Morphology of the Atria: Part One

If you are an adult sonographer transitioning into the field of pediatric echosonography, you will quickly learn that knowing the didactics of various pediatric defects is just the tip of the iceberg. There are about 30 major defects and numerous less major defects, and all of these defects may occur in conjunction with each other e.g. double outlet right ventricle with d-transposition, or pulmonary atresia with or without ventricular septal defect.

Just adding to the confusion, pulmonary atresia with septal defect in certain circumstances may be be considered a severe type of tetralogy of fallot.

Do not panic! If you are new to this field I will try to give you some tips on how to make your ultrasound scan professional and readable to a pediatric cardiologist.

Your job is to identify each structure of the heart and determine flow dynamics. You do not diagnose the defect involved, but if you perform your job correctly, you will enable the cardiologist to diagnose the echocardiogram appropriately.

For the typical echocardiographer (unless you work at a childrens hospital), you will be evaluating PDA’s (patent ductus arteriosis), ASD’s (atrial septal defects), VSD’s (ventricular septal defects), or pulmonary hypertension most of the time. When a complex defect occurs, take a systematic approach to the heart, and you will do just fine.

A systematic Approach to Imaging
Divide the heart into three sections. The atria and all of its venous connections; the “ventricular mass” or the ventricular section of the heart that may be one or two ventricles where one ventricle may be hypoplastic and one dominant; the arterial outflow portion which may consist of an independent aorta and pulmonary artery, or a common arterial trunk in which the aorta and pulmonary artery are merged, or an aortal trunk in which the pulmonary ateries derive from this structure.

It is not easy to identify the right atrium from the left atrium via transthoracic echo. It is much easier with TEE (transesophageal echo). Do not assume that if the cavae flow into an atrium that it is the right atrium. By the same token, do not assume that if the pulmonary veins drain into an atrium it is the left atrium.

For example, in the case of total anomalous pulmonary venous return, the pulmonary veins drain into a “confluence”, or large truncal vein that will drain into the right side of the heart at some point.

The right atrium is differentiated from the left based on the atrial appendage.

The right atrial appendage is a large “triangular” shaped structure with pectinate muscles that surround the orifice. The left atrial appendage is a smaller, tubular shaped structure.

The atria may be “concordant” with their particular ventricles (RA to RV, right atrium to right ventricle, LA to LV, left atrium to left ventricle), they may be a “mirror image” or transposed, or they may be “isomeric” which means that there are two right atria in one presentation, or two left atria in another presentation.

Most frequently, the RA and LA are positioned normally, and the AV valves and the RV and LV are positioned abnormally. It has been my experience that the AV valves tend to follow their respective ventricles (MV to LV, TV to RV.)

What Does All That Mean?
Typically, the RA and LA are positioned normally. Unless you do a TEE, as an echocardiographer it would be very difficult to tell the RA from the LA.

Pay more attention to the AV valves (mitral and tricuspid valves) and the ventricles.

There might be an absent TV valve, absent MV valve or an overriding single valve (endocardial cushion defect), along with any configurations of the ventricle: dominant LV (most frequently 80%), or a dominant RV, or rarely a true single ventricle.

Further, there may be a double inlet type of AV valve configuration, in which the AV valves drain into one ventricle.

As an echocardiographer, evaluate the pulmonary vein connections, the venae connections and document them. Determine whether there are two ventricles. If one is hypoplastic, try to determine which one is dominant. The LV is often dominant. The RV is a very coarsely trabeculated chamber.

Where is the outflow going? Determine which outflow artery is the aorta, and which one is the pulmonary artery. The aorta is typical in appearance and has the brachiocephalic, left carotid and left subclavian arteries that arise from it. The pulmonary artery has two large branches, the right and left pulmonary arteries that arise from it.

Do not confuse the “ductal arch” or the PDA with the aortica arch. The PDA is often so enlaged that it may look like the aortic arch. The ductal vessel is singular, and has no other arteries that arise from it.

It is important that you take a systematic approach to any echo scan. The normal heart has four chambers, four valves and eight blood vessels that run in and out of it. Evaluate each chamber and vessel, and do it in a coordinated approach.

Which is the right ventricle, and which is the left ventricle? Which outflow arteries are attached to which ventricle? Where do the cavae drain? Where do the pulmonary veins drain? Are there two outflow arteries (aorta and pulmonary artery)? Is there a PDA? Is there an ASD? Is there a VSD?

Take it one step at a time and you will do a very nice echocardiogram.

Ken Heiden