Dr.S.Venkatesan MD

Surprises are hall-marks of medical science . The cardiologists do  get  it ,   in enough doses   from  echo  labs  on a regular basis !   . One such thing is  the total ECG-ECHO myocardial  territorial  mismatch following  a STEMI .  Human myocardial segments are divided by cardiologists  by 17 segments by echocardiogram . Long before  echo came into vogue ,  electro-cardiologists  divided the  heart electrically into three zones to  localise MI . (Anterior , inferior and  the  poorly defined entity  lateral walls* ) .Inferior and posterior  segments are  almost used interchangeably. So , when we have 17  echo  segments to be fit into these three electrical category !   were  bound to have  some overlap . The issues of fitting in septal segments is really complex as septum  is a three dimensionally engulfs all three electrical surface of the heart .

* By the way , anatomists  never agreed about existence of walls in heart.They simply said  , heart has smooth  surfaces that blends with one another.  We cardiologist have  built imaginary walls and struggling to come out it !

We will   try to answer the question that’s been asked here .  “Inferior MI”  by ECG   . . . “Anterior MI”  by  echocardiography . How common is that ?

Possible causes for this wrong call

Technical errors  in  acquiring echo  imaging plane  or  it’s interpretation is the commonest . Many  times  ,  obliquely obtained long axis view  wrongly and strongly  suggests  a septal  MI  instead of   inferior posterior MI. This is  because  in  apical 4  chamber view  bulk of   septum  (Basal and mid third )  lies   in the  infero-posterior region .

Perhaps ,  misunderstanding this  septal  geography is  the  commonest cause for  erroneously  calling inferior MI as anterior  in echocardiography . (A simple clue is the presence of MR . (It  fixes the infarct in infero-posterior zone with 90% accuracy )

Rotation  and  posture of heart

Alignment of the septum to the rest of the chambers  can influence  , how three inferior leads is going to look  at the septum (There can be  considerable errors  -Electrical myopia ? as these leads are located distantly )  . The plane of the septum is such that  in horizontal hearts  septal electrical activity  will be directed infero posteriorly inscribing a q waves in inferior leads rather than anterior leads . One can expect such ECG /Echo discrepancy in the following subset as well

• Post CABG patients (Any pericardiotomy will make the septal motion  erratic )
• Obese persons
• COPD

There are three  more  situations  ,  which   mystified me   with  definite  ECG/ECHO  mismatch

1. LVH and STEMI  is always an engima . Counter clockwise rotation when accopany  LVH  that masks anterior MI  electrically . It  however inscribes a   q wave in inferior leads.
2.  In dominant LCX lesions  ( with at-least  one  major OM    )  and  left main bifurcation  STEMIs  ,  combination of  anterior and inferior  wall motion defects are  quiet common . When a such  a  MI evolves ( with or without  revascularization )   regeneration of R wave can be  time shifted . Septal R wave may appear  much earlier and inferior R may follow or vice versa . .Further,  anterior MI  may  evolve as  Non q MI  making it  ECG blind ,   still  echo may pick up the WMA . So there can be important  ECG-ECHO mismatch in myocardial segmental geography .
3. Further , WMA  need not  always be an  infarct  .Any new episode of ischemia  can result in WMA . Hence a patient  with inferior Q waves  in ECG may experience anterior wall motion defect meagerly  due to fresh episode of   ischemia (This we should not attribute  to  old anterior  MI. It is also possible intra-myocardial conduction delays can elicit remote wall motion defects.

Final message

By general rule  , ECG  correlates  well  with  ECHO  for localising myocardial segments   . At times ,  it  can  really be tricky , and we  get into above situation  in echo labs.

While ,  it is common to observe  ECGs  to mimic  inferior MI  at the first look  and  subsequently echo  revealing  anterior  infarct ,  the reverse is also very much possible .

The  mechanisms are varied and technical  issues are for more frequent than true clinical discrepancy .The issue has important management implications.

Of course ,  coronary angiogram will pin point the   anatomy , still  it also has  strong limitations in localizing myocardial segments (to which it supplies ) especially with multi-vessel  CAD and  collateral dependent circulation .