing for symptomatic bradyarrhythmias (Oyama et al 2001, Hildebrandt et al 2009, Estrada 2015), biopsy of intracardiac masses and extraction of parassites (Saunders 2015). This article focuses on those procedures that are performed in congenital cardiac disease.

Pulmonary valve stenosis
Pulmonic stenosis (PS) is the most common congenital heart disease in dogs (Oliveira et al 2011, Schrope 2015) and can be classified into supravalvular, valvular and subvalvular stenosis. Of the three, valvular pulmonic stenosis is the most commonly reported in dogs (Schrope 2005). The valvular stenosis can be differentiated into 2 main types. Type A is characterized by normal annular size, fusion of the valve leaflets that are mildly thickened and post stenotic dilation of the pulmonary artery. Type B is characterized by hypoplasia of the annulus and the main pulmonary artery and various degrees of leaflet thickening/ immobility with minimal fusion (Bussadori et al 2000). Cases with aspects of both type A and type B stenosis also exist.

  When considering whether balloon valvuloplasty (BVP) is indicated for a patient, the severity and type of stenosis, the presence of clinical signs and the concurrent cardiac defects/ systemic disease are considered. The severity of the stenosis can be determined by several methods but the most used one is the indirect measurement via continuous wave Doppler in echocardiography. By measuring the peak velocity of the pulmonary artery flow, the peak pressure gradient through the stenosis is estimated; mild stenosis is defined as a 10-50 mmHg pressure gradient; moderate stenosis is when the gradient is 50- 80 mmHg and severe stenosis when the gradient is >80 mmHg (Kittleson 1998, Martin and Dukes-Mc Ewan 2010).

  Generally, dogs with mild PS have normal life span and remain asymptomatic throughout life. BVP is not recommended in these patients. Dogs with severe PS are at increased risk of developing clinical signs, including syncope, exercise intolerance, congestive heart failure and sudden death (Francis et al 2011). BVP is strongly recommended in these patients as studies have confirmed that dogs live longer after intervention (Johnson et al 2004, Ewey et al 1988). In patients with moderate stenosis, recommendations are not clear; however a study suggests that dogs with a pressure gradient above 60 mmHg and tricuspid regurgitation have an increased probability of cardiac death and BVP is usually considered in these patients (Francis et al 2011).

  The type of stenosis can affect the success of BVP and should be considered when the success rate of BVP is discussed. Dogs with type A stenosis have a greater improvement in severity of PS compared to type B stenosis (Bussadori et al 2001). Current recommendation, however, is to consider performing BVP in all patients with severe obstruction regardless of the type of stenosis (Schrope 2005).

The procedure
Two approaches are possible for BVP, the jugular vein or the femoral vein, and the access to the vein can be achieved by surgical cut-down (which allows visualization of the vessel) or transcutaneous approach (Seldinger technique).

  Regardless of the type of approach and the vein used, with the patient under general anesthesia, a vascular sheath is placed in the vein and this provides access to the vessel for the catheters and guidewires that are used during the procedure.

  Under fluoroscopic guidance, a first catheter is passed into the heart and pulmonary artery and the pressures within the pulmonary artery, the right ventricle and the right atrium are measured and recorded. The catheter is then replaced by an angiographic catheter, usually a pigtail, that is placed in the right ventricle. An angiogram is performed and measurement/ accurate localization of the pulmonary valve annulus is achieved. Based on the measurement, the balloon catheter size is selected. The angiographic catheter is then replaced by an exchange guidewire, with the help of an end-hole catheter, that is passed into the pulmonary artery. The selected balloon catheter is then advanced through the guide-wire across the stenosis and inflated. The inflation is typically repeated at least twice. A “waist” of the stenosis against the balloon is visualized at the beginning and it is reduced after inflation indicating that the “ballooning” was successful. If the waist persists then the balloon is re-inflated or is replaced by a bigger balloon. Then the balloon catheter is removed and pressures are re-assessed (Schrope 2005). Once all catheters have been removed from the heart the vascular introducer is also removed and, in the case of the transcutaneous approach, pressure is applied for 15 minutes. In the case of the surgical approach, the jugular vein is ligated and the skin is closed routinely.

  The procedure is considered successful if the gradient across the stenosis is reduced by at least 50% after BVP (Thomas 1995). Follow-up echocardiograms are usually performed 24h, 3 months and 6-12 months post-surgery while, beyond this point, re-examinations depend on the degree of residual stenosis. Uncommonly, significant re-stenosis can occur (Schrope 2005, Bussadori et al 2001).

  Newer treatment options have been developed to address the variable results of balloon valvuloplasty on annular hypoplasia and/or severely thickened (dysplastic) valves. These include high-pressure balloon valvuloplasty (that uses balloon dilation catheters with higher inflation pressures than normal balloon catheters for better results) (Belanger et al 2018), cutting balloon valvuloplasty (balloons that have microblades on their outer surface causing controlled tears at the valve that is then ruptured more effectively by a high-pressure balloon) (Goya et al 2018, Markovic and Scansen 2019) and intravascular stent placement of the right ventricular outflow tract (RVOT) (Scansen 2017, Scansen 2018, Markovic et al 2020, Borgeat et al 2021). Results with these techniques are variable and, especially as far as the two latter techniques are concerned, are not performed routinely.

Patent ductus arteriosus (PDA)
PDA is recognized as one of the most common congenital heart diseases in dogs with an incidence that ranges between 17% and 26% (Oliveira et al 2011, Schrope 2015, Brambilla et al 2020). Female and toy-breed dogs (Maltese, Pomeranians, Poodles, Yorkshire Terriers) are overrepresented. The disease is usually suspected based on the clinical signs and the characteristic murmur and it is confirmed by echocardiography. In the typical PDA, with a left to right shunt, blood flows from the aorta to the pulmonary artery through the ductus. The increased blood flow causes pulmonary overcirculation, volume overload of the left atrium and left ventricle and leads to left-sided CHF. Some dogs are born with or develop pulmonary hypertension due to the chronic volume overload and reversal of flow through the PDA (from the pulmonary artery to the aorta) may occur (right to left or reversed PDA) (Kittleson and Kienle 1998, Buchanan 2001).

  If ligation or occlusion is not performed, most dogs with typical PDA develop congestive heart failure (CHF) within the first year of age; for this reason, ductal closure is recommended in these cases (Kittleson and Kienle 1998, Buchanan 2001). On the other hand, occlusion is contraindicated in dogs with right-to-left shunting because of severe pulmonary vascular disease or in dogs with concurrent cardiac conditions that rely on the PDA for survival (e.g. tetralogy of Fallot) (Kittleson and Kienle 1998, Buchanan 2001).

  Various techniques for PDA closure have been described and include surgical PDA ligation, thoracoscopic PDA occlusion and transcatheter occlusion. Selection of the method of closure depends on the patient’s size, PDA size and morphology as well as cost of the procedure.

  The first report of PDA surgical closure in a dog was reported in 1952 while, since 1994, when the first transvascular PDA closure was performed, minimally-invasive percutaneous techniques have been employed in dogs and have become the treatment of choice for most cardiologists (Stokhof et al 2000, Glaus et al 2002, Sisson 2003, Gordon and Miller 2005, Campbell et al 2006, Hogan et al 2006, Nguyenba and Tobias 2007, Smith and Martin 2007, Nguyenba and Tobias 2008, Achen et al 2008, Henrich et al 2010, Hildebrandt et al 2010, Blossom et al 2010, Gordon et al 2010, Singh et al 2012). A variety of occlusion devices intended for human use, such as thrombogenic or embolization coils (Stokhof et al 2000, Gordon and Miller 2005, Campbell et al 2006, Schneider et al 2007, Henrich et al 2010, Hildebrandt et al 2010, Blossom et al 2010, Gordon et al 2010), Amplatzer vascular plug (Smith and Martin 2007, Achen et al 2008), Amplatzer Duct Occluder (Glaus et al 2002, Sisson 2003), and different routes, such as transvenous and transarterial (Stokhof et al 2000, Schneider et al 2001, Sisson 2003, Hogan et al 2006, Achen et al 2008, Henrich et al 2010, Hildebrandt et al 2010, Blossom et al 2010, Stauthammer 2015), have been used. More recently, the Amplatzer Canine Duct Occluder (ACDO), a device developed specifically to fit the shape of the canine PDA, has replaced the previously used devices, for its ease of use, degree of PDA closure and low complication rate (Nguyenba and Tobias 2007, Nguyenba and Tobias 2008, Gordon et al 2010, Singh et al 2012) and it is now the device of choice for the majority of PDA occlusions38,44.

The procedure
The ACDO is a self-expanding device, made of 2-3 layers of nitinol mesh, with two distinct disks: a smaller distal disk designed to expand into the pulmonary artery and a bigger proximal disk that expands into the ductal ampulla. The two disks are separated by a short “waist” that occludes the PDA (Nguyenba and Tobias 2007).

  With the patient under general anesthesia and in lateral recumbency the femoral artery is surgically isolated and cannulated with a vascular sheath. Under fluoroscopic guidance, a sizing pigtail angiographic catheter is introduced and advanced into the aorta immediately cranial to the junction between the aorta and PDA. This catheter has radiopaque reference markers near its tips and allows accurate measurement of the minimal ductal diameter (MDD) for ACDO selection. Angiography is performed by injecting iodine contrast solution with an angiographic injector and allows visualization of the ductal morphology as well as accurate measurement. If transesophageal echocardiography is available PDA sizing as well as procedure monitoring can be achieved without the need for angiography limiting (Silva et al 2013) or eliminating (Porciello et al 2014) fluoroscopy exposure.

  Following ACDO size selection, a flexible exchange wire is placed across the PDA from the aorta into the main pulmonary artery with the aid of an angle-tipped end-hole catheter. A guiding catheter and its dilator are then advanced over the exchange wire across the PDA into the pulmonary artery. The dilator and the exchange wire are then removed and the selected ACDO is advanced through the guiding catheter until the first disk is deployed within the main pulmonary artery. The partially deployed ACDO and the catheter are then retracted and the second disk is released into the ductus ampulla. Correct ACDO position and stability are confirmed and the ACDO is detached. The catheter and vascular sheath are then removed and the femoral artery is repaired (Nguyenba and Tobias 2007). The incision is then closed routinely.

Subvalvular aortic stenosis (SAS)
Aortic stenosis is one of the most common congenital cardiac diseases in dogs and cats (Oliveira et al 2011, Schrope 2015). Subvalvular stenosis is the most common form of the disease while valvular stenosis is more rarely seen. In large breed dogs and particularly in certain breeds (Boxer, Bull Terriers, Bullmastiffs, German Shepherds, Dogues de Bordeaux, Golden Retrievers, Newfoundlands and Rottweilers) the disease is more common (Ontiveros and Stern 2021). Subvalvular stenosis is characterized by the presence of fibromuscular nodules, a ridge or a ring of fibrous tissue located underneath the aortic valve that cause obstruction of the left ventricular outflow tract. Dogs with severe disease are at increased risk for sudden death that occurs commonly early in life (within the first 3 years of age) (Kienle et al 1994).

  Over the years, treatment options have included: medical therapy (use of beta-adrenergic blockers (i.e. athenolol)), transcatheter balloon valvuloplasty and open-heart resection of the fibrotic ridge or ring. However, the available treatment options remain controversial; surgical and interventional options have not showed a survival benefit compared to treatment with atenolol (Orton et al 2000, Meurs et al 2005) and, similarly, treatment with atenolol did not influence survival compared to control group in dogs with severe stenosis (Eason et al 2014). The use of cutting balloon catheter (a balloon that features 3-4 microsurgical blades on its outer surface) combined with high-pressure balloon valvuloplasty (balloons with increased inflation pressures) has been described in dogs (Kleman et al 2012) and results show that this procedure is a potential option for symptomatic dogs with severe disease (Schmidt et al 2010, Kleman et al 2013, Sykes et al 2020). The rationale behind this procedure is that the microblades of the cutting balloon cause controlled tears to the obstruction/ vessel and allow to the high-pressure balloon to stretch and dilate it more effectively. Previous reports show that reduction in severity gradient is possible in some dogs with severe disease and owners report improvement of clinical signs. However, data comparing the benefits of this procedure to medical treatment or no treatment are currently lacking.

The procedure
With the patient under general anaesthesia and lateral recumbency the right carotid artery is accessed and cannulated with a vascular introducer. Under fluoroscopy, a pigtail catheter is advanced through the aortic valve into the left ventricle and aortic root for pressure measurements and angiography. The diameter of the subvalvular obstruction and the aortic annulus are measured, and the appropriate cutting balloon and high-pressure balloon are selected. A thin guidewire is advanced through the pigtail catheter into the left ventricle and the catheter is removed. The cutting balloon is then advanced over the guidewire and inflated 2-3 times rapidly at the level of the subaortic obstruction. The cutting balloon and the guidewire are then removed.

  The pigtail catheter is placed again into the left ventricle and an exchange stiff guidewire is placed into the left ventricle through the pigtail. The pigtail catheter is then removed and the high-pressure balloon catheter is advanced over the guidewire and across the center of the SAS. The balloon is inflated 2-3 times and both the balloon dilation catheter and the guidewire are removed. Left ventricular and aortic pressure measurements and another angiography (to evaluate the degree of aortic regurgitation) are performed. Finally, the vascular introducer is removed and the carotid artery is ligated. The incision is then closed routinely (Kleman et al 2012).

Other congenital cardiac defects

Atrial and ventricular septal defects
Percutaneous transcatheter closure of atrial and ventricular septal defects has been reported in dogs (Sanders et al 2005, Bussadori et al 2007, Margiocco et al 2008, Gordon et al 2009, Gordon 2015, Durham et al 2015).

  Atrial septal defects (ASD) are relatively rare congenital anomalies in dogs and cats and long-term prognosis is usually good for small-sized, isolated ASDs (Margiocco et al 2008). Large ASDs (> 12 mm) and the presence of concurrent congenital or acquired disease may worsen prognosis and intervention may be indicated (Gordon et al 2009). Ventricular septal defects (VSD) are common in cats and dogs and are typically located in the perimembranous portion of the septum, high in the ventricular septum beneath the aortic and pulmonary valve. Most animals with isolated, small, “restrictive” VSDs have a very good prognosis and normal life expectancy (Bomassi et al 2015).

  Device occlusion is usually recommended when a hemodynamically significant shunt exists causing moderate to severe cardiac remodeling and the anatomy of the defect is favorable for occlusion (sufficient tissue rim around the defect). Devices that have been used for this purpose include various types of Amplatzer (Amplatzer Atrial Septal Occluder, Amplatzer Patent Foramen Ovale (PFO) Occluder, Amplatzer Multi-Fenestrated Septal Occluder, Amplatzer muscular Ventricular Septal Defect Occluder) and detachable coils (Gordon et al 2009, Gordon 2015). Hybrid techniques have also been described where a surgical approach (usually through direct access to the left atrium) is combined with the transcatheter techniques (Gordon et al 2009, Saunders et al 2013, Gordon 2015).

  Access to the heart is obtained through the femoral or jugular vein and fluoroscopy and transesophageal echocardiography are used for device selection, wire and catheter exchange, secure device positioning and release.

Tricuspid valve stenosis
Tricuspid valve stenosis is a rare congenital malformation in dogs and cats (Oliveira et al 2011, Schrope 2015). It is more commonly seen in Labrador Retrievers, Dogue de Bordeaux, German Shepherds, Irish Setters and Great Danes. Severe cases develop right-sided congestive heart failure (CHF) and syncope/episodic weakness that affect quality of life and life expectancy (Kittleson 1998, Navarro-Cubas et al 2017). There are only limited reports of balloon valvuloplasty in dogs with tricuspid stenosis (Brown and Thomas 1995, Kunze et al 2002, Navarro-Cubas et al 2017, Lake-Bakaar et al 2017) that show improvement or resolution of clinical signs post-procedure. However, recurrence of clinical signs, in particular ascites, may occur in these dogs and the procedure is contraindicated in patients that have moderate-to-severe tricuspid regurgitation (TR) to begin with, due to the risk of TR worsening and development of severe CHF following valvuloplasty (Lake-Bakaar et al 2017).

Cor triatriatum dexter (CTD)
CTD is a rare congenital defect associated with the failure of the right sinus venosus valve to regress during embryogenesis causing presence of an intra-atrial membrane in the right atrium. This membrane may be imperforate or perforate to varying degrees and causes obstruction of the blood flow through the caudal vena cava towards the right atrium. Clinical signs include hepatic congestion and ascites or cyanosis due to right-to-left shunt flow through a patent foramen ovale (Kittleson and Kienle 1998, Moral et al 2016).

  Several minimally-invasive techniques have been described for CTD treatment and include conventional balloon dilation, cutting-balloon dilation and intravascular stent implantation (Adin and Thomas 1999, Atkins and DeFrancesco 2000, Leblanc et al 2012, Barncord et al 2016). Hybrid balloon dilation has also been described (Uemura et al 2019).

Double-chambered right ventricle
Double-chambered right ventricle (DCRV) is a rare congenital cardiac anomaly resulting from anomalous muscle or fibromuscular band that divide the right ventricle into two compartments: a proximal, high pressure, chamber and a distal, low pressure, chamber, close to the pulmonary artery (Winter et al 2021, Schober et al 2017). Due to the obstruction the proximal chamber becomes hypertrophied and as the disease progresses clinical signs develop like exercise intolerance, syncope, right-sided CHF and sudden death. To prevent or alleviate clinical signs early intervention is indicated. Techniques like balloon angioplasty, cutting balloon followed by high-pressure balloon dilation, hybrid surgery-interventional procedure and open-heart surgical excision have been described. Most of the interventional procedures reported in veterinary literature provide temporary but not long-term relief of clinical signs and surgical approach seems to be the preferred treatment for DCRV patients (Winter et al 2021, Schober et al 2017).

Conclusion

Techniques that are currently commonly performed to treat congenital diseases in small animals were reviewed in this report. Interventional cardiology continues to expand in veterinary medicine and more techniques, that are routinely used in the human field, are becoming available for animal treatment as equipment and training advance.

Conflict of interest
The author declare that there is no conflicts of interest

Corresponding author:
Tonia Mavropoulou
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