AF

Afib is common and is associated with increased mortality in critically ill patients.

Systolic function:

LV contractility is reduced by 3 mechanisms. A short diastole:

  1. means LV emptying begins before aortic pressure has fully decreased with the increased afterload reducing SV.
  2. gives less time for LV filling with reduced preload causing a reduced SV.
  3. means there is less time for calcium to be taken up into the sarcoplasmic reticulum after its release in systole so the next systole will have less calcium available for contraction.

SV and CO are generally preserved until HR is >100. At higher rates SV falls progressively more quickly meaning CO falls despite the increase in HR.

Diastolic function

Short diastole and the loss of atrial contraction reduces LA emptying and LV filling meaning LVEDP falls while LAP rises. The LA also dilates.
Patients with diastolic impairment are relatively dependent on LA contraction for LV filling and so tolerate Afib poorly.

Measurements

Systolic function

The commonly used echo measurements of systolic function are dependent on preload, afterload and HR. (EF, FS, FAC, SV etc)
Evidence demonstrates that 10 consecutive cardiac cycles should be measured and averaged for accuracy.
A single beat can be measured if the 2 preceding R-R intervals are the same and the selected beat is representative of mean HR. 
10 consecutive beats is the simplest.

Diastolic pressure

LA volume increases with increased LVEDP and Afib so cannot distinguish between the 2.
Doppler measurements should be taken of 5-10 cardiac cycles (quality of the signal is more important than them being consecutive).
1 measurement can be used if the cycle length is between 700 and 1000 ms (HR 60-85) but this is not validated in IPPV.
E velocity correlates poorly with PAOP in Afib (E velocity falls in mild relaxation failure but increases with increased LAP from Afib). Changes in E velocity in an individual patient will correlate with changes in PAOP however.
E deceleration time correlates well with PAOP if the EF is <45%. A EDT of <150 ms predicts a PAOP >15 (sens 71%; spec 100%). <120 = PAOP >20 (sens 100; spec 96).
IVRT performs well but HR dependent. IVRT <65 predicts PAOP >15.
E/Vp (Vp is the slope of the aliasing velocity of LV inflow using colour M-mode in LV inflow) shows good correlation with PAOP but is complicated to measure. E/Vp >1.4 = PAOP >15.
E/Em is probably the best to use in Afib when balancing accuracy with drawbacks. E/Em >11 = PAOP >15 (sens 75; spec 93).

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