ASD is  the most common acyanotic heart disease  with  left to right shunt . Highest qp/qs  are  seen  with ASDs

The shunt  begins  from left  atrium  and goes on to complete a circuit.


In this circuit all chambers  enlarge except the LA . (Inspite of the fact about 200-300 % cardiac output traverses this chamber )

Why ?

Post -test

The most popular answer in the above poll  is LA is  a transit chamber .

If it is so . . .  RA is equally  a transit chamber ,  why it enlarges significantly ?

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  • Second heart  sound is widely split because the pulmonary  hangout interval* is wide
  • Fixed because , the RV stroke volume does not show the normal  respiratory changes.

This is  due to  dynamic phasic   shunting across the IAS  ( For example  : During inspiration ,  if RA,RV volume gets augmented by 50ml from IVC  inflow  , in expiration this  IVC  augumentation is removed  but a  50 ml augmentation from  left atrium takes place  , this keeps the RV diastolic ,  as well as  systolic  volume relatively constant.) This makes the 2nd heart sound fixed .

* Hangout interval is the gap ( in time ) between the  arterial pressure curve and the  respective pumping chamber pressure curve (RV, LV)  at the level of incisura.

Incisura is the  notch   on the descending limb of arterial pressure curve  ,  when the  pulmonary  or aortic valve closure occurs . When we analyse  the simultaneous pressure  recordings of  RV ,LV/Pulmonary artery/Aorta , the arterial  pressure curves  faithfully accompanies the chamber pressure curve  along  the dome of the chamber pressure  curve  till it descends  , where  it dissociates  ,  from the chamber pressure curve and hangs out for a certain  milliseconds. This time interval is  called as hang out interval (Named by Shaver et all )

What is the normal pulmonary hangout interval and systemic hangout interval /

Pulmonary hangout interval  is  60-80ms

Aortic hang out interval  is    20 ms

Why does it happen ? What does it signify

It happens because ,   even as the chamber pressure falls below  the  arterial pressure  ( Note: Semi lunar  valves  close at this  cross over point ) blood continues to  rush  forward ,  with momentum in to the pulmonary and systemic circulation, in spite of the closed semi lunar valves. This keeps the arterial pressure  not only to be  sustained little  longer  but also  slightly higher .

This  interval  is an  indirect( inverse) marker for vascular  impedance  of the distal  draining  circulation .The impedance is same  as vascular resistance  for all practical purposes.Since pulmonary circulation is  a low impedance circulation , it has a wide hangout interval and  the systemic  circulation vice versa.

How much of S2 widening is contributed by RBBB in ASD ?

This is not known .But it has a minor role in prolonging S2 split. This is because , the RBBB in ASD is most often  incomplete and  peripheral one  .( Pesudo RBBB due to RVOT dilatation )

What happens  to S2  when  pulmonary arterial hypertension develops in ASD ?

  • It  is often narrow and fixed . Pulmonary arterial hypertension makes the pulmonary circulation to  behave like systemic , hence the impedance becomes  high and the hang out interval is significantly lost  and second sound is narrowly split. (But fixity may  be maintained.)
  • It also depend upon the RV function and associated RBBB. RV dysfunction and  RBBB both tend to widen the split.

*Mild PAH usually does not alter the S 2 splitting



Is there any other cause for wide and fixed splitting of second heart sound ?

Having known the reasons for widening and fixity it is easy to understand  , a  patient with right heart failure and  RBBB   can have a wide and fixed split .

Widening is due to RBBB (Delayed activation of RV ) . Fixity is due to severe right heart failure makes the RV out put relatively constant .(As RV inotropism is not good  enough to handle the inspiratory augmentation of RV end diastolic  volume.)

Why in VSD the second heart sound is not wide and fixed  split even though  hemo- dynamically it fulfills the same hemodynamic scenario  ?

Guess the answer .It will be posted soon.

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Atrial  septal defects  are one of the commonest forms of congenital heart disease.

  • The commonest being the ositum secundum ASD     ( Which is in fact is a defect in the development of septum primum)
  • The next common is ostium primum defect which is a part of AV canal or atrio ventricular septal defect.

Other forms of ASD include

  • SVC type /Also  called sinus venosus type of ASD .
  • IVC type
  • Coronary sinus defect -Also called partial or complete forms of unroofed coronary sinus

asd svc asd sinus venosus

SVC type ASDs

They are in the strict sense can not be called as ASD. This is because there is no defect in  any of the  embryological  inter atrial septal component.

There is no direct communication between RA and LA, instead   a window  or passage of communication   between pulmonary vein and SVC. Right upper lobe pulmonary vein  is usually the culprit .Some times more than one PV  communicates with SVC.

The exact area of this PV-SVC window occur between anterior surface of right upper lobe PV with postero lateral surface of SVC.

PAPVC partial anomalous pulmonary venous drainage can be considered an integral part of this defect as RUPV is linked with SVC.

Can we have a combination of SVC ASD and OS ASD ?

This is possible .But two embryological errors need to occur. This is often seen as a large OS ASD with deficient or absent superior rim. So whenever superior rim of IAS is deficient a PAPVC and a SVC ASD should be looked for.

Clinical features

  • SVC type ASDs  generally shunt lesser blood than OS ASDs. (Often<2:1) .This is because it is not the LA that is communicating with RA instead only a
  • It is usually a single PV (some times 2) that shunts from left to right.

There is a distinct possibility of missing this lesion in routine echo.Minimal RA,RV enlargement may give us a clue.The classical subcostal or  4 chamber  view in echocardiography may not visualise  these defects.

So, whenever one encounters mild dilatation  of RA and RV and the IAS appears  intact, a meticulous search and a focused echo in the superior aspect of IAS is warranted. Angled  superior views may pick up this defect.A transesophageal echocardiogram (TEE) is often required to  confirm it.

Therapeutic issues

  • Device closure is not possible
  • Surgery involves little more technicality than ASD OS.
  • Small defects can be patch closed.
  • Some times the SVC has to be disconnected from the PV and anastamosed separately on right atrial appendage. SVC resection  will  aid the surgeon in proper patch closure.
  • Post operative follow up is necessary as SVC obstruction or PV obstruction may be a delayed consequence


svc asd sinus venosus



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                                               The superior aspect of   coronary sinus and a portion of the left atrium share a common wall .Embryological defects in this area result in a communication between left atrium and coronary sinus. This defect  descriptively called as unroofed coronary sinus .This entity is most commonly associated with persistent left SVC. Extreme form of unroofing is some times termed as absent coronary sinus.

How to diagnose it ?

A high degree of anticipation is necessary in all patients with ASD or LSVC. A dilated coronary sinus in routine echocardiography warrants full investigation. A contrast echocardiography with agitated saline injection in left cubital vein will clinch the diagnosis as contrast enters LA after opacifying the dilated coronary sinus. During  right heart catheterisation catheter course entering coronary sinus and  advancing into LA through the fenestrations (Unroofing)  will confirm the defect.

How do you classify unroofed coronary sinus ?

The morphologic type of URCS was classified as Kirklin and Barratt-Boyes

 Type   I, completely unroofed with LSVC;

 Type II, completely unroofed without LSVC;

 Type III, partially unroofed midportion;

 Type IV, partially unroofed terminal portion

What is the clinical relevance of this entity ?

This entity should be suspected in every patient with persistent LSVC, ( and LSVC should be suspected in every patient with ASD). The hemodynamics is that of an ASD but if sufficient mixing of LSVC blood and LA blood takes place the child will have mild cyanosis.Some times when the coronary sinus is totally absent it will present as a typical dusky ASD picture which can closely mimic a TAPVC clinically.

Surgeons have a greater role in recognising and treating this entity. A typical repair will be done like this


Links to some of interesting articles on this topic

Raghib circulation 1965

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