The Right Atrium



The Right Atrium






SECTION 1 ANATOMY OF THE RIGHT ATRIUM






FIGURE 5.1 3D image of the right atrium processed from a computerized tomographic image.






FIGURE 5.2 Stick diagram of the RA and important structures within. Blue arrow, fossa ovalis; red arrow, coronary sinus and thebesian valve; aqua arrow, tricuspid valve; green arrow, eustachian valve; SVC, superior vena cava; IVC, inferior vena cava.


The right atrium (RA) is the entranceway to the heart, receiving blood from the inferior and superior vena cava (SVC) and the coronary sinus. Embryologically, it is generally divided in two morphologically distinct parts. The superior-posterior portion or the sinus venous portion is relatively smooth and nontrabeculated, while the inferior-anterior portion or atrial appendage portion is trabeculated. The two areas are separated by the crista terminalis, which begins at the SVC and extends to the inferior vena cava (IVC) in a crescent-shaped fashion. The right atrial appendage is located in the trabeculated region. The sinus venous portion includes the SVC and IVC, the entrance to the RA or vestibule, and the interatrial septum. As placental blood enters the RA from the IVC, it is baffled by the eustachian valve (named after Bartolomeo Eustachi or Eustachius) toward the foramen ovale and into the systemic circulation. The eustachian valve is a residual of the right valve of the sinus venous and usually involutes during childhood. The eustachian valve sometimes persists to a variable degree in adulthood and can create confusion for imaging. After birth, the foramen generally closes, and systemic venous blood is collected in the RA and pumped toward the right ventricle in late diastole.

Sometimes, there is a prominent veil-like filamentous network in the lower aspects of the right atrium known as the Chiari network (named after Hans Chiari). It has variable attachments (in contrast to the eustachian valve) including the crista terminalis, the upper regions of the RA, the interatrial septum, the floor of the RA, and the opening of the coronary sinus. The exit of the coronary sinus is guarded by the thebesian valve (named after Adam Christian Thebesius). This thebesian valve is sometimes present and other times completely absent.1,2,3,4


SECTION 2 FUNCTION OF THE RIGHT ATRIUM

The right atrium contains many important components, and each has its own set of functions and potential problems. The right atrium receives all of the systemic blood from the body including the inflow from the heart itself through the coronary sinus. After receiving the systemic venous flow, the right atrium has reservoir, conduit, and pump functions. Blood enters the right atrium passively, is stored there briefly in mid-diastole, and is pumped into the right ventricle in late diastole. In the embryonic stage, IVC blood or placenta blood is baffled by way of the eustachian valve allowing oxygenated and nutrient-rich blood to cross the foramen ovale and enter the systemic circulation.

The right atrium contains the sinus node and the interatrial conduction pathways leading to the AV node and to the left atrium (Bachmann bundle). The AV node has endocardial landmarks that allow this structure to be avoided during electrophysiologic procedures. The bundle of His has characteristic electrical potentials in the right atrium close to the AV node. These landmarks allow identification of the AV node in individuals with AV node reentry tachycardia
and subsequent ablation if needed. Ablation of the AV node and pacemaker is sometimes needed in individuals with uncontrolled ventricular responses to atrial fibrillation. The counterclockwise pathways of atrial flutter are located in the right atrium. Pacemakers usually have atrial attachments in the right atrial appendage. Theos of the coronary sinus is located on the floor of the right atrium. Electrophysiologists place pacemaker leads in this vessel as a portal to the lateral wall of the left ventricle that allows for bi-ventricular pacing. Accessory bundles course from the sinus node around the AV node by way of the right atrium and lead to conduction tissue beyond the AV node creating ventricular preexcitation.

In open heart procedures, the right atrium is the first area that is cannulated to withdraw venous blood for the pump oxygenator. The os of the coronary sinus offers an avenue for retrograde cardioplegia. The fossa ovalis is the portal to the left atrium for electrophysiologic and structural heart procedures via transseptal puncture. In congenital heart disease, the atrium can be used as a pathway to the pulmonary circulation when tricuspid or pulmonary atresia is present.

Inflow of blood can deliver undesirable objects, abnormal cells, tumors, or thrombi to the right atrium and subsequently to the right ventricle and the lungs. Tumors growing in the vena cava may appear in the right atrium. Primary cardiac tumors or tumors of adjacent tissues may be visible in the atrial wall. The right atrium is generally the cardiac chamber with the lowest mean pressure; therefore, internal and external pressure changes readily alter its shape and hemodynamics.4 The abnormalities below illustrate some of these occurrences. The right atrium contains many important components, and each has its own set of functions and potential problems. The right atrium receives all of the systemic blood from the body including the inflow from the heart itself through the coronary sinus. After receiving the systemic venous flow, the right atrium has reservoir, conduit, and pump functions. Blood enters the right atrium passively, is stored there briefly in middiastole, and is pumped into the right ventricle in late diastole. In the embryonic stage, IVC blood or placenta blood is baffled by way of the eustachian valve allowing oxygenated and nutrient-rich blood to cross the foramen ovale and enter the systemic circulation.



SECTION 3 THE NORMAL RIGHT ATRIUM






FIGURE 5.3 The normal right atrium should be measured from the apical four chamber view. The minor axis of the right atrium is measured from the interatrial sepal to lateral wall from a line that is perpendicular to the long axis of the right atrium (yellow double headed arrow). This dimension is parallel to the plane of the tricuspid valve annulus. The volume of the right atrium is measured by tracing the blood to atrial wall border with the base being a line across the tricuspid annulus. The disk summation technique will result in a volume. These volumes are calculated from a single plane in contrast to those calculated from the biplane left atrial calculations. Normal is different for males and females in contrast to the left atrial dimensions. Normal values for the right atrial minor axis is 1.9 ± 0.3 cm/m2 for women and 1.9 ± 0.3 cm/m2 for men. For the major axis normal is 2.5 ± 0.3 cm/m2 and for women and 2.4 ± 0.3 cm/m2 for men. Normal volume indices for males is 25 ± 7 mL/m2 and for females is 21 ± 6 mL/m2. Note that these values are indexed for body surface area. The above example is from a female. (From Lang RM, Badano LP, Mor-Avi V, et al. Recommendations for chamber quantification by echocardiography in adults: an update from the American Society of Echocardiography and the European Association of Cardiovascular Imaging. J Am Soc Echocardiogr. 2015;28:1-39.)


The normal right atrium should be measured from the apical four-chamber view. The minor axis of the right atrium is measured from the interatrial septal to lateral wall from a line that is perpendicular to the long axis of the right atrium (yellow double-headed arrow). This dimension is parallel to the plane of the tricuspid valve annulus. The volume of the right atrium is measured by tracing blood to the atrial wall border with the base being a line across the tricuspid annulus.

The disk summation technique will result in a volume. These volumes are calculated from a single plane in contrast to those calculated from the biplane left atrial calculations. Normal values are different for males and females. Normal values for the right atrial minor axis is 1.9 ± 0.3 cm/m2 for women and 1.9 ± 0.3 cm/m2 for men. For the major axis, normal values are 2.5 ± 0.3 cm/m2 for women and 2.4 ± 0.3 cm/m2 for men. Normal volume indices are 25 ± 7 mL/m2 for males and 21 ± 6 mL/m2 for females. Note that these values are indexed for body surface area. The above example is from a female.5


SECTION 4 ABNORMALITIES OF THE RIGHT ATRIUM


PATENT FORAMEN OVALE

The incidence of patent foramen ovale (PFO) in the general population has been well studied. In 1984, Hagan found in 1000 normal hearts at autopsy that the incidence was ˜27%. Various estimates have been made in various settings, but they all are more or less in this range. Large defects have different implications than small defects.6

Embryologically, PFO occurs when the left-sided septum primum and the right-sided septum secundum do not fuse. In the region of the fossa ovalis, this leaves overlapping membranes that in certain circumstances allow venous blood, clots, air, and other right atrial contents to cross from right to left. This overlapping occurs to varying degrees resulting in a variety of anatomic configurations. If they do not completely overlap, a secundum atrial septal defect occurs. When a PFO is present, these overlapping membranes can best be visualized by transesophageal echocardiography in the ˜70° view. In ordinary circumstances, this communication remains closed under the influence of slightly higher pressures on the left side. These membranes respond to competing pressures. When the right-sided pressures are elevated, the septum primum component of the PFO bulges to the left and uncouples the relationship between the membranes opening the pathway between the two chambers. The length of the pathway appears as a tunnel of variable lengths. Microbubbles injected into a peripheral vein ordinarily do not cross to the left-sided circulation. If a PFO is present, microbubbles will cross to the left side under certain circumstances. Maneuvers that transiently increase right atrial volume and pressure, such
as coughing, laughing, sneezing, and deep breathing or releasing a Valsalva maneuver, increase the right atrial volume and pressure and facilitate crossing of the microbubbles from right to left. The most dreaded consequence of this physiology is the paradoxical embolism whereby an embolus in transit in the right atrium can cross the PFO in the setting of transient elevated right atrial pressure and enter the left-sided circulation. Paradoxical embolism can have various consequences in the left circulation including stroke.7,8,9,10 Sometimes the foramen may be open from left to right when the atrium is dilated and the left atrial pressure is very high.

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May 5, 2019 | Posted by in CARDIOLOGY | Comments Off on The Right Atrium
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