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Cardiac Anatomy - The Left Atrium

Image Courtesy of St. Jude Medical

Left Atrium

          As blood leaves the lungs, it returns to the heart through the pulmonary veins. Blood flow from the veins is deposited into the left atrium where it passes through the mitral valve into the left ventricle.
          The left atrium is one of the most structurally complex chambers in the heart. There are usually four pulmonary veins that connect to the posterior wall of this chamber. The area where the veins connect to the atrium is a region of intertwined venous and heart muscle tissues. Many physicians feel that stretching of the tissues where the veins connect to the atrium that happens over time may be a contributing factor to the rhythm atrial fibrillation.
          Beyond the veins, the left atrium itself has many similarities to the right atrium. Below is a list of some of the structures that can be found in the left atrium.

Left Atrial Sturctures

          The structure of the left atrium is relatively simple. The chamber walls are generally smooth except where the left atrial appendage, or auricle, attach to the body of the atrium.(1)   Here, small pectinate muscles denote the ostium of the appendage from the body of the atrium itself.(1)

• Pulmonary Veins

          There are a number of veins that return oxygenated blood flow from the lungs to the left atrium. In most circumstances, there are 2 veins that attach to the left (lateral) side of the chamber and 2 that connect to the right (septal) side.(1)  There are times when up to 3 veins have been documented inserting into the septal LA.(1)   It has also been demonstrated that the left veins will sometimes sprout from a single common ostium.(1)

          The upper, or superior veins on each side tend to connect a little more anteriorly while the lower or inferior veins will generally be found in a more posterior position. These veins are often referred to using the following terminology:
• Left Superior Pulmonary Vein
• Left Inferior Pulmonary Vein
• Right Superior Pulmonary Vein
• Right Middle Pulmonary Vein (when present)
• Right Inferior Pulmonary Vein

• Atrial Septum

          The atrial septum divides the right and left atrial chambers. This region is important for the structures that are associated with the septal wall and how the each plays a part in the different arrhythmias that may be encountered during an electrophysiology procedure. The septal wall itself can provide key information on the identity of some abnormal rhythms. Atrial flutter is one specific rhythm where what is occurring on the septal wall is an important piece of the puzzle especially when compared to the activity of the lateral wall. This topic will be expanded upon in the chapter on atrial flutter.

• Bachman’s Bundle

          Bachman's Bundle acts as the primary conduction pathway between the right and left atrium. This connective bundle of fibers lies on the posterior aspect of the mid to upper right atrial septum. Conduction across Bachman's Bundle is bidirectional and this region is often the location of left to right breakthroughs for left atrial arrhythmias. Mapping early activation to Bachman’s Bundle is often a good indication that the left atrium may be of importance for the specific arrhythmia being analyzed.

• Foramen Ovale / Fossa Ovalis

          During fetal development of the heart, the Foramen Ovale is an opening between the right and left atrium. This opening allows blood to pass from the right atrium through to the left atrium, into the left ventricle and out to the body effectively bypassing the lungs. At the moment of birth, the baby will take his first breath and the lungs will begin functioning. As the blood flow shifts through the right ventricle into the lungs, it now enters the left atrium by way of the pulmonary veins. This new direction in blood flow and higher pressures in the left side of the heart closes the small flap of tissue that covers the foramen ovale.

          Once the foramen is closed, it becomes known as the Fossa Ovalis. In most people, the fossa ovalis becomes fixed and access to the left atrium is no longer possible via this route. In a small number of patients, the fossa remains patent and can be easily pushed open by even a small amount of pressure applied by a catheter. If the physician in the lab ever indicates that the patient has a patent fossa, he is simply stating that he can access the left atrium without having to perform a trans-septal puncture.

          Normally, the fossa is not patent and a trans-septal puncture is required. This technique for entering the left atrium is performed by placing a special needle at the fossa ovalis under the guidance of intracardiac ultrasound / echo. The Intra-Cardiac Echo, or ICE catheter, allows the physician to visualize the paper thin fossa clearly providing for safe positioning of the trans-septal needle. If you are ever observing this procedure, watch the ICE display as the physician positions the ultrasound catheter to clearly view the septum. The fossa is an extremely thin flap of tissue that may be easily distinguished from the rest of the thicker tissue that makes up the rest of the septum.

• Left Atrial Appendage

          The left atrial appendage attaches to the superior lateral aspect of the anterior wall of the left atrium. The ostium of the appendage, also referred to as the auricle, is marked by small pectinate muscles. The appendage varies in size and may often drape over the body of the atrium.

• Ridge of Marshall

          The Ridge of Marshall is located between the base of the left atrial appendage and the ostium of the left superior pulmonary vein. This muscular ridge often provides a challenging approach to atrial fib ablations.

• Mitral Isthmus

          The Mitral Isthmus is named as it is one of the regions within the left atrium that is commonly used for atrial flutters originating within the LA. This region is found on the lateral wall bounded by the mitral valve on the inferior aspect, extending superiorly to the ostium of the left inferior pulmonary vein.

• Mitral Valve (AV Valve)

          As with the Tricuspid Valve, the Mitral Valve appears best when viewed from an LAO projection. Viewed from the angle, the entirety of the valve opening is presented to the observer. This provides the characteristic "clock" view that is used to describe the orientation of different types of flutter. When perceived as traveling from 12 o'clock, or the top of the valve, down the lateral aspect reaching the 3 o'clock and six o'clock aspects in that order, the left atrial flutter would be described as clockwise. Flutters proceeding in the opposite direction would be described as counter clockwise.

          There are two cusps to the mitral valve, the posterior and anterior cusps.(1)  Becuase the mitral valve only has two cups, it is also referred to as the bicuspid valve.

(1) Information found in "The Netter Collection of Medical Illustrations, Volume 5; The Heart" by Netter
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