Clarification of Question by
sabrinapdx-ga
on
22 Feb 2006 22:21 PST
the question stems from the following common scenario: a person has
normal LV dimentions, normal LV EF and significantly reduced Cardiac
Index. I'm having a hard time understanding how and why that happens.
in thinking about it purhaps the first and more basic question is how
is a normal ejection fraction maintained in diastolic failure?
ejection fraction is the amount of blood released from the heart's
ventricle when it contracts.
In the scenerio of diastolic dysfunction (which is the only time I
THINK that this occurs) the muscle is too stiff to expand properly
enough to fill with an appropriate volume - I can understand how that
would work with frank starling laws to decrease contractability and
ultimately lower EF, but that's not what happens, EF is maintained.
Talking myself through this - using unrealistic numbers for
conceptual ease of use. If a normal heart fills with 10 cc blood and
ejects 60% of it = 6 cc. In diastolic falailure if the heart fills
with only 5 cc blood and pumps out 60% of it = 3cc, that's how you
would accomplish the actual difference in cardiac output.
I think I've finally grasped the concept that has been alluding me
1)in diastolic heart failure the the heart never really fills. 2) I
think in terms of systolic failure, where a fair amount of blood is
left in the ventricle with every beat - but in diastolic failure
that's just not happening - I think that was the fundimental failure
in my ability to grasp this concept.
Almost all of the blood is being ejected in diastolic failure (normal
EF), which is significantly less in terms of volume than a normal
heart (reduced CO). the ability to compensate for the low output is
decreased due to the facts that 1) increasing heart rate further
reduces preload and 2) as you decrease preload you would also decrease
contractility (frank-starling).
In terms of being mathematically confusing to me: stroke volume
measures the actual volume ejected : ((end diastolic volume) - (end
systolic volume)) Where as ejection fraction : ((end diastolic volume)
- (end systolic volume)) * (100%)) / (end diastolic volume) measures
the percentage of blood in the ventricle that was ejected. So to get
to the cardiac output = (stroke volume)(heart rate) it now makes sense
to me based on what I postulated above how the stroke volume and
ejection fraction can be significantly different, not just in
diastolic failure but also certian constrictive settings or valvular
dysfunctions when the percentage of volume ejected may be high but the
actual volume ejected is low.
I know that this is not clarification in the way that you asked, but I
think I may have answered my own question!!! Can you please look this
over and see if that seems conceptually correct to you. If it does
consider the question answered - if nothing else this is an excelent
medium for me to figure stuff out.
