Pathophysiology

Foetal Circulation

Foetal circulation relies on 3 shunts to direct blood away from the developing lungs and liver. The lungs are filled with fluid during development which causes a high resistance to pulmonary blood flow.

During gestation, the Foramen Ovale acts as a flap valve and is held open by the difference in pressure between the right and left atria. The left atrium has a low pressure because there is little blood returning from the lungs whereas pressure in the right atrium is higher as it receives systemic venous return.

Transition to Neonatal Circulation

Duct dependent lesions

• The timing of presentation of CHD can be related to whether lesions are duct-dependent or not. Some neonates with CHD remain dependent on the Ductus Arteriosus to allow mixing of blood to maintain systemic or pulmonary circulations.
• Presentation of these lesions is usually around the time the DA closes, which occurs due to withdrawal of prostaglandin E2. In the foetus, prostaglandin E2 production is very high because of the placenta and this plays a role in keeping the Ductus Arteriosus open. At birth, prostaglandin levels fall due to proper functioning of the lungs, changes in neonatal blood pressures in the systemic and pulmonary circulations and removal of the placenta.
  • They can either present be with subtle symptoms when the duct is just beginning to close such as poor feeding, sleepiness and tachypnoea
  • Or can present as the collapsed neonate with cyanosis and shock when closure has occurred whereby a Prostaglandin infusion is required to maintain patency.
• For some duct dependent lesions, a Blalock-Taussig (BT) shunt is used to maintain blood flow to the pulmonary artery
  • Synthetic graft is used to create a shunt between the right subclavian artery and the right pulmonary artery
  • Another surgical shunting method is to create a conduit between the right ventricle and the pulmonary artery (RV-PA Conduit)

Learning Bite

Foetal circulation relies on three shunts which close shortly after birth. Some CHD lesions depend on DA patency so present with cyanosis/shock when this closes. These are the duct-dependent lesions.

Presentations of CHD

In order to characterise congenital heart abnormalities, it is useful to split them into how they may present to the ED. The most common presentations of CHD in the neonatal and early childhood period are shock, cyanosis or congestive heart failure.  It is important to note that these present differently in children compared to adults.

Table showing timing of presentation of CHD. [3] Abbreviations TAPVD: Total Anomalous Pulmonary Venous Drainage, TOF: Tetralogy of Fallot, TGA: Transposition of the Great Arteries, HLHS: Hypoplastic Left Heart Syndrome, AS: Aortic Stenosis, PDA: Patent Ductus Arteriosus, ASD: Atrial Septal Defect, VSD: Ventricular Septal Defect.