Congenital heart diseases are defined as anatomical and morphological defects of the heart or great vessels at the time of birth. Congenital defects are formed primarily during the different phases of embryonic development or at birth or secondary to a drug or toxin. Most of the congenital heart diseases are inherited. Most common congenital heart diseases are patent ductus arteriosus, subaortic stenosis, pulmonic stenosis and tetralogy of fallot followed by other less commonly reported conditions like atrial septal defect, ventricular septal defect, dysplasia of atrioventricular valves and persistent right aortic arch. Most of the congenital diseases progress towards cardiac failure that can be managed by drugs or its correction by interventional procedures.
Etiology and Prevalence
Congenital heart diseases are defined as anatomical and morphological defect of heart or great vessels at the time of birth. Congenital defects are formed primarily during the different embryonic development phases or at birth or secondary to a drug or toxin. Most of the congenital heart diseases are inherited. Most common congenital heart diseases reported in various studies are Patent Ductus Arteriosus (PDA) , Subaortic Stenosis (SAS) and Pulmonic Stenosis (PS), Tetralogy of Fallot (TOF) followed by other less commonly reported conditions like Atrial Septal Defect (ASD), Ventricular Septal Defect (VSD), Tricuspid Valve Dysplasia (TVD), Mitral Valve Dysplasia (MVD) and Persistent Right Aortic Arch (vascular ring anomaly). Subaortic stenosis was commonly reported over PDA in some geographical areas. It might be due to local availability of breeds. Other congenital heart diseases like cor triatriatum and Peritoneal Pericardial Diaphragmatic Hernia (PPHD) are also reported. Higher prevalence rate of congenital heart diseases was reported in purebred animals than mixed-breed animals. Sex and breed predisposition of congenital heart diseases was also reported in dogs (MacDonald, 2006).This article describes in detail about the different congenital heart diseases and their diagnostic approach and its management.
Congenital heart diseases are classified according to anatomical defects and pathophysiology. According to pathophysiology it is divided into three categories primarily causing volume overload (PDA, MVD, TVD and VSD), pressure overload (SAS and PS) and cyanosis (TOF and Reversed PDA) (Fig.1).
Fig.1: Classification based on pathophysiology of congenital heart diseases
According to anatomical defects it is divided into extracardiac arteriovenous shunt (PDA), ventricular outflow obstruction (SAS and PS), intracardiac shunt (VSD and ASD), Atrioventricular valve malformation (MVD and TVD) and anomalies causing cyanosis (TOF) (MacDonald, 2006).
Patent Ductus Arteriosus (PDA)
Patent ductus arteriosus (PDA) is one of the most commonly observed defect in young animals and also reported in adult animals too. Ductus arteriosus is a shunt connecting pulmonary artery and proximal descending aorta during foetal life to bypass the non-functional lungs and it will be constricted and completely closed by fibrous tissue forming a structure called ligamentum arteriosum immediately after birth. This ductal closure is controlled by different factors like increased pulmonary blood flow which drops the pulmonary vascular resistance, increases oxygen level to and decreases Prostaglandins E2 levels. Failure of ductal closure is controlled by defective formation of elastic fibers and noncontractile fibers in aortic wall. It is otherwise called as persistent ductus arteriosus. Ductal diameter determines the different consequences and different patterns of PDA. Normally the pulmonary vascular resistance is low when compared to aorta (systemic circulation). This can lead to shunting of blood from aorta to pulmonary circulation, referred as left-to-right shunting (most common). When pulmonary vascular resistance increases than aortic (systemic) circulation blood will shunt from pulmonary artery to aorta, which is referred as right-to-left shunting or reversed PDA.
PDA has been reported in both dogs and cats. Higher frequency reported in female over male dogs. Some of the breeds predisposed to PDA are Chihuahua, Cocker Spaniel, German Shepherd Dog, Labrador Retrievers, Maltese, Poodle, Pomeranian and Yorkshire terrier (Ware, 2014).
Aortic Stenosis (AS)
Aortic stenosis is narrowing of left ventricular outflow tract dimension at the level of valve or below valve or above valve level. Narrowing of ventricle outflow tract below the valve level is called as subaortic stenosis (subvalvular form). Subaortic form is the second most common congenital heart defect in dogs. A fibrous ring or band or muscular form just below the aortic valve affects the left ventricular emptying. Stenosis of left ventricular tract leads to increased pressure overload on the left ventricle which forms concentric hypertrophy. Blood velocity also increased due to stenosis. Myocardial oxygen demand will be increased due to myocardial hypertrophy, increased wall tension and decreased capillary density. Certain large breed dogs are highly inherited with modifying genes (autosomal dominant) with these conditions in which include Newfoundland, Boxers, Golden Retriever, Rottweiler, German Shepherd Dog and Great Dane (Ware, 2014).
Pulmonic Stenosis (PS)
Pulmonic stenosis is a most commonly reported condition in case of small breed dogs. Common breeds affected are Beagle, Boxer, Chihuahua, Cocker Spaniel, English Bulldogs and Terriers etc. Narrowing of pulmonary arteries produces right ventricular outflow obstruction by simple fusion of valvular cusps or dysplastic valves. Dysplastic valves are asymmetrical and thickened and partially fused with hypoplastic valve annulus. Valvular form is most common (pulmonic valve dysplasia). Right ventricular hypertrophy due to increased pressure overload further leads to myocardial ischemia and finally to congestive heart failure. Most of the dogs are asymptomatic until diagnosis even with right side heart failure or with symptoms like exercise intolerance and syncope (Ware, 2014).
Ventricular Septal Defect (VSD)
Ventricles are separated by inter ventricular septum which made up of muscular portion and membranous portion nearby apex and base of the heart respectively. Septal defects are more common in the membranous portion. Septal defects are primarily causing volume overload which is determined by size of the defect and the resistance of the ventricles. Left to right shunting is most common because the left ventricular pressure is five times greater than right ventricle. English bulldog is having higher incidence than other purebred and mixed breed dogs. In some animals defects are closed within first two years of life. This closure might be due to myocardial hypertrophy around septal defect. Animals with small to moderate defects are maintains normal life span (Tilley et al., 2008).
Atrial Septal Defect (ASD)
Atrial septal defect are commonly present with other cardiac malformations as well. Defect in the region of the fossa ovalis is more common in dogs.
Atrioventricular Valve Malformation
Congenital malformation of mital valve and tricuspid valve includes dysplastic changes and stenotic lesions. Dysplasia of atrioventricular valves characterized by shortened fused or thickened elongated chordae tendineae and direct attachment of valve cusp to papillary muscles and thickened short or cleft valve cusps or prolapsed valve leaflets. Mitral valve dysplasia is most common in cats and some large dog breeds like Bull Terrier, German Shepherd Dog, Great Dane, Golden Retriever, Newfoundland and Dalmatian. Tricuspid dysplasia is most frequently reported in large breed dogs especially in male Labrador Retrievers. Tricuspid dysplasia is inherited in Labrador Retrievers (Ware, 2014).
Tetralogy of Fallot (TOF)
Tetralogy of Fallot consists of pulmonic stenosis, right ventricular hypertrophy, ventricular septal defect and dextraposition of aorta. Ventricular septal defect can be quite large in this condition and in some cases pulmonary artery is hypoplastic. Polygenic inheritance of this condition was identified in the Keeshound. Cyanosis, syncope, dyspnea, exercise intolerance and stunted growth are common symptoms reported (Ware, 2014).
Eisenmenger’s syndrome is a right to left shunting which is formed by large resistive defects like patent ductus arteriosus or septal defects or atrioventricular canal defect with an initial large left to right shunt caused pulmonary arterial hypertension. Due to larger defect, blood flow through pulmonary artery develops pathological changes that raised pulmonary vascular resistance higher than systemic vascular resistance. This condition results in cyanosis, secondary polycythemia and arterial hypoxemia (MacDonald, 2006).
Bounding pulse from femoral artery and precordial thrill over left side of the heart usually present with left to right shunting PDA in dogs. In case of reverse PDA more prominent pulse observed in right side apical region of heart. Hyperkinetic pulses are classic sign of PDA. Caudal cyanosis and polycythaemia (PCV greater than 60) are important signs of reverse PDA. Continuous machinery murmur heard on auscultation over cranial left heart base is a hallmark of PDA (left to right shunting) prominent at mid to late systole. These type of murmurs are absent in case of reverse PDA. Systolic murmurs normally heard over the left heart base area (PMI) are occasionally noticed in right heart base area and weak and late rising femoral pulses (pulsus parvus et tardus) noticed in left heart base area in aortic stenosis. A systolic crescendo-decrescendo type of murmur would be heard at left side heart base area on auscultation. In case of severe pulmonic stenosis precordial thrill can also be palpated over left side of thorax. Clinical signs of stenotic lesions include respiratory distress, tachypnea, syncope, exercise intolerance and cough. The characteristic auscultatory findings of holosystolic murmurs are noticed in the right heart base in severe ventricular septal defect and tetralogy of fallot. Symptoms noticed in left side congestive heart failure are dyspnea, tachypnea, pulmonary edema, exercise intolerance and syncope. Symptoms noticed in right side congestive heart failure are hepatomegaly, ascites, pleural effusion, pericardial effusion, limb edema, subcutaneous edema, distended jugular vein and caudal vena cava (Ware, 2014).
Elelctrocardiographic changes in patent ductus arteriosus are left ventricular and atrial enlargement ECG patterns higher amplitude R waves, deep Q waves. Increased duration of QRS and P waves are also observed. Normally ST segment abnormalities and ventricular tachyarrhythmias are reported in subaortic stenosis. Increased R amplitude, deep S waves, right axis deviation, atrial fibrillation and ventricular arrthymias are reported in pulmonic stenosis and tetralogy of fallot. Right ventricular enlargement patterns like deepening of S wave and right deviation of mean electrical axis are reported in pulmonic stenosis and tetralogy of fallot (Tilley et al., 2008; Ware, 2014).
Dorsoventral position provides three characteristic changes of PDA viz. include bulging of aortic arch, enlarged left atrium and pulmonary outflow tract. Left ventricular enlargement may also be noticed. In case of reverse PDA, enlarged right ventricle and pulmonary artery in lateral thoracic radiography. Based on the severity of stenosis, radiographic changes like dilatation of aortic arch (post stenotic dilataion) and left ventricular and atrial enlargement are observed in aortic stenosis. Dorsoventral view reveal reverse D shape due to right ventricular hypertrophy and enlarged pulmonary trunk at 1 o’clock position in pulmonic stenosis (Tilley et al., 2008).
Echocardiography is one of the highly sensitive noninvasive method used to diagnose and grade the different congenital anomalies. Right parasternal long axis and short axis (heart base) views are used to visualize left atrial and ventricular enlargement due to volume overload and pulmonary artery size respectively. Shunts can be visualized using blood flow pattern of color Doppler echocardiography in left cranial view of heart base. Reverse shunting can also occur due to pulmonary hypertension which leads to right ventricular hypertrophy. Contrast echocardiography was a useful technique for the diagnosis of reversed PDA. It is used to visualize anatomical changes like left ventricular hypertrophy and thickened papillary muscle and hyperechoic myocardium with calcium deposition due to fibrosis and ischemia. Doppler echocardiography can be used to measure blood flow velocity and pressure gradient which gives exact relative changes based on the severity of the condition. Pressure gradients above 100mmHg are considered as severe stenosis and pressure gradients between 80-100mmHg represents moderate stenosis. Anatomical changes like right ventricular hypertrophy and severity of pulmonary outflow obstruction can be visualized in 2D echocardiography. Doppler echocardiography is used to measure pulmonary regurgitation velocity. Echocardiography reveals left ventricular enlargement with or without right ventricular enlargement. Doppler studies are used to demonstrate the shunt flow between ventricles (Boon, 2011).
Cardiac Catheterization and Angiography
Routine technique used to assess the ductal size and other anomalies. In selective angiography, the site used to inject contrast material includes the aortic arch and pulmonary artery to confirm the diagnosis of left to right and right to left shunt respectively. Non selective angiography is used to diagnose right to left shunting. Cardiac catheterization and selective angiography can also be used to find out stenosis and other related cardiology defects. Vascular access for cardiac catheterization is usually via jugular vein or carotid artery in the neck region and femoral artery or vein in the inguinal region.
Patent Ductus Arteriosus
An average of 50-65 per cent of untreated dogs dies within twelve months. Treatment can be achieved by surgical correction or coil embolization. In most of the cases surgical correction is reported as successful. Coil embolization is a noninvasive procedure which can be performed without any complications by experienced cardiologist with short hospitalization procedure. In case of small dogs, catheterization of femoral artery is a difficult task. According to Stauthammer et al. (2015) low profile Amplatz canine ductal occluder prototype was successfully used to correct patent ductus arteriosus in a dog. Surgical procedure which needs thoracotomy has higher chances of haemothorax, hydrothorax and pyothorax and this limits its usage on a regular basis. Mortality reported in surgical procedure is around 8-12 per cent.
Eason et al. (2014) reported that usage of beta blockers in severe subaortic stenosis did not influence the survival rate of animals. Beta blockers are used to minimize the frequency and severity of arrhythmias and myocardial oxygen demand. Exercise restriction is advised in animals with moderate and severe subaortic stenosis. Prophylatic antibiotic therapy is needed before starting any procedures to prevent bacteremia.
Beta blockers are used to minimize the arrhythmias and myocardial oxygen demand. Exercise restriction is advised in animals with moderate and severe pulmonic stenosis.
Balloon valvuloplasty is a procedure to dilate the stenosis and reduce outflow obstruction by interventional catheterization. It reduces the pressure gradient about 50% and is favorable for 3-6 months (short term). Before doing valvuloplasty anatomical position of coronary artery is important to be considered. Sometimes abnormally developed pulmonary artery is encircled by coronary artery. Single aberrant coronary artery is found in Boxers and Bull dogs. Balloon valvuloplasty cannot be performed in dogs with aberrant coronary artery with significant annular hypoplasia (Francis et al., 2011). Conservative balloon valvuloplasty was carried out for the treatment of pulmonic stenosis in bull dogs with an aberrant coronary artery (Cripps and Dukes-McEwan, 2010). Fonfara et al. (2010) opined that conservative balloon valvuloplasty might be safe treatment for pulmonic stenosis in dogs with aberrant coronary artery in English bulldogs and might improve quality of life.
Ventricular Septal Defect
A low profile flexible Amplatzer Duct Occluder II was used to correct ventricular septal defect by interventional catheter procedure and combination of surgery (Saunders et al., 2013).
Tetralogy of Fallot
Open heart surgery is indicated in this condition. Clinical signs like severe erythrocytosis and hyperviscosity (rear limb weakness, lethargy, shortness of breath and seizures) of blood can be corrected by a method of periodic phlebotomy which removes blood (5-10ml/kg BW) simultaneously and administration of equal amount of isotonic fluid. Cote et al. (2001) reported that initial removal of blood (10 % of patient blood volume) without replacing fluids can be performed as an alternative method to reduce volume of packed red blood cells.
Other Cardiovascular Defects
Vascular Ring Anomalies
A persistent right aortic arch is the most commonly reported cardiovascular ring anomaly in German Shepherd Dog and Irish setters. During embryonic development, esophagus is surrounded on right side with aortic arch and left side with ligamentum arteriosum. Clinical signs include stunted growth and regurgitation immediately after weaning of dogs. This defect prevents solid food entry and forms esophageal dilation cranial to the vascular ring defect causing altered esophageal motility. Aspiration pneumonia is the secondary complication characterized by fever, cyanosis and respiratory symptoms which include wheezing, and coughing. Contrast radiography was a useful technique to visualize the esophageal stricture location. Surgical correction and medical management was advisable. In some cases persistent regurgitation was reported after surgical correction. These cases were managed with liquid or semisolid food eaten in an upright position (MacDonald, 2006).
Uncommon defect caused by an abnormal membrane that divides left (sinister) or right atrium (dexter). In cor triatriatum dexter, the coronary artery and caudal vena cava empty into right atrium below the intra-atrial membrane (MacDonald, 2006).
Peritoneal Pericardial Diaphragmatic Hernia
Congenital defect in pericardial sac is called as peritoneal pericardial diaphragmatic hernia (PPDH). This defect is common in ventral region of diaphragm and pericardium that allows abdominal structures to enter inside the pericardium. Abdominal structures commonly herniated are omentum and liver lobes and less commonly intestines, stomach and spleen, commonly reported in cats than dogs. Common clinical signs observed are dyspnea, tachypnea, vomiting and anorexia. Surgery is warranted in symptomatic animals (MacDonald, 2006).
The clinical diagnostic workup starts with thoracic auscultation which is used to localize and characterize the different type of murmurs. Diagnosis of congenital heart diseases includes physical examination, auscultation, electrocardiography and cardiac imaging techniques like radiography, echocardiography, cardiac catheterization and angiography. Cardiac catheterizations with selective angiographic procedures are used for diagnosis and treatment. Echocardiography is the promising noninvasive tool used for definitive diagnosis of congenital heart diseases. Transesophageal echocardiography is an important tool to aid in the interventional cardiac procedure. The most common congenital defects like Patent ductus arteriosus and pulmonic stenosis can be corrected by surgical or interventional cardiac procedures. Beta blockers have been used to manage mild to moderate level subaortic stenosis. Congestive heart failure is managed with drugs like Angiotensin converting enzyme inhibitors, diuretics, iotropics and inodilators. Surgical corrections of defects are limited because of its postsurgical complications. This article is an overview of diagnosis and management of congenital heart diseases in dogs.