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The Aortic dissection : What you wanted to know about true and false lumen !

Posted Mar 24 2009 3:58pm

Why is identifying false and true lumen important ?

This helps the interventional cardiologist to plan the specific therapeutic procedure .

aortic-dissection-table

Is it really difficult to differentiate the two ?

O ne may wonder , why is that  difficult  to identify  the true aortic lumen by echo, after  all  , the LV empties the blood into true aortic lumen ! Yes  , in aortic root dissections  identifying the true from false lumen is rarely an issue.

aortic-dissection-4

The issue becomes  important and complicated as the propagation of dissection goes in a random and erratic way into the ascending aorta and arch and downwards.The situation could further get  complicated  by the fact there could  be multiple communication between the two lumens .Some of these communication are  hemodyanically patent others form  a simple anatomical continuity.The size and the configuration of true and false lumen are not uniform it is highly  variable.In the aortic root the size of the true lumen is usually  large and when it reach the descending aorta  as in type3 the whole thing could be reversed.

The enigma  of  these lumonomics, is that some of the native branches of aorta , would  either be, subtended by false or true lumen. This is a real tricky issue for the surgeons . If a aortic vessel branch (Say bronchial artery . . .) is perfused successfully by the  hemodynamically active false lumen should we meddle  that  at all ?


circumferential-dissection1What are the types of false lumen ?

Usually single septae divide the aorta into two , one false lumen and true lumen.There can  be other types.

Triple lumen aorta:This is usually seen in the aortic root following dissection .Usually there is two false lumen and and one true lumen in the centre

Double barreled aorta: A circumferential   aortic dissection with a central true lumen surrounded by a  circumferential false lumen  mimicking a double barrel on within the other.

aortic-dissection

What determines blood flow within false lumen ?

  • Site of  intimal tear
  • Length of tear
  • Plane of cleavage  . Superficial  subinitmal tear with minimal  medial thickness is likey to give in easily  as the blood  dissects the plane  so it more often manifest as a flap  rather than sustained  dissection
  • Number  of exit points (It is often assumed  aortic dissection  there is typically one entrance and one exit point .

but  more  often  multiple exit points can occur. Some points can have both two and fro flow as it may act as both as entry or exit points

What  is the importance of identifying  point, exit point , true  lumen false lumen etc ?

  • This is vital for planning   repair  of  the segment
  • optimising side branch blood flow
  • some time one may require to create an exit point  for providing useful thermodynamics   of false lumen that could give branch to a vital area.

Why false lumen is  prone for thrombosis ?

  • Sluggish flow within false lumen
  • Plane of cleavage of intima  and media  create an  irregular surface that  trigger  tissue factor mediated thrombus.
  • Free floating cob webs   intimal  remnants may accelerate thrombus formation

What is the clinical significance of  finding  a thrombosed false lumen ?

Large thrombus can occur within false lumen.The presence of which , sometimes an advantage as

it limits further progression of false lumen (An organised thrombus is sort  of  natural  stent graft !)

many of these patients do well with medical management.

C J Sanderso Thorax 1981; 36:194-199;

Can thrombus occur in true lumen also ? How common it is ? If so what is the mechanism ?

Yes , but it is rare  as the velocity  is  more .But it can occur in following situations.

  1. Preexisting atherosclerosis can be  a milieu for  insitu thrombus
  2. Thrombus in true lumen  can occur at the entry point where there is intimal tear ,  which  projects  into true lumen. that can  deccelerate the  flow(Rare)
  3. Thrombus in the false lumen may project into true  lumen  through another tear.
  4. Migration of false lumen thrombus may occur distally and reenter the  true lumen.

What is a cobweb ?

Cob web are the residual ribbons of dissected internal elastic lamina of aorta .
They are variably called as aortic bands, strands ,  septae, flaps etc.

What is the significance of the junction between false and rue lumen ?

The classic false lumen is crescent shaped. True lumen is either round or oval(Gibbous moon)
Tunction between false and true lumen has some characteristic feature.It mimics  the letter Y. The mainstem of Y correspond to main(  Normal full thickness)aortic  wall of the true lumen.The  oblique lines represent the outer wall of the false lumen and the septae dividing true (Fig 3)

dissection-41

What is the natural history of false lumen after surgical correction ?

Surgeons often leave the false lumen insitu , especially beyond the arch in type A dissection.

If false lumen is large  >70% of aorta , secondary dissections may occur in the long term.

Which is the best imaging modality  for  assessing dissection of aorta  ?

Even though MR angiogram and CT scans are shown to be good imaging tools in the evaluation of  dissection of aortamany practical issues creep in doing MR or CT angiogram.Many of these patients are too ill and will be on multiple arterial and venous lines Doing an MRI is  too dificult a task .Further these imaging modalities require a another arterial access .Requires contrast injection and  CT has in addition , radiation hazard.

TEE is a simple investigation can be done even in unstable patients in the bedside .Further also help us  us evaluate the aortic valve function and associated complications of dissection. TEE will be very useful peroperative also in assessing the repair.

*But MRI  and CT can give a long axis , saggital cuts of aortic dissection depicting the entry and exit points in a single image

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