Septation

Question 1

Steps of atrial septation

Answer 1

Septum primum: from dorsal LA wall (atrial roof) => myocardial lengthening => AV canal endocardial cushions

Forms ostium/foramen primum: Completely occluded when fuses w dorsal + ventral endocardial cushions

Ostium/foramen secondum: perforations in superior portion of septum primum

Septum secundum: from dorsal atrial wall, on the R of septum primum and L of sinus venosus => AV cushions

Fossa ovalis/foramen ovale: remaining opening, covered by septum primum

Fusion of septum primum + secondum forms interatrial septum after birth

Question 2

Types of ASD

Answer 2

PFO

Ostium primum

Ostium secundum

Sinus venosus

Unroofed CS

Question 3

PFO

Answer 3

failure of anatomic closure

Persistent ↑ RAP after birth OR atrial stretch → prevent fusion

Central part of IAS

Question 4

Ostium primum defect

Answer 4

incomplete fusion of septum primum + AV cushions

Form of AVSD

ventral part of IAS above AV valves

more common in cats

Question 5

ostium secondum defect

Answer 5

a. Failure to grow/shorten septum secondum
b. Overabsorption of septum primum

central part of IAS

most common in dogs => boxers, dobie, samoyed

Question 6

Sinus venosus defect

Answer 6

abnormal attachment of R PVs to Cr/CaVC
a. 2nd to incomplete resorption of sinus venosus
b. PV connection → RA
c. Abn connection more than true ASD

Question 7

Unroofed coronary sinus

Answer 7

failure of development of wall btwn LA + coronary sinus
a. Associated w/ persistent LCrVC

Question 8

Types of CS defects

Answer 8

I: completely unroofed + LCrVC
II: completely unroofed, no LCrVC
III: partially unroofed mid portion
IV: partially unroofed terminal portion

Question 9

What is AV canal

Answer 9

Atrioventricular canal = canal between atria and ventricles during embryologic formation
* Layer of fibrous insulation => interrupt myocardial continuity btwn atria & ventricles

Question 10

AV canal formation

Answer 10

• Proliferation of mesoderm => dorsal + ventral endocardial cushions (major cushions)
  o Dorsal endocardial cushion => fuses w septum primum => ostium primum closure
  o Dorsal + ventral endocardial cushions => fuse to divide in L + R AV canals
  Forms AV septum: normally separates LVOT and RA
  If no fusion, cannot fuse w septum primum => ASD
  o Fusion w atrial and ventricular septa => forms MV + TV

Question 11

Membranous septum

Answer 11

btwn LVOT and RA

Question 12

Spiral septum formation

Answer 12

Formation of paired endocardial cushions
* Truncal cushions (distal): from R and L sides => fusion will form the truncal septum

 Divides truncus in 2 channels: pulmonary channel and truncus arteriosus

• Bulbar/conal cushions: continuous w truncal cushions
  o Will eventually fuse and form the primitive outflow tracts
  o Divides in anterolateral (RVOT) and posterolateral (LVOT) portion