Sinus node which orchestrtes the rhythm of life gets it’s blood supply by a small blood vessel arising from either RCA or LCX. (55 :45%) . The course of sinus node branch is highly variable .
There are three distinct pattern observed.
- Posterior encircling of SVC .(50%)*
- Anterior encircling of SVC(40%)
- Form a “garland like ” anastomosis on either side of SVC (10%)
* Some refer to as clockwise and counterclockwise course.
SA node is a spindle shaped structure with a length up to 20mm . Extending from cranial to caudal aspect.The pecularity of the blood supply to SA nodal artery is , it enters the SA node either in it’s superior aspect or inferior aspect never in the mid part. There can be water shed area in the either ends depeding upon the entry.This can have electrophysiological and pathological significance .
The other consistent feature is that , the major trunk of SA node artery courses through the central core of SA node.In fact , many times pathologists recognise ther SA node , with the help of this arterial course.
Is there a collateral blood supply to SA node ?
It is not common . Rarely atrial branches of LCX /RCA can have extensive anastomosis with SA nodal branches .The hemodynamic significance of which is not known.
Ischemic SA nodal disease has become an important entity . As the cardiologists are preoccupied with opening occluded coronary arteries in cath labs , it is not surprising to note, there is little ongoing research in the anatomy and physiology of SA nodal blood supply.
We have to go back in time to get some great articles on the subject
At this point of time , we should realise the 1ooth anniversary of SA nodediscovery passed of silently . Kieth and Flack found the SA node with bare eyes in the year 1907 , when none of the present-day investigations including ECG and X RAY were not even conceptualised !
With tributes to those humble pathologists like M.J Davies, R.H .Anderson, T.N. James,M. Lev ,J.L.Titus who followed the foot steps of Kieth and Flack ,
Here is a link to one of the great articles on the blood supply to SA node
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC514694/pdf/brheartj00203-0035.pdf
Related point : How do you recognise the SA nodal artery in coronary angiogram ?
During RCA angiogram it is many times confusing to identify the SA nodal branch . In RAO and LAO views the plane of exit of SA nodal branch from RCA will be determined by the course it is going to take .(Anterior vs posterior encircling pattern). The conal branch which is often the first branch of RCA , also behaves aberrantly many times. So, we can’t have a rule of thumb in identifying SA nodal branch .
When SA node branch originates from LCX it has to take a long route but once it reaches the SVC/RA junction it takes one of the above described course. It is not clear whether LCX fully understands it’s responsibility , when RCA ignores it 45 % of times . There is reason to suspect the commitment and dedication of LCX because it rarely supply the SA node by a seperate branch. It is often the left atrial CX that comes to the rescue and give a twig to the SA node .
Considering the complexity of SA nodal blood supply , one can understand why some develop premature sinus node failure. One can never determine with evidence , how much of SA node destruction is due to ischemia and how much is due to age related degeneration and fibrosis.
Dr.S.Venkatesan MD 