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Cardiac Anatomy - The Right Atrium

Image courtesy of St. Jude Medical

Right Atrium

          The right atrium is the upper chamber on the right side of the heart. Besides being the first cardiac structure to recieve blood returning from the body, it is also the home of the structure that controls how fast the heart beats. This structure is called the sinus node. The sinus node normally sends out electrical signals at a rate of 60 - 100 times per minute. When you engage in strenuous activity the heart rate will usually increase and when you rest, your heart will beat at a slower rate.
          There are numerous other structures within the right atrium that are important to cardiac electrophysiology for a variety of reasons. Each of these structures is listed below with a brief description of what the structure is and how it is involved in the field of EP.  It is important to gain a good working knowledge of the right atrium and the various structures contained within. Not only is the right atrium the origin of the primary cardiac rhythm all of us have, it is also hosts a wide variety of abnormal rhythms. The vast majority of EP procedures will, in one way or another, involve the right atrium.

Right Atrial Structures

  • Sinus Node:

           In most people, the sinus node is located at the SVC / RA junction in the right atrium where the posterior lateral wall connects to the superior vena cava. As the primary control for the heart, this structure regulates how fast the heart beats. It does so by recieving input from the vein by way of the Vegus nerve. The information from the brain is regulated by the need for oxygen and nutrients for the cells throughout the body. During times of increased physical activity, the cells of the body require more oxygen. This need is transmitted to the brain which in turn sends signals to the sinus node instructing it to increase the number of times it generates signals that will cause cardiac cells to contract or depolarize.

  • Crista Terminalis (Terminal Crest and Terminal Sulcus):

          This ridge of tissue develops during the fetal growth of the heart. Once the heart is fully developed, the crista becomes a non-conductive ridge that extends from the lower posterior lateral wall up to the region just below the sinus node. Because conduction does not occur across the crista terminalus, the location of it may be defined by identifying split potentials recorded from catheters placed on the lateral wall.

  • Pectinate Muscles

          There are numerous Pectinate Muscles that run laterally from the anterior Crista Terminalis towards the right atrial free wall. Posterior to the Crista, the atrial tissue is much smoother.

  • Right Atrial Appendage

          The atrial appendage is a triangular region of the right atrium that consists of pectinate muscles.  An alternate term that you may hear periodically is the atrial auricle. Epicardially, the atrial appendage is not always easy to differentiate from the surface of the heart. It is also used as an access point for some cardiac surgeries.1

  • Inter-atrial & Intra-atrial Pathways

          Specific Internodal conduction pathways in the atria may exist based on strong physiologic and some anatomical evidence.1 There are three of these inter-atrial pathways that traverse the atrium anteriorly, superiorly and posteriorly.  One of these pathways becomes an intra-atrial pathway and passes through the septal wall at Bachman's Bundle.  Note that the terminalogy of intra- vs inter- helps to distinguish objects or events that occur within one chamber (inter-) and objects or events that incorporate more than one chamber (intra-).  Thus, the pathways that remain in the right atrium only are interatrial. The pathway that crosses the septum into the left atrium is an intraatial pathway.

  • Bachman's Bundle

          The primary conduction pathway between the right and left atrium is referred to as Bachman's Bundle.  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.

  • 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 IntraCardiac 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.

  • Limbic Band / Ridge

          Surrounding the Fossa Ovalis is a ridge of tissue referred to as the Limbic Band or the Limbus Fossae Ovalis. (1)  This ridge is the second obstacle that the sheath "hops over" as it is positioned for trans-septal crossing.

  • Coronary Sinus (CS)

          Venous blood return from the cardiac tissue returns to the right atrium by way of the Coronary Sinus. The Coronary Sinus is an important structure in the EP world as it provides information on both left atrial and left ventricular activity without requiring a trans-septal approach.

          The Coronary Sinus lies in the posterior grove that is located between the left atrium and left ventricle. A catheter placed in the CS will be able to pick up signals from both chambers. This information allows for rapid information collection on electrical activity from these chambers. In normal sinus rhythm, the activation sequence of the CS shows first on the proximal electrodes (9-10). Each subsequent electrode pair shows the next activation until the distal most pair of electrodes (1-2) show the latest activation. If a tachycardia occurs where the CS does not show the normal proximal to distal, (referred to as concentric) activation, it suggests that exploration of the left atrium may be warrented. The abnormal CS patterns that do not show proximal to distal sequence is referred to as eccentric activation sequence.

          The primary veins that empty into the Coronary Sinus are the Vein of Marshall which runs along the lateral wall of the left atrium, the middle cardiac and the posterior left ventricular veins. Of these three, the Vein of Marshall is the only that does not have a valve where it joins onto the CS.1

  • Thebesian Valve

          The ostium of the Coronary Sinus may have a ridge of tissue that is referred to as the Thebesian Valve. This valve originates from the embryonic Right Venous Valve(1) and may obstruct entry to the CS.

  • Eustachion Ridge / Valve

          The Eustachion Ridge, like the Thebesian Valve, also originates from the embryonic Right Venous Valve and may, or may not be present.  This ridge of tissue originates along the anterior border of the IVC where it joins the right atrium and runs posterior towards the CS ostium where it may connect with the Thebesian Valve. Large, well developed ridges are often perforated by openings that form a lattice like pattern. When present, this network of tissue is referred to as the Network of Chiari.(1)

          The Eustachion Ridge can play a significant role in atrial flutter ablations. A large well developed ridge may present some difficulties during ablation of the right atrial isthmus. In larger "barrel chested" male patients, achieving complete ablation of the tissue along the base of this ridge may require utilization of a Ramp or Ramp1 type sheath. The sheaths of this type lift the ablation catheter over the ridge and direct the tip of the catheter to this difficult to reach area.

  • Membranous Septum

          Located along the superior / posterior aspect of the Tricuspid Valve is a small region known as the Membranous Septum. This tissue seperates the right atrium from the left ventricle. If the Membranous Septum was crossed, the catheter would enter the Left Ventricle jus below the non-coronary cusp of the Aortic Valve.(1)  Crossing the Membranous Septum is rarely performed as it poses greater risks than a standard retrograde approach to the Left Ventricle.

  • Tricuspid Valve

          The Tricuspid Valve is one of two atrioventricular valves. These valves lie between that atria and the ventricle and are attached at a fiberous ring that is referred to as the Annulus Fibrosus. The atrial side of the valves is smooth while the ventricular side is somewhat irregular due to the insertion of the Chordae Tendineae.(1)  These structures help the valve maintain its form and thus, its function. The valve is divided into three leaflets, the Anterior, Septal and Posterior leaflets. Each leaflet is anchored along the annulus and are not completely seperate from each other. (1)

  • Triangle of Koch

          The Triangle of Koch is a region of the lower right atrial septum that is defined by the Tendon of Todaro, the ostium of the Coronary Sinus and the Tricuspid Valve annulus. Within the upper portion of this triangle the AV Node may be found. This region acts as an important landmark for approaching ablation of AV Nodal Reentrant Tachycardias (AVNRT).  The atrial end of the slow pathway inserts near the ostium of the Coronary Sinus while the fast pathway lies closer to the apex of the triangle near the AV Node.(1)

  • Atrial Ventricular (AV) Node

          The AV Node can be found at the apex of the Triangle of Koch as defined above. The internodal tracts blend into the tissue of the node on the atrial side.(1)  On the ventricular side, the common AV Bundle or the Bundle of HIS is located. This bundle then subdivides into the right and left bundle branches.

  • Right Atrial Isthmus

          The Right Atrial Isthmus is not, in itself, a cardiac structure. It is the region that lies between the Inferior Vena Cava and the Tricuspid Valve Annulus. It is noteworthy in that this region is the primary target for treatment of Typical Atrial Flutter. This region lies anterior to the IVC and is best visualized in the Right Anterior Oblique (RAO) fluoro view.

  • Inferior Vena Cava

          The IVC provides a route for blood return to the heart from all the regions of the body that lie inferior to the heart. While there is very little about the Inferior Vena Cava that is specific to cardiac electrophysiology other than a convenient entrance to the Right Atrium, it can anchor the Eustachion Ridge which can provides difficulties in patients with a large, well developed ridge.

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