Looking beyond coronary stenosis : Epicardial to endocardial blood flow is not a free vascular water fall !
Posted Nov 02 2011 1:37pm
If we think we have unraveled all the mysteries of human coronary blood flow we are sadly mistaken . Cardiologists generally 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 a 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
The coronary perfusion pressure
Coronary flow reserve
Coronary wave forms
Sub endocardial vs subepicardial flow ratio
Effect LVH on myocardial flow
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 .
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 !)