Normal Anatomy and its Relationships at Birth

The relative orientation of right and left ventricles is determined by cardiac looping. Morphologically, the left ventricular endocardium is smoother with finer trabeculations than the right ventricular endocardium which is highly trabeculated. Normally the morphologic right ventricle lies to the right and anterior relative to the morphological left ventricle, referred to as "D-loop". Reversal in direction of looping results in the morphological right ventricle lying posterior and to the left of the morphological left ventricle, which is then referred to as "L-loop".

Normally the aorta and its aortic semilunar valve are connected to the morphologic left ventricle and found posterior and rightward. The pulmonary trunk and its pulmonic semilunar valve are committed to the morphologic right ventricle and found more anterior and leftward.

Cardiac malpositions and heterotaxy syndromes

Cardiac malposition refers to an abnormal location of the heart within the chest or relative to the abdominal organs. Abnormal sidedness is highly associated with other complex cardiac anomalies. In the period between 1988 and 2002, at Children’s Hospital in Boston, 724 patients were diagnosed with cardiac malposition (1%) and had a mortality rate of 19% (ch.39, Keane).

Three terms are commonly used to describe the location of the heart within the chest:

• Dextrocardia indicates that the heart is located in the right side of the chest rather than left.
• Mesocardia refers to displacement of the heart toward the right, but not completely in the right chest.
• Levocardia refers to a heart located wholly in the left side of the chest.

These positions can be further classified based on the visceroatrial sinus which refers to the relative position of the right and left atria to abdominal organs:

• Situs solitus—normal connection of visceral venous vessels (superior and inferior vena cava), which positions the stomach and spleen toward the left and appendix toward the right. The right lobe of the liver is normally larger than the left and lies on the right side.
• Situs inversus—situation occurring when the positions of abdominal organs are reversed. In this case, the stomach and spleen lie on the right, the right (dominant) lobe of the liver lies to the left, and the appendix and inferior vena cava to the left. The left (rather than right) lung is typically tri-lobed.
• Situs ambiguous—a group of anomalies characterized by a lack of sidedness. The abdominal organs may be positioned to either the right or the left side with the liver typically lying in the midline. In addition, these patients are often found to be either polysplenic or asplenic; the former typically having left-side dominance of atrial and pulmonary structures and the latter expressing right-side dominance.

Importantly, the position of the heart in the chest and with relation to the abdominal organs does not indicate anything about the specific segments or connections between organs. As such, an understanding of the physiology and clinical manifestation of these defects requires segmentation and analyses of the heart, as well as knowledge of non-cardiac abnormalities.