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Posts Tagged ‘cardiac cycle’

Normal  left atrial  (LA) volume is about  22ml/sq.meter body surfacearea  at all ages.Maximum LA volume in physiology is  about 46ml in  females and 56 ml  in males( Average 35 ml)

LV stroke  volume  for each beat is  about  70  ml . . . so where does  the remaining 35ml come from ?

Answer .

  • Pulmonary veins ?
  • Residual LV end systolic volume ?
  • Mix of the two ?

It is logical to assume about 35 ml of fresh  blood  from 4 pulmonary vein*  rushes into LV with every diastolic cycle  .It  never stays in LA  .It just uses LA as a transit  route ,

*In diastole the four PVs,LA  and LV all act like one single chamber .

 

Is this reasoning correct ?.

If we believe the continuity equation this explanation is correct . However still what  we need to know the fate of residual  LV volume (End systolic LV  volume which is also  about 35 ml that would be  in queue for ejection into Aorta  for the next beat !)

Further , we know the LV end systolic volume is not constant .During exertion it  can reach  negligible levels (<10 ml) .At times of vigorous contractions  it can touch near zero as well . Then , It become vital for the  pulmonary venous reservoir  to be act as a  major donor for  LV blood volume for  every ensuing beat.

If the hemodynamics of pulmonary vein LA interface is tricky even in physiology ,  one can imagine the complexities  if the LV diastolic function and left atrial compliance  is affected

Debit and credits of  LV end -diastolic volume .

Let us assume LVEDV is about  1o5  ml .LA blood volume is  roughly one third of LV volume .For every beat equal amounts of fresh blood  from pulmonary vein . These two (LA+PV)  adds to the  residual  blood in LV  to make LVEDV 105   ml . From this 70 ml is ejected as stroke volume leaving behind 35 ml.


lvedv lvesv stroke volume wiggers cycle left atrial volume pressure volume loop residual diastasis

Image template from http://www.cvphysiology.com

 Further questions

LA Chamber volume and blood volume need not be same .What  I struggle to understand is , total anatomical  LA volume  measures  35ml , while the amount of blood it is supposed to hold is also about the same .Does  it mean the LA is completely filled with blood . . . air tight !

Will the LA compliance make it accommodate twice or thrice the blood volume during exercise ?

What is quantum of residual end diastolic  LA volume ?

 

Reference.(Normal LV and LA volumes )

echopedia

 

 

 

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The mechanical atrial function   during atrial fibrillation remain a mystery . In fact , the general  belief  is during  AF  the mechanical function of atria is zero. This is why AF  is promotes stasis and   LA clot formation. It may appear theoretically correct  , still   AF especially coarse  still imparts some amount of  mechanical motion .But this usually does not translate to any useful hemodynamic function .

If atrial booster pump is lost (which is said to be 25 % of  LV filling )  suddenly one expects dramatic symptoms  especially if there is associated LV dysfunction or aortic valve disease .

But in real world AF is well tolerated arrhythmia in most  .  We know by land  mark trials AF  is as good as sinus rhythm  if the rate is  is under control

This is a definite evidence the AF  may not compromise  LV filling   even if   it nullifies  the  atrial contractility .

There is one  more evidence for  retention of atrial mechanical activity in spite of AF .It is well recognised , pre-systolic accentuation is preserved  in many cases of mitral stenosis with AF.

*Crazy hemodynamics : For an attached LA clot to  dislodge ,   one needs some amount of LA contraction isn’t ?  Unfortunately  a fibrillating  atria always  tend to  have this one ! This again is a senseless  proof for some  mechanical activity of LA during AF !

How is this possible ?

Is it a  purely volume dependent filling   ? ( Or )  is it  the  Intrinsic LA starling forces that do not depend electrical atrial activation .

This is definitely an  issue to ponder over . A good LV contraction makes the atria empty more completely . This would  somehow  mean , LV relaxation  is facilitating atrial function . During  AF the LV  handles effectively  the additional burden  imposed by the loss of   25  %  booster pump of atria ( Accelerated LV relaxation ? )  A  constantly  changing  RR interval makes LV diastolic function a more complex event .

Final message

Atrial fibrillation is  a well tolerated  arrhythmia in vast  majority of patients  . This  implies either of the two things.

  1. The so called  physiological atrial  booster pump is redundant  or dispensable in otherwise healthy heart
  2. The booster pump is indeed important  . . . but it is less  affected by AF as long as the rate is under control !

It is to be  strongly emphasized , Heart rate and  LV function  will ultimately determine  , how one is going to tolerate the AF  !

It is  a small gesture  from LV  to LA  at it’s hour of crisis  . . . in return  for  it’s lifetime assistance  as a booster pump ! 

Postamble

How  rate control  prevails over rhythm control in spite  zero atrial contractility in the  former  ?

Comments welcome !

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