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 Increased heart rate and CBF  



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Author9 Posts
  #1

Hi everyone,

When the heart rate increases, and since the diastole period is decreased in relation to systole, coronary blood flow is decreased (since CBF occurs only in diastole when the vessels are dilated and not compressed by ventricular muscles contraction)!

But, somewhere else, they said that when heart rate increases, ventricular muscles consumption of oxygen increases, causing increase in adenosin (and other metabolic products) secretion... and these products cause coronary vasodilation, causing the CBF to increase!!

Can someone explain this?!

Thanks,




  #2

During the acute phase, the increase in CBF due to the metabolytes is greater than the decrease due to shorter dyastole. In time, though, it moves toward the other end of the equation.

That's the case, for example, of the tachycardia induced cardiomyopathy we see in chronic atrial fibrilation with fast ventricular response. If you create a complete heart block and put a definitive pacemaker, lowering the ventricular rate, the cardiomyopathy is reverted.



  #3

Well explained.. Experience shows grin



  #4

Thanks for the explanation..

But, does that mean there is a CBF during systole?!


  #5

well that CBF will be due to autoregulation.....


  #6

Not during systole. Always during dyastole.



  #7

ok during systole there is a slight blood flow in the right ventricle as the right ventricular contraction is slightly less than left ventricular contraction but overall the main blood flow is still higher during diastole in Right ventricle....


  #8

Overall, Most of CBF occurs during diastole. But still as much as 30% of CBF occurs during systole.

Duration of Diastole is the critical factor in deciding CBF. In conditions of stress/ exercise, where myocardial O2 demand increases, but diastolic duration decreases, CBF is maintained by adenosine and other metabolites that vasodilate coronary vessels and can increase blood flow to upto 5 times of baseline! ( remember that unlike other tissues, heart can not increase O2 extraction from blood and thus has to depend on increased flow to meet the increased O2 demand)

Hope this helps.



  #9

Q=O2 consumption/A-V pO2 difference. so flow increases as O2 consumption increase. CBF is autoregulated, & only way to increase O2 delivary is to increase blood flow, as extraction is almost complete even in resting condition one has to increase flow to deliver more O2.
on the left side coronary BF is almost 0 during systole but on the right side there is some flow during systole, still it is less than it is in diastole.





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