Posts Tagged ‘anrep effect’

Can the LV  ejection fraction change with every  heart beat  ?

EF % is one of  the  glamorous  cardiac functional indices that has  caught  the  imagination of both patients and physicians. How accurate it is ? How reproducible it is ? How many methods are available  to arrive at EF % ?

Picture courtesey http://rachel.worldpossible.org/ocw.tufts.edu Munther Homoud, M.D

 How many of us  realise  it can  potentially   change  with  every  heart beat ? *

Apart from the heart rate dependency ,  the echocardiographic error can be amplified  by

  • Difficulty in identifying  the  leading and trialling edges  of endocardium
  • Patient posture errors
  • Edge detection errors in 2D
  • Pap muscle shadowing .
  • Angle errors
  • Sub optimal echo windows  when EF is measured  in the  bed side  in critical care units
  • Mental status of performing sonographer/cardiologist  (One who chops  2D shells hurriedly and obliquely !  )

All these make this index a highly  variable parameter(  next only   to your  city temperature ! ) This happens whether you measure EF  with M Mode, 2D Simpson , 3D volumetric etc .

* The term  “beat to beat” changes may be  a little exaggerated  statement .It is used   to convey the point of   ” huge  variability” of this parameter.  It  means there can be variations of EF %  with varying heat rate.

The heart is not an Independent organ rather, it is a slave to preload and afterload !

How to overcome the limitation  of EF ?

To overcome this  error a new  parameter called myocardial performance index (MPI) which accounts for heart rate came into vogue . (Did it come really ? Ihaven’t seen a single cardiologist  do this in his clinic ) . 3D volumetrics,  velocity vector imaging , and many other innovations has been added.  Nothing  was  able to replace the EF % . Because of complexities in the newer  modalities  most cardiologists (including  the author  )  continue to romance  the  much flawed EF %  .

Simplicity  shall   reign supreme   .  .  . in spite of  inaccuracies ,  in any walk of life  !

 How does  EF  change  beat to beat ?

The answer is simple . The contractility of heart is dependent   upon the previous  diastole ,  during which heart fills. Heart is primarily an elastic organ. Whenever the  filling is  is more   ventricle is stretched  more ( diastolic filling is the stretch ) and the subsequent force of contraction is more . This is the basis of famous frank starling law.

LV filling is dependent on RV filling which in turn depend on venous return ..Venous return is a function of  vascular tone and the persons physical activity .

Apart from this  adrenergic drive make the heart contract vigorously . This is the reason ,  many patients  with  severely compromised  LV function  in ICU  , supported  with  inotropic agents  show vigorous contraction of heart .(Basis of doubutamine  stress test )

** Every one of us is aware about the huge influence  the preload  has ,  on LV contractility .  Surprisingly,  it   can also  swing  with changing  after load . This fact is often  under recognised .This is called Anrep effect .

So , imagine the scenerio . . .the heart is simply  a “squeezing- slave”  of   pre load and  after load  !  . . . And still we are happy with assessing the cardiac function ,  in isolation without giving any respect to the loading conditions.

Final message

EF ,  would rank  first among all  medical  investigations ,  that is  significantly  flawed , still  continue  to  enjoy huge popularity  ! It has little value as a  screening   test for assessing  LV function in  general  population . But ,  it  has an  important role to assess  the damage following   MI and in  the  follow up of patients with   significantly  compromised LV function.

Cardiologist are aware of this fact ,  but most non cardiologists , especially  Anesthetists  and Surgeons  revere  the  EF% with    sanctity  . This is definitely un-called for . It is the duty of the cardiologists to pass on  this  message to their colleagues in other fields.

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