Ventricular septal defect (VSD) in the dog and cat

Ventricular septal defect (VSD) is a congenital heart anomaly characterized by the presence of one or more openings in the interventricular septum that separates the left and right ventricles of the heart.

AETIOLOGY

VSD is the result of an abnormal development of the interventricular septum. During normal septal development a proliferation of connective tissue allows the union between the ventral septal tissue and the atrioventricular cushions. When such proliferation does not occur normally, part of the primary interventricular foramen may remain patent, giving rise to one or more interventricular defects.

PREVALENCE

VSD is one of the most common congenital disorders of the cat. In the dog, the breeds considered at risk are the West Highland White Terrier, English Bulldog and English Springer Spaniel; however, given the multitude of breeds affected, the hypothesis of a congenital origin of this disease tends today to be discarded in favour of the presence of a random alteration occurring during the embryogenetic process.

CLASSIFICATION

VSDs are classified according to the anatomical location of the defect (Fig. 1):

• Perimembranous VSD: the most frequent one, involving the fibrous portion of the septum.
• Membranous VSD: caused by septal malalignment (tetralogy of Fallot).
• Canal type VSD (inlet): located in the proximity of the mitral and tricuspid septal leaflets.
• Subpulmonary, infundibular, supracristal or conal VSDs: located below the pulmonary semilunar valve.
• Muscular VSD: located on the trabecular portion of the interventricular septum.

PATHOPHYSIOLOGY

The amount of blood passing through the VSD and the direction of the shunt are determined by the size of the defect and by the pulmonary resistance present.

The presence of a small opening identifies a so-called restrictive defect; this term, used in human literature, indicates the ability to "restrict" the increased left ventricular pressure without transmitting it to the right ventricle. In such cases, the right ventricular systolic pressure remains lower than the left ventricular systolic pressure and the shunt maintains a left-to-right direction. In the presence of restrictive defects a left side volume overload is present, proportional to the size of the defect and to the magnitude of the shunt present.

The defect is instead defined as non-restrictive when the opening is large and there is no resistance against the blood flowing through it; in non-restrictive defects the blood flow distribution is only regulated by the resistance provided by the pulmonary and by the systemic circulation; in such cases the shunt volume becomes important and the right and left ventricular pressures tend to equalize. Non-restrictive defects are always associated with secondary pulmonary hypertension.

In adults, the pulmonary vascular resistance is usually much lower than the systemic resistance, as it gradually decreases from birth until the third or fourth month of life. Should this not be the case, a condition called Eisenmenger complex (often referred to, incorrectly, as Eisenmenger syndrome) is present, characterized by the presence of a congenital defect with right-to-left shunt and primary pulmonary hypertension.

CLINICAL SIGNS

Patients with smaller openings are usually asymptomatic and the only clinical sign present is a harsh holosystolic heart murmur, mostly audible cranially, on the right, close to the sternum. In association with the VSD murmur, a systolic murmur in the pulmonic area consequent to pulmonary hyperflow may be detected.

Occasionally, perimembranous defects result in a loss of aortic valve structural support and aortic valve regurgitation may be present. In such cases the VSD systolic murmur  is associated with a diastolic murmur (systolic-diastolic murmur) which at auscultation can simulate a continuous murmur (to-and-fro murmur).

Clinical signs, when present, are those characteristic of left heart failure (exercise intolerance, coughing, dyspnoea). Large VSDs are rarely diagnosed in veterinary medicine as affected puppies die prematurely: patients who survive may experience an attenuation of the murmur, up to its complete disappearance, following reversal of the shunt.

DIAGNOSIS

RADIOLOGY

Pulmonary vascular hyperflow( pulmonary overcirculation) is present with small/moderate-size ventricular septal defects, associated with a variable enlargement of the left atrium and ventricle (Fig. 2). When pulmonary hypertension is also present this can  cause dilatation of the main pulmonary artery, reduction in the pulmonary artery  and vein caliber and right ventricular hypertrophy.

ECHOCARDIOGRAPHY
When VSD is suspected, echocardiography is the examination of choice, not only to confirm the diagnosis but above all to define the size, location and haemodynamic consequences of the defect. The echocardiographic examination allows direct visualization of the defect or, with colour Doppler, the identification of a turbulent flow through the opening itself. The suggested views vary depending on the location of the VSD. The right parasternal five-chamber view (standard 2) allows the visualization of most perimembranous defects; it does not however allow the visualization of muscular, supracristal and inlet defects (Fig. 3). The short axis right parasternal views allow instead a complete scan of the left ventricle, from apex to base, and allow a more accurate visualization and localization of each defect (Fig. 4). Doppler imaging allows the assessment of the flow direction and of the peak velocity through the defect. By calculating the pulmonary  (Qp) and systemic (Qs) blood flow it is also possible to classify VSDs as restrictive (Qp/Qs <1.5), moderate (1.5< Qp/Qs <2) and non-restrictive (Qp/Qs >2).

THERAPY AND PROGNOSIS

In the presence of restrictive VSDs, no specific medical treatment is necessary and the long-term prognosis is favourable. In the presence of heart failure, a normal congestive heart failure treatment is used.

The percutaneous closure of the VSD using an Amplatzer occluder (Amplatzer muscular VSD occluder) has been described. The feasibility of the intervention is influenced by the size and location of the defect. In the presence of larger defects and in the absence of Eisenmenger’s syndrome, banding of the pulmonary artery is possible. This procedure increases the right  ventricular afterload, thereby reducing the magnitude of the left-to-right shunt. This technique is not of easy execution and the result depends on the surgeon’s experience.

In the presence of Eisenmenger’s syndrome the prognosis is poor. In this case the treatment is symptomatic and the goal is to control both polycythaemia and the hyperviscosity syndrome. 

Suggested reading

1. Bonagura JD, Lehmkuhl LB, Congenital heart disease. In: Fox PR, Sisson DD, Moise NS, Textbook of Canine and Feline Cardiology. Ed. Saunders, Philadelphia, 1999: 499-505.
2. Bussadori C, Carminati M, Domenech O. Transcatheter closure of a perimembranous ventricular septal defect in a dog. J Vet Intern Med. 2007; 21(6): 1394-400.
3. Bussadori C, Pradelli D. Cardiopatie congenite. In: Santilli RA, Bussadori C, Borgarelli M, Manuale di cardiologia del cane e del gatto. Ed Elsevier, Milano, 2012: 180-194.
4. Kittleson MD, Kienle RD, Septal defect. In: Kittleson Md, Kienle RD, Small Animal Cardiovascular Medicine. Ed. Mosby, St Louis, 1998: 233-239.
5. Margiocco ML, Bulmer BJ, Sisson DD. Percutaneous occlusion of a muscular ventricular septal defect with an Amplatzer muscular VSD occluder. J Vet Cardiol 2008; 10 (1): 61-6.
6. Ware WA, Patologie cardiovascolari congenite. In: Ware WA: Malattie dell'apparato cardiovascolare cane e gatto. Ed. Utet, Torino, 2008: 241-243.