Atrial septal defect (ASD)

What is ... ?: 

In dogs, as in people, the heart is made up of 4 chambers: the right atrium, the right ventricle, the left atrium, and the left ventricle.  As the name implies, an atrial septal defect is a defect or hole in the muscular wall -the atrial septum- that normally separates the right and left atria.  It is often referred to in layperson's terms as a "hole in the heart," which is correct but potentially misleading: there is no hole causing blood loss out of the heart, but rather just an opening between two regions within the heart that should normally be separated from each other.

The result of an atrial septal defect is unnecessary recirculation of blood inside the heart.  An atrial septal defect can therefore be considered a needless shortcut: part of the blood meant to go out to the circulation instead returns within the heart.  This disturbance is inefficient, and the workload on the heart increases in order to maintain an adequate circulation.  With large atrial septal defects, the result may be exhaustion of the heart and congestive heart failure, but with small atrial septal defects, dogs often do not know they have it and there is no long-term adverse consequence.

How is ... inherited?: 

The mode of inheritance is not known for atrial septal defects, but many congenital cardiac defects are believed to have a polygenic mode of inheritance, with variable expression. This means that many individual genetic contributors must exist to result in an atrial septal defect.

What does ... mean to your dog & you?: 

The main determinant of the impact of an atrial septal defect is its size.  A small defect usually will be of no significance to a dog, and indeed most dogs with small atrial septal defects live normal lives.

With larger defects, however, there will be abnormal blood flow from the higher pressure left side of the heart across the defect to the right side. This causes more work for the heart , which can eventually lead to congestive heart failure. Signs may include respiratory difficulties, fainting, tiring with exercise, abnormal cardiac rhythms, and abdominal swelling due to fluid retention from the disrupted circulation.

How is ... diagnosed?: 

Often, as with most heart defects, the first indication of a problem is when the veterinarian hears a heart murmur during a puppy's first physical examination for vaccinations (8-10 weeks of age). In the most serious cases, with very large atrial septal defects, there may be visible symptoms, such as exercise intolerance or respiratory difficulties, that prompt a visit to the veterinarian, where an atrial septal defect is discovered.

A heart murmur alone is never conclusive for atrial septal defects, because many other cardiac defects cause similar murmurs, and many pups may have healthy hearts and a murmur that goes away after a few weeks as part of normal growth.  Therefore, if the features of a heart murmur make the veterinarian suspicious of the possibility of an atrial septal defect, then he or she will recommend tests, especially thoracic radiographs (chest X-rays) and an echocardiogram (also called cardiac ultrasound or sonogram of the heart).  These noninvasive tests can identify clearly whether an atrial septal defect is present, and if it is, whether there is reason for concern (and whether treatment is necessary).


How is ... treated?: 

In humans, surgical closure of an atrial septal defect is commonplace, and can be undertaken either with open-heart surgery or with minimally-invasive catheter-based interventions.  In dogs, such operations are challenging and rarely undertaken.  In the absence of symptoms, most dogs with atrial septal defects do not receive treatment.  Thankfully, most dogs with atrial septal defects have small defects that are of no consequence.  Dogs whose atrial septal defect is sufficiently large to cause symptoms receive oral medications at home to alleviate the symptoms rather than treating the underlying cause.

For the veterinarian: 


  1.    MURMUR: classically, soft, mid-systolic ejection murmur; loudest in pulmonic area, left cranial thorax.  The murmur is due to overcirculation -left-to-right shunting ASD- through a normal pulmonic valve, causing a murmur of relative pulmonic stenosis.  Audible splitting of the second heart sound is possible but often very subtle.
  2.    ELECTROCARDIOGRAM: often normal; evidence of atrial enlargement (abnormally large P waves) is possible.
  3.    RADIOGRAPHS: may be normal with small defects; with larger defects, evidence of left and right atrial enlargement is common; evidence of right ventricular hypertrophy, and pulmonary overcirculation, also are variably present dependent on magnitude of shunting.
  4.    ECHOCARDIOGRAPHY: diagnostic test of choice for confirming presence and magnitude/size of atrial septal defect, and ruling in or ruling out concurrent defects.
Breeding advice: 

Affected individuals and their parents should not be used for breeding. Siblings should only be used after careful screening.



Wey A.  Atrial septal defect.  In Cote E, ed.  Clinical Veterinary Advisor: Dogs and Cats, 2nd ed (St. Louis, MO: Mosby Elsevier, 2011) pp. 113-115.
Oyama MA, Sisson DD, Thomas WP, Bonagura JD.  Congenital heart disease.  In Ettinger SJ, Feldman EC, eds.  Textbook of Veterinary Internal Medicine, 7th ed (St. Louis, MO: Saunders Elsevier, 2010) pp. 1250-1298.

What breeds are affected by ... ?