Posts Tagged ‘SA node’

Pacemaker current is   strangely  referred  by physiologists  as  funny current (I f ) . I am yet to find the exact reason .  This is the current  that  sustain  our life right from the day 22  of  embryonic life when the  cardiac jelly beats for the first time.   SA node  solemnly  follow our  entire life  before  making  a  bid-adieu !

pacemaker  potential sa node 5


pacemaker current if funny current poential 002

pacemaker current if funny current poential 003

What is contribution of  If  current in the overall Pace-making  activity ?
This  has not been quantified . The fact that ,  Ivabradine induced  If  current  blockade does not result in serious bradycardia indicate  , SA node has alternate reserve currents as well . ( SA node  is a such a mystery  structure , it would never be a  surprise , if we  find many more  “not so funny”  currents !)

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This is an  ECG which  I reported  yesterday in my clinic . I thought it was a  near perfect example for sinus node premature beat .

sinus premature beat spb 2

(Of course I need to explain  why the  P morphology  slightly  differs )

A  sudden unexpected  QRS  complex is often called as  ectopic beat . If it occurs prematurely (ie earlier than anticipated )  it is called as premature beat. If it occurs late it is refereed  to as escape beat .Please note the difference is not absolute .

Sinus node is a dramatic bundle of energy with divine powers that  drives rhythm of life !

The pacemaker cells are arranged in a compact fashion with  differential properties from cranial cells firing fast and caudal cells little slower. The neural control is under constant Neuro/electro/humoral  servo control mechanism.It is well known the pacemaker shifts it’s firing location within the SA node in fairly regular fashion .The entire SA node has rich adrenergic and  cholinergic  innervation , with  a dominant control by the later . (This is  why the intrinsic heart rate is  in the tachycardia  range (around 116 )  when SA node is denerved  pharmacologically )


sinus premature systole spd sinus node ectopics002

SA node ,  being  a complex structure ,  it is not surprising to note  few beats to fire  slightly late  or  prematurely.If it occurs late it is called sinus pause ,  if it occurs early it is sinus premature beat , if  both occurs  interchangeably  we refer it as  sinus arhhytmia. (Read  about sinus pause here)

What is the clinical significance  of   SPD ? (Sinus premature depolarisation )

It is a  very benign entity that it is  merely an  academic fascination . By  stretching my  imagination  I  can  correlate  it  with few possible  clinical issues.

  • May be it has potenital to trigger a  SA nodal reentry tachycardia  or In appropriate sinus tachycardia/bradycardia.
  • It may be imporatnt in sinus node modification process.
  • However ,the main issue is  thee  cardiac physicians  in their enthusiasm should not mistake it for some serious  cardiac arrhythmia !

Related article


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I guess ,the art of delivering medical lectures is gradually deteriorating . This is not because of lack of young brains in teaching profession .It is primarily due to onslaught of technology  and multiple  scattered source of knowledge . I do remember some of my physiology  professors take class  in  the first year medical school  in the early 1980s  .

I wonder  I  could go back in time machine to hear the voice of Dr Kieth who delivered this grand lecture of anatomy of heart in the year 1918 .in the famed auditorium of  Royal college of surgeons . We should profusely than the BMJ for providing the text of that lecture free to us in  almost 100 years later.

By the way  . . .  for those who do not know  ,  Kieth is one of the inventor of SA node the pacemaker of the heart .




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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.

  1. Posterior encircling  of  SVC .(50%)*
  2. Anterior  encircling  of  SVC(40%)
  3. 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


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.

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