Posts Tagged ‘coronary blood flow’

If  we   think we have  unraveled  all the  mysteries  of   human coronary  blood flow   we are  sadly  mistaken . Most cardiac physicians spend  their  prime life  in opening the  obstructed coronary arteries  playing  a role of coronary plumber.

Like any plumber ,  it is not going to be  one time job and   our patients  would  have to hire their  services  periodically  . Many times  it turns  out to  be a 108/  911 call  as well !

Unfortunately , hem0dynamics  of  coronary blood flow  do not follow  the principle of  water flowing across  a domestic  pipeline.The most dramatic   difference  between  the  coronary blood flow and   water pipe  is ,   in the later  ,   as  the water is being ferried   across the house  ,   neither the building   nor the   pipe    contracts    (Unfortunately all our understanding , derivations and research were  based on simple physical  modules  of  hydrodynamics in a static  delivery  system )

Pressure flow relations especially in biological system is  not  simple. Since  our  foundations on principles of   blood flow  is based on this simplistic model  ,  every assumption  could be proven  wrong. This  is what  is happening now . Nothing seems to work  in a  learnt  manner.

A patient  with  100% occlusion  walks comfortably  without damaging his muscle.While an other patient  would  develop cardiomyopathy even if the occlusion is  gradual   and  incomplete  ! Hemodynamic  logic tells us blood flows from high pressure to lower pressure  zones  like a water fall !

But coronary waterfall is not a simple and smooth affair. It is not a free fall  ,  even as the water falls there are  pumpy  interruptions .When these  pumpy ride  occur  even in physiology one can imagine  the pathological states  , when  the coronary  artery is blocked ,  the myocardium is  scarred and the systemic blood pressure fluctuates .

While every  organ welcomes   the systole  ,  as they are fed  with  blood  during this time of cardiac cycle  . Heart  is only organ which sacrifices  its own blood flow during this phase  as the systolic contraction  interrupts the blood flow .

Determinants of coronary blood flow

What we learnt over the years has been too simplistic. It is not the  patency of vascular  system that matters. The coronary micro vasculature, the metabolic demand, the neuro  humoral regulation etc.  For  most cardiologists  the epicardial  patency   or stenosis remains the only relevant  issue

The reality is  much complex  to comprehend

  1. The coronary perfusion pressure
  2. Coronary flow reserve
  3. Coronary wave forms
  4. Sub endocardial vs subepicardial flow ratio
  5. Effect LVH on myocardial flow
  6. Coronary venous tone and arterial ischemia.

Now,  we have an entirely new concept which proposes (Rather proven concept !)   the  integrity of  myocardial contraction and relaxation on the coronary blood flow. This land mark paper in circulation has identified  six wave forms of coronary blood flow This include 4 positive  waves and two negative waves

Questions need to be answered 

During diastole  myocardium relaxes . Only if  the myocardium  relax   optimally  the compressive effect of systole  on coronary  coronary   micro vasculature is reversed  ,  intra coronary resistance  falls so that coronary blood flow can occur smoothly. We do not know  whether diastolic dysfunction would  affect the diastolic coronary filling waves  jeopardizing the coronary flow.

Myocardial viability is important for one more reason  , in the distribution  of   coronary blood flow .A dysfunctional muscle can not receive  and  inject  the blood  deep into  sub  endocardium (Note this becomes  important  when  revascularising   severely  dysfunctional segment )

Does myocardium has a  calf muscle analogy   and  behave like  a  powerful  intramuscular perfusing pump .

A breakthrough concept  from Davies et all in circulation .  These are not new ( Buck -Berg  ?)thought  about this decades ago .  The interest is rekindled in recent years  ,  as  complex angioplasties  following myocardial infarctions  failed  to improve outcome and relive symptoms in many .

During primary PCI ,  no- reflow  often  denotes a meaning  of  failed  PCI .The issue involved  is  hydrodynamics of intra myocardial  blood flow .The following  article partly  answers the  issue  underlying no re flow .http://circ.ahajournals.org/content/113/14/1721.full.pdf+html

Final message

Young  physicians  need to  spend  more time  in  basic  cardiac sciences . Lest, what  we  do  in cath lab blindly  will become a laughing stock  ! We have to go back to the golden years of  research in cardiac physiology  (1960 -1970s)  . Mastering coronary  angioplasty  may increase the blood  flow  up to the  myocardium ,  but pushing the blood beyond the muscle requires more sense  and effort .

A simple  hemodynamic  model based  on  physical  principles alone is a  greatest error we make in cardiac science . * Further, human heart muscle is not only influenced by the quantum of blood  it receives  but to the great extent the content of blood.The blood caries all the ill effects of  systemic diseases and  damage   the vessels and muscle .The interaction  between the  blood and  the muscle  is never  an issue in  the pure  physical labs .( Even animals misbehave !)

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Even as we make rapid strides in  conquering coronary atherosclerosis by all those fancy gadgets , the  fundamental coronary  hemodynamic principle   is poorly understood . Hence  there is no surprise  for the  “perennial ambiguity”  in the indication  and effectiveness of  coronary revascularization .

Why the hell ,   reliving  a  coronary obstruction  may  not provide the   expected hemodynamic benefit  or do not prevent future  heart attack  in many ?    One of my patients  asked ?

I told him . Wait , do not get  excited , we also do not know  . . .We are just beginning to understand mysteries of coronary  circulation.

It is a well documented fact  ( but a  debatable )   that  lesser the  severiity of a  lesion more likely it is prone for an acute coronary  event .( Vulnerability , shearing stress or is it a simply a statistical mirage !) While the  vulnerability aspect is  complex , the hemodynamic  impact of  coronary  lesions  is   relatively better  understood. Here is  an important  documentation from Dr B . K  Koo from  Seoul , South Korea  who has elegantly shown the behavior of  fractional flow reserve (FFR ) in various grades of  stenosis  .This study was done in jailed side branches following PCI.

FFR  shows a surprise   relationship  with severity of coronary stenosis  . Even severe lesions showed equal if not more flow  reserve ?

and mild lesions might have lost all its reserve.

 How is it possible ?  Can it be true ?

Yes , it is indeed a  fact . God generally  keeps a stong link between anatomy and physiology  , structure and function . But he adds a rider and keeps  a reserve in every  human cell   meant for  emergency  back up . FFR is  one aspect of this , we have  partially discovered .  When we fail to understand this we are bound to get confused and make a wrong decision in cath lab.

Simply stated ,  flow across a coronary  artery is much more depedent on the status of microvascualture  than the hurdles they face in the epicardial highways !

Link to this original article from JACC .

How to do the FFR procedure ?


Soon to follow . . .  If less severe lesions are more  dangerous why we are ignoring it in cath lab ?

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