Certain principles
It is to be understood, LA is a separate chamber only in systole ie ,when MV is closed. In diastole the pulmonary veins, the LA, LV all act like a single virtual anatomical /physiological chamber.
The principle of the atrial booster pump contributing to 25 % of LV diastolic filling may be an undisputable fact, but in the real world in normal persons fall in stroke volume and cardiac output during AF , is not a consistent feature. The well-contracting LV (Rather relaxing ) compensates spectacularly for the loss of the atrial booster pump by increasing its suction power.
In such a scenario, the LA suddenly fibrillates, what will happen?
In a decompensated heart AF can have serious consequences. Not otherwise.
Then,this question comes. Is it the rate or rhythm that is responsible for this hemodynamic deterioration?
It is intuitive to blame the rhythm, as it eliminates the booster pump, but it is prudent to infer, the sudden increase in the rate, does more hemodynamic harm. This is one of the reasons, the rate vs rhythm debate, rhythm control is still struggling to prove its superiority in spite of its apparent hemodynamic superiority.
Can LA pressure increase during its reservoir function independent of LV function (When MV is closed)?
This is very tricky, we rarely discuss about the dissociation between LVEDP and LA mean pressure.LA compliance is to be discussed with reference to the atrial cardiac cycle.
Is it the LV systolic or diastolic function that Impacts LA mean pressure ?
Obviously , it is the diastolic, but the diastolic funtion is intrinsically interwined with its systolic function is a major topic by itself .
A lesson from severe mitral stenosis and AF.
Increase in mean LA pressure at the onset of AF is instantaneous, the quantum of increase is unpredictable. Though, It is logical to expect, acute pulmonary edema is a rare complication in MS, at the onset of AF.
Final message
The normal LA hemodynamics and its pressure-volume loop is a complex topic. It might make some sense, to know AF paralyses only one component of its triple function, ie Reservoir, conduit, and pump. LA can, still handle this arrhythmia effectively with assistance from LV. This is very much true in an otherwise healthy heart.
Dr.S.Venkatesan MD 