​Atrial Septal Defect (Secundum Type) in Dogs- A Study for 15 Years

Congenital heart disease (CHD) is a morphologic defect of the heart or associated great vessels present at birth that are caused by alterations or arrests in particular phases of embryonic development of the fetal heart (Macdonald, 2006). Atrial septal defect (ASD) is one of the common congenital heart diseases (CHDs) in humans, but considered relatively rare in veterinary patients. However, advancement of modern diagnostic facilities such as 2d-echocardiography and doppler techniques is facilitating detection of these defects earlier in awake animals (Chetboul et al., 2006a). Dogs may be presented asymptomatic or with signs of heart failure and are diagnosed for ASD alone (Chetboul et al. 2006a) or the atrial septal anomaly may be associated with other congenital cardiac abnormalities like ventricular septal defect and tricuspid dysplasia (Guglielmini et al., 2002). The present paper puts on record about clinical and diagnostic aspects of ASD in dogs.

The study was carried out among the dogs of various breed and gender that were presented with the history and signs suggestive of heart failure and also those that were routinely referred for complete check up to cardiology ward, Veterinary Clinical Complex of CVSc., Rajendranagar during a period of 15 years (2006 to 2020). All these dogs were subjected for detailed clinical examination followed by physical examination. Chest auscultation was carried out in a sound proof environment, on specific points at left and right cardiac region. Thoracic plain radiography was performed on both the right lateral and ventro dorsal views. Further, 2d-echocardiography was performed using GE logiq Doppler machine and 6S cardiac probe to diagnose ASD in these dogs. Both short and long axis of cardiac chambers were evaluated to detect the septal anomaly on B-mode and whereas, dimensions of left ventricle was evaluated on M-mode echocardiography. Later Doppler evaluation of ASD dogs was also carried out to study the blood flow direction through the septal defect. These dogs were medically managed with angiotensin converting enzyme inhibitors, diuretics and positive ionotropic drugs and all the cases showed clinical improvement.

2dimensional echocardiography was performed as right parasternal transthoracic echocardiograms in unsedated suspected dogs by placing them in right lateral recumbency. Access to the right side of the thorax was facilitated by using a table with a special cut-open to allow the transducer to be directed upward towards the site of maximal cardiac pulsation (Allworth et al., 1995). Transducer is located parasternally between right third and sixth intercostal spaces between sternum and costochondral junction (Thomas et al., 1994). M-mode recordings were taken at the high papillary level and the measurement of left ventricular dimension at end–diastole (LVEdD) and end-systole (LVEsD) was made intraluminally from the trailing edge of the septal wall image to the leading edge of the ventricular free wall. End–diastolic and end–systolic measurements of the thickness of the Inter Ventricular Septum (IVSd, IVSs) and left Ventricular Posterior Wall (LVPWd, LVPWs) were made using trailing edge (Allworth et al., 1995). Further, colour flow Doppler was also done to assess the directional flow as per the technique suggested by Dominique and Marc-Andre (2008).

Signalment and signs
 
A total of 1358 dogs of various breed and gender that were presented to cardiology ward of VCC of CVSc Rajendranagar were diagnosed for various acquired (93%) and congenital cardial diseases (7%), accounting for 1263 and 95 cases, respectively. Out of the total congenital diseases (95), ASD was diagnosed in 7 cases (7.5%). Congenital anomalies of the cardiovascular system are defects present at birth and often lead to perinatal death in dogs. However, in some cases, congenital heart diseases are asymptomatic and undetected until later in life, so the percentage of dogs with congenital heart diseases that survive to adulthood to breed can be rather high (Garncarz et al., 2017 and Caivano et al., 2018). These dogs were aged between 2y-6y and the majority of breeds include Boxer (43%) followed by German shepherd, Cocker spaniel, Daschund and Pomeranian. Boxer and Domestic shorthair was the most common canine and feline breeds affected (Chetboul et al., 2006a). The main studies report different prevalence of congenital heart diseases (CHDs) in the affected breeds, depending on the popularity of the breed in a country in a given period of time (Ghirlanda et al., 2014). Knowing the epidemiology of CHDs plays an important role in maintaining dog health and in preventing the diffusion of CHDs in the dog population (Garncarz et al., 2017).

Auscultation of cardiac area revealed, soft systolic murmur on left heart base (78%) and pulmonic valve area (56%), low intensity heart sounds (48%) and whereas, physical examination determined distended jugular vein (32%), jugular pulse (24%) and ascites (29%).  Most of the dogs (53%) were presented with the history and signs suggestive of heart failure and few (21%) were asymptomatic. The common clinical manifestation recorded among symptomatic cases include, reduced exercise tolerance (18.2%), dyspnoea (11.3%), general weakness (10.4%), jugular pulse (6.2%), cough (5.2%) and syncope (2.8%). According to the location of the defect within the interatrial septum, three types of ASD have been defined; primum ASD is located in the most apical portion of the atrial septum; secundum ASD involves the fossa ovalis region and third type that is present in the upper portion of the septum is sinus venosus ASD (Chetboul et al., 2006a). Of all these types, secundum ASD is the most commonly seen septal defect among dogs (Guglielmini et al., 2002; Chetboul et al., 2006a; Macdonald, 2006). In dogs, most of the ASDs are secundum type and without clinical signs, but some are presented with certain common signs that included systolic murmur over the left heart base, exercise intolerance, dyspnoea, syncope and cough. Intensity of a systolic heart murmur over the left base is directly proportional to the size of the defect (Chetboul et al., 2006a). Systolic murmur at the left heart base also helps to differentiate the type of the congenital heart disease, for that matter, a low intensity left heart base systolic murmur signifies an isolated ASD whereas a high intensity murmur suggests SAS, either isolated or associated with a concurrent CHD. The ASD is a common cause of left heart base systolic murmur among Boxers (Chetboul et al., 2006b). A low intensity of heart beats, jugular pulse, tachypnea and ascites are some of the common manifestations among ASD dogs (Cavalcanti et al., 2009).
 
Radiography
 
Radiographically, varied degrees of right ventricular / atrial enlargement, pleural effusion and prominence to the pulmonary vessels indicating pulmonary overcirculation were noticed on lateral and ventrodorsal views (Fig 1-2).The diagnosis of congenital heart disease in dogs is not only important for the sake of the affected pet, but also to prevent these defects in successive generations by eliminating these patients from the breeding pool (Macdonald, 2006). Survey thoracic radiography revealed pleural effusion and right atrium enlargement (Cavalcanti et al., 2009).



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Echocardiography
 
2d-echocardiography revealed gross dilatation of right atrium and right ventricle, as well as identification of the defect as a loss of echogenicity at the interatrial septum. The right parasternal B-mode examination revealed interatrial communication of varied size, enlargement of main pulmonary artery and discrete, thickened mitral and tricuspid valves but with normal left atrium:aorta base ratio (Fig 3-4). M-mode aspects of left ventricle lumen include, increased left ventricle lumen at end diastole with hyperkinetic interventricular septum (IVS), increased fractional shortening, systolic function and paradoxal septal motion (Fig 8). The echocardiography demonstrates interatrial communication and colour Doppler confirms a left-to-right shunt with a continuous flow on spectral Doppler echocardiography at the defect region. Dilated right atrium, right ventricle and main pulmonary artery, an increased systolic function associated with a high fractional shortening, thickened atrioventricular valve leaflets with mild to moderate regurgitation were the echocardiographic findings among ASD dogs (Cavalcanti et al., 2009). ASD is considered relatively rare in veterinary medicine. Its prevalence has been reported to vary from 0.7 to 3.7% of all canine congenital cardiovascular anomalies (Guglielmini et al., 2002). Echocardiographic and Doppler techniques offer a good view of the morphology of the myocardium and thus, facilitating earlier detection of ASD in asymptomatic awaken animals (Chetboul et al., 2006a). Whereas, the colour flow Doppler study demonstrated the left-to-right shunt and a continuous flow was observed on spectral Doppler echocardiography (Fig 5 to 6) and regurgitation jet at the tricuspid valve area (Fig 7). The echocardiography combined with color-flow Doppler mode was the method that is chosen to confirm ASD and to determine the severity of the shunt (Oliveira et al., 2011). Isolated ASD usually result in a left-to-right shunting across the septal defect, because the left side heart chambers have a higher pressure than that of the right heart, thus the extra blood from the left atrium may cause a volume overload in right heart chambers, may lead to dilatation of right heart (Chetboul et al., 2006b). Similarly, the abdominal ultrasound evaluation revealed anechoeic space with floating viscera, ascites and mild to moderate hepatomegaly with hepatic congestion. Right heart failure may further lead to ascites, pleural effusion, pulmonary overcirculation, paradoxal septal motion and enlargement of the main pulmonary artery (Oliveira et al., 2011). Adult dogs with ASD may develop cardiac insufficiency in due course, secondary to myxomatous mitral valve degeneration (Cavalcanti et al., 2011).












 
Medical management
 
Following institution of therapy with angiotensin converting enzyme inhibitor (ACEi), diuretics and calcium sensitizers, alleviation of signs were noticed from day 7-14 with an improvement in energy levels, physical activity and overall, well-being among the present symptomatic dogs. All the dogs survived a near normal life and the survival period ranged between 3 yr to 6 yr after the diagnosis and initiation of therapy. Treatment of small septal defects may not be necessary. Palliative treatment and medical management of septal defects of larger size and for those with heart failure signs include arterial vasodilation to reduce the severity of the shunt along with diuretics and angiotensin converting enzyme inhibitors. Positive inotropic drugs may be needed if there is myocardial failure, often as a result of severe aortic insufficiency (Cavalcanti et al., 2011). Usually, the prognosis depends on the severity of the defect, the dogs with isolated and small-sized ASD is usually good (Gugleilmini et al., 2002).

ASD in dogs is relatively rare congenital heart disease that is diagnosed incidentally as majority of cases were asymptomatic or presented with non cardiac signs. The echocardiography helps to diagnose and the prognosis is usually fair to good if the defect is small and without any other congenital heart disease. However, the diagnosis of any congenital heart disease in dog population is also important to eliminate further breeding. 

All authors declare that they have no conflict of interest.