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.
- The so called physiological atrial booster pump is redundant or dispensable in otherwise healthy heart
- 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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Dr.S.Venkatesan MD 
I think it’s goes about time of filling the chambers. Even if the heart is healthy, rhythm is sinus, and atria have good contractility, when HR is to fast (i’m not sure but it’s about over 170), cardiac output starts decrease. This border changes when there is heart faliure. Chambers need more time to fill up. For zero atrial contractility, we should not forget about pumping force of variable pressure in chest during ventilation and stretching of atrials during ventrical systole (and compressing while ventrical diastole).The second mechanism is independent of atrial contractility. It significantly affect the filling of the chambers.
Second thing is treatment. HR control + anticoagulants, are often more safe than farmacological rhythm control.
If i’m wrong, please correct me
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