In these pathologies (total anomalous pulmonary venous return, wide VSD, single AV valve, single ventricle, TGA, truncus arteriosus), the pulmonary system is not protected by stenosis (Figure 14.17 and Figure 14.21).



Figure 14.21: Diagram of a cyanotic R-to-L shunt involving a mixture of venous and arterial blood in a double-outlet single ventricle (SV). The image shows a case of hypoplastic right heart syndrome with tricuspid atresia (green arrow). The SV is of left ventricular anatomical type. Blood flows from the RV to the LV through a VSD (blue arrow). The pulmonary artery (PA) and aorta (Ao) stem from the same ventricle. Blood flows between the right and left-side circulation through a wide atrial septal defect (ASD).

Distribution of blood flow between the aorta and PA is directly dependent on the balance between systemic and pulmonary arterial resistance. Anatomical variations determine whether the highest level of blood flow is sent to the pulmonary or systemic vascular tree [2].

In heart diseases where arterialised blood mixes with venous blood, the degree of cyanosis (SaO₂ ≤ 85%) is dependent on the ratio between the pulmonary blood flow (Qp) and systemic blood flow (Qs). Since cyanotic children adapt to low SaO₂, the aim is to achieve a Qp/Qs ratio close to 1:1 rather than the highest possible SaO₂. This often equates to SaO₂ of 75-85%. Seeking optimal SaO₂ would risk excessive pulmonary blood flow. This would result in ventricular decompensation and low systemic blood flow combined with ischaemia and metabolic acidosis [2].

Choices in terms of haemodynamic management are determined by whether the systemic or pulmonary blood flow is predominant. Children with pulmonary overload are particularly difficult to stabilise: hyperventilation and hyperoxaemia further increase pulmonary blood flow and cause ventricular decompensation, while hypoventilation and low FiO₂ increase the shunt and cyanosis.

 

Bidirectional cyanotic shunt

Mixture of arterial and venous blood, excessive pulmonary blood flow Aim for Qp/Qs close to 1:1, even if SaO2 remains low

© BETTEX D, BOEGLI Y, CHASSOT PG, Juin 2008, June 2008, last update May 2018
 

 

 

References