Why is identifying false and true lumen important ?
This helps the interventional cardiologist to plan the specific therapeutic procedure .
Is it really difficult to differentiate the two ?
One 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.
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 ?
What 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.
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.
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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.
- Preexisting atherosclerosis can be a milieu for insitu thrombus
- 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)
- Thrombus in the false lumen may project into true lumen through another tear.
- 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)
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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