Atrial septal defect

Atrial septal defect (ASD) is the second most common congenital heart defect after ventricular septal defects (VSDs) and the most common to become symptomatic in adulthood.

They are characterized by an abnormal opening in the atrial septum allowing communication between the right and left atria. Due to the low pressures of the atria, the lesion is typically asymptomatic until adulthood despite 2-4 times the normal pulmonary blood flow. Gradual (high output) congestive cardiac failure may eventually develop, becoming symptomatic by the age of 30 years.

Epidemiology

Atrial septal defects account for ~10% of congenital heart disease ⁷. There may be greater female predilection.

Associations

Atrial septal defects are seen in association with the following:

• Down syndrome (secundum ASD) ¹¹
• Holt-Oram syndrome
• Ellis-van Creveld syndrome
• mitral valve prolapse
• Lutembacher syndrome
• anomalous pulmonary venous return (especially with sinus venosus defects)
  • total anomalous pulmonary venous return (TAPVR)
  • partial anomalous pulmonary venous return (PAPVR)

Clinical presentation

Most patients are asymptomatic but as cardiac failure develops they may present with shortness of breath, palpitations, and weakness ⁷. Chest auscultation classically reveals an ejection systolic murmur heard at the left upper sternal border, attributed to increased flow across the pulmonary valve rather than blood shunting across the defect itself ⁷. 

ECG

There are characteristic ECG findings in patients with an atrial septal defect.

• incomplete right bundle branch block (RBBB morphology with QRS duration between 110-120 ms) 
  • increased specificity with crochetage sign in the inferior (II, III, aVF) leads
• right precordial (V1-3) "defective T waves"
  • describes biphasic morphology, initial T wave flattening or inversion and sharp, positive upstroke to terminal positive deflection ⁹
  • peak of the T wave delayed when compared to lateral precordial leads
• notching of the terminal upstroke of the R wave (crochetage sign) correlates with the size of ASD and implies a greater degree of shunting ¹⁰
• left axis deviation
  • characteristic of ostium primum defects and anatomical distortion of the left bundle branch fascicles
  • associated first degree AV block
• right axis deviation
  • suggests ostium secundum defect
• low atrial ectopic rhythms
  • negative P wave polarity in lead II
  • found in sinus venosus ASDs

Pathology

Classification

An understanding of complex cardiac development is important in understanding the numerous possible abnormalities that can arise. There are four major types of atrial septal defect (ASD) ⁴, distinguished according to their location within the septum:

• secundum ASD
  • 60-90% of all ASDs
  • usually an isolated abnormality
• primum ASD
  • 5-20%
  • associated with cleft anterior mitral valve leaflet (partial atrioventricular septal defect)
• sinus venosus
  • 5%
  • associated with anomalous right pulmonary venous return to the superior vena cava or right atrium
• coronary sinus type ASD ("unroofed coronary sinus")
  • <1%

A patent foramen ovale (PFO) is a form of atrial septal defect.

Radiographic features

Plain radiograph

• can be normal in early stages when the atrial septal defect is small
• signs of increased pulmonary flow (pulmonary plethora or shunt vascularity)
  • enlarged pulmonary vessels
  • upper zone vascular prominence
  • vessels visible to the periphery of the film
  • eventual signs of pulmonary arterial hypertension
• chamber enlargement 
  • right atrium
  • right ventricle
  • note: left atrium is normal in size unlike VSD or PDA
  • note: aortic arch is small to normal

Treatment and prognosis

Atrial septal defects do not cause any impairment in cardiac function in utero and even most neonates are asymptomatic. The defect can be closed surgically or percutaneously (e.g. atrial septal occlusion device). However, careful evaluation has to be made to ensure lack of development of elevated right heart pressures or a right to left shunt before any intervention.  

Complications

In approximately 10% of untreated patients, pulmonary hypertension develops. In this situation, flow through the shunt eventually reverses and becomes right-to-left leading to cyanosis, known as Eisenmenger syndrome.

Other complications include:

• paradoxical emboli
• cardiac conduction defects, e.g. atrial fibrillation, atrial flutter