Admixture lesions

Admixture lesions

Admixture lesions  are cyanotic congenital heart diseases (CCHD) with near equal saturations in aorta and pulmonary artery. They include:

• Total anomalous pulmonary venous connection (TAPVC)
• Total anomalous systemic venous connection (TASVC)
• Single atrium
• Tricuspid atresia
• Single ventricle
• Truncus arteriosus

In these lesions, there is chance for complete mixing of both systemic and pulmonary venous return so that final mixed sample is delivered to both aorta and pulmonary artery so that their saturations are near equal. In TAPVC the mixing occurs in the right atrium while in TASVC it occurs in the left atrium. In single atrium, single ventricle and truncus arteriosus, the admixture occurs in the corresponding regions.

In normal individuals systemic circulation and pulmonary circulation are in series so that there is no mixture of deoxygenated blood, except for the small amount of blood (less than 3%) which drains the bronchial veins into the pulmonary veins [1].

In CCHD with admixture physiology, a cardiac defect facilitates complete mixing of deoxygenated systemic venous blood and oxygenated pulmonary venous blood in a common  receiving chamber [1].

If the cardiac index is within the normal range, effective saturation of blood after completed admixture will be related to the amount of oxygenated blood coming from the lungs. Hence the systemic arterial saturation will reflect the pulmonary blood flow in an admixture lesion [2].

Often the mixing in the common chamber is not complete and can result in different saturations in pulmonary artery and aorta. If the pulmonary venous return streams preferentially into the aorta, it is known as favourable streaming as aortic saturation will be higher. If pulmonary venous return streams selectively into the pulmonary artery, it is unfavourable streaming as aortic saturation will be lower. Typical examples for both these types of streaming can be drawn from different types of TAPVC. In supracardiac TAPVC draining into superior vena cava, the pulmonary venous return can stream selectively to the tricuspid valve, right atrium, right ventricle and pulmonary artery. This constitutes unfavourable streaming. In infradiaphragmatic TAPVC draining to inferior vena cava, streaming can occur selectively to the left atrium across the atrial septal defect. This will be favourable streaming.

References

1. Tharakan JA. Admixture lesions in congenital cyanotic heart disease. Ann Pediatr Cardiol. 2011 Jan;4(1):53-9.
2. Burchell HB. Total anomalous pulmonary venous drainage: clinical and physiologic patterns. Proc Staff Meet Mayo Clin. 1956 Mar 21;31(6):161-7.