Cardiac Embryology

Objectives
 
At the completion of this chapter the user will be able to:
 
  • Discuss the Normal Anatomy Variants of the Heart
  • Describe the Common Pitfalls
  • Discuss the Types of Artifacts
  • Describe Common Artifacts
  • Explain the Common Methods to Avoid Misinterpretation of Artifacts
Introduction
 
Normal structures or variants, which can be misinterpreted as pathological conditions, are called anatomic pitfalls. In contrast, artifacts are errors in interpretation of images due to the inherent properties of the ultrasound technology being used. It is important to be familiar with the normal structures that appear to be abnormal and common artifacts to facilitate recognition and avoid unnecessary interventions.

Anatomic Pitfalls

Cardiac Embryology

 

An understanding of cardiac development facilitates the understanding of anatomic pitfalls as many normal variants are remnant structures from in utero development and circulation. Further, some minor abnormalities of embryological development can result in common but usually inconsequential a findings seen with transesophageal echocardiography (TEE).

The right and left atria and the sinus venosus evolve in the 4th week of embryonic development.  Initially the sinus venosus receives blood from left and right sinus horns, which attach to the developing heart posteriorly. The veins to the left sinus horn are then obliterated and the remnants of the left sinus horn become the coronary sinus, which is found posteriorly in the AV groove. Conversely, the right sinus horn enlarges and forms the smooth-walled part of the right atrium. As the right atrium expands, the smooth atrial tissue displaces the primitive trabeculated atrial tissue into the adult right atrial appendage (RAA) (which may create pectinate muscles). The right sinus horn further develops into vestigial structures seen in the adult heart:

1) Superiorly, the structure between the smooth atrial tissue and the trabeculated part of the right atrium is the crista terminalis.

2) Inferiorly, the valve of the inferior vena cava or the eustachian valve is formed , which serves to direct blood flow from the inferior vena cava across the foramen ovale in utero.

3) Inferiorly, the valve to the coronary sinus , the thebesian valve is formed. The chiari network is also thought to be derived from sinus venosus remnants.

Embryology of the Heart - 4th Week Gestation
 
Also within the atria, migration of the atrial septum to create separate right and left atria leads to the characteristic appearance of the foramen ovale, with the possibility of a patent foramen ovale. The septum primum and ventricular septum grow towards and fuse with the endocardial cushion.  The septum primum obliterates the fossa ovalis.  The septum primum base dissolves forming an ostium secundum.  The septum secundum grows towards the endocardial cushion and obliterates the ostium secundum.   Failure of the septum primum and septum secundum forms a patent foramen ovale.  Patent foramen ovale is present in 25-30% of patients.
Embryologic Formation of Atrial Septums

In the left atrium, the smooth tissue of the pulmonary veins is incorporated into the wall of the left atrium; this tissue displaces the trabeculated tissue into the left atrial appendage (LAA), usually creating significant muscle ridges within the LAA . Further, the junction of the left atrial appendage and the left superior pulmonary vein appears to be a ridge of tissue, which has been called the coumadin ridge.

In contrast to normal development, a failure of cardiac vein development and reabsorbtion may lead to a persistent left superior vena cava, which connects to and creates a dilated coronary sinus.   The animation to the right shows the anterior cardiac vein (ACV), posterior cardiac vein (PCV), umbilical vein (UV), and vitilline vein (VIT) in the embryo as they progress to the superior vena cava (SVC), inferior vena cava (IVC), coronary sinus (CS), and left superior vena cava (LSVC).  The LSVC does not disappear in persistent left superior vena cava.  A bracheocephalic vein may be present, absent, or a small remnant may be present.
Cardiac Vein Development
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The growth of the ventricles is due to the outward growth of the myocardium and diverticulation of the internal walls. Therefore the ventricles appear trabeculated , usually with the right more heavily trabeculated than the left. The most significant muscle band in the right ventricle is the moderator band, found in the apical third of the right ventricle. The heart also becomes suspended from the cranial and caudal ends, within a pericardial sac. Occasionally a left ventricular band can be observed as well.