Right atrium (RA)

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The right atrium (RA) (plural: atria) is one of the four chambers of the human heart, and is the first chamber to receive deoxygenated blood returning from the body, via the two venae cavae. It plays an important role in originating and regulating the conduction of the heart.

Gross anatomy

The right atrium receives deoxygenated blood from the superior vena cava (SVC), the inferior vena cava (IVC), the coronary sinus (covered by the Thebesian valve), and the Thebesian veins.

It is grossly the shape of an irregular ellipsoid, with the exception of the right atrial appendage, which arises anteriorly. The right atrial appendage overlies the aortic root and the proximal right coronary artery (RCA).

The right atrium is separated from the left atrium by the interatrial septum. In cases of congenital cardiac malformations, the morphologic right atrium can be identified by the fossa ovalis in the interatrial septum.

The right atrium leads into the right ventricle through the tricuspid valve.

Boundaries 

  • superior end: SVC orifice 
  • inferior end: IVC orifice
  • posterior wall: interatrial septum, faces anterior and to the right, marked by depression (fossa ovalis) in lower part of the septum with crescenteric upper margin called the limbus)
  • anterior and right lateral wall: cardiac wall
  • left lateral extent: right atrioventricular ostium 

Parts

  • atrial cavity divided into 2 main parts by crista terminalis 
    • smooth muscular ridge from the roof just in front of and below the SVC opening (between SVC and right auricle) and extends along lateral wall to the anterior lip of IVC
  • sinus venarum cavarum 
    • cavity posterior to crista terminalis 
    • drains IVC and SVC
    • smooth wall 
  • atrium proper 
    • anterior half of the chamber (anterior to crista terminalis)
      • represents true auricular chamber of embryonic heart 
    • includes right auricle 
      • upper end of right atrium to the left of the SVC
      • overlaps commencement of aorta and clasps infundibulum of right ventricle 
    • wall is ridged by pectinate muscles fanning out from crista terminalis and also extending into the right atrial appendage. 
      • the atrial lead of an external pacemaker is frequently located in these muscles.

Openings

  • SVC opening superiorly 
    • superiorly to the right of the crista
  • IVC opening inferiorly 
    • guarded by ridge that extends from anterior lip of IVC (remains of valve of IVC / Eustachian valve)
  • coronary sinus opening inferiorly
    • medial to the IVC opening in posterior wall of right atrium
    • bordered by small fold (valve of coronary sinus / thebesian valve) 
  • right atrioventricular ostium
    • guarded by tricuspid valve

Relations

  • superior: SVC
  • inferior: IVC
  • left lateral: right ventricle, aortic root and valve
  • right lateral: right lung and pleura, phrenic nerve, pericardiophrenic artery and vein
  • posterior: left atrium, right pulmonary veins 

Arterial supply

Venous drainage

  • variable veins drain the atrial walls
  • tiny myocardial Thebesian veins drain directly into the right atrium

Nerve supply

  • sinoatrial node
    • primary cardiac pacemaker 
    • subepicardial location near junction of SVC and right atrium
    • regulated by vagus nerve and cardiac sympathetic plexus 
  • atrioventricular node (AVN)
    • located between coronary sinus ostium, septal leaflet of tricuspid valve and tendon of Todaro (which connects the Thebesian and eustachian valves) 

Variant anatomy

  • thickened Eustachian valve of the IVC 
    • thickened ridge due to incomplete regression of Eustachian valve 
  • chiari network
  • incomplete closure of the foramen ovale
  • variable size of the fossa ovalis 
  • coronary sinus may be doubled
  • coronary sinus may drain into the left atrium
  • SVC may be joined to the left atrium 

Radiographic features

On contrast-enhanced chest CT and cardiac MRI, the right atrium when measured on axial slices can be considered enlarged when the transverse diameter is ≥67 mm (male) and ≥64 mm (female) .

Development

Develops from the right horn of the sinus venosus. Originates as two chambers, an anterior and a posterior.

Practical points

CT evaluation of the right atrium:

  • the patient should be encouraged to avoid the Valsalva maneuver, which may bring unopacified blood from the IVC into the right atrium, and may mimic a thrombus
  • to avoid streak artifact from dense CT contrast, a multiphasic study with 100% contrast followed by a 20-50% contrast and saline chaser should be administered

Size 

Echocardiography

Measurements of the right atrium are primarily acquired from the transthoracic apical four chamber view. While more time intensive, planimetry is preferred to estimations of area based on long/short axis dimensions. Measurements are obtained as follows :

  • major dimension
    • measured parallel to the interatrial septum from the center of the tricuspid annulus to the superior atrial wall
  • minor dimension
    • perpendicular to the long axis, measured from the interatrial septum to the atrial free wall
  • end-systolic area
    • the atrial endocardium is traced, spanning the lateral to septal aspects of the tricuspid annulus

The lower and upper limits of reference values for the right atrium are as follows:

  • major dimension: 3.4-5.3 cm
  • minor dimension: 2.6-4.4 cm
  • end-systolic area: 10-18 cm

History and etymology

"Atrium" is the Latin word for "court", referring to the central area in a Roman house from which one could enter various chambers. It was entered through the "ostium" of a Roman house.

Related pathology