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What are the types of ischemia that can be observed during excercise stress testing

Posted Jun 30 2010 10:50am

Exercise stress test is done  in two major situations.

1.In apparently normal individuals , to detect any hidden ischemia or to aid in the diagnosis of  chest pain before a coronary event occurs.

2.  The second group :  It  is done after a coronary event , to assess the over all risk  . Currently   EST has   become a  popular  investigation  after a  coronary  angiogram ,  to assess the physiological impact of  a given lesion .

(Note the complete reversal of the concept over the years !) Eg -Is a  70% LAD lesion ,  really obstructing the flow ? EST can give a clue.

When interpreting the EST , we always set our mind to the “sole  concept of   inducible ischemia”  , but , in reality  the mechanism of ischemia especially in post ACS patient  is a complex one .

Number of mechanisms of ischemia may occur during an EST

  • Resting (Basal)* ischemia
  • Resting ischemia getting worsened with exercise.
  • Inducible ischemia in the affected segment
  • Inducible ischemia in a new remote  territory
  • Non inducible ischemia**
  • Residual ischemia post ACS.
  • Differential ischemic burden in  various coronary beds
  • Collateral dependent ischemia
  • Coronary spasm mediated (Micro or macro)
  • A combination any of  the above

Among the above , the most common mechanism  in post  coronary event  population   is  ,  basal ischemia  which is  aggravated by exercise .

**Is there a entity called ” non”  inducible ischemia ?

Current cardiology literature do not  give us the liberty to label like this !  But , when there is inducible ischemia there  must  be  ”non” inducible ischemia as well .

Note : All rest angina are , in fact ,  forms of non inducible ischemia .* Technically a patient  with a resting ischemia should not undergo an EST. but many times we won’t be aware of this.

Apart from spontaneous rest angina  and unstable angina there are areas of myocardium  that show chronic low grade ischemia  ( Hibernating muycardium ) .It is also possible some  segments of myocardium  are immune to development of ischemia , in spite of reduced flow.These are the lesions missed by EST and reported as false negative.

  • This can be up to 20% of all EST performed .
  • We are not clear,  why in some individuals ischemia can not be induced by exercise !
  • Some  cells may  have  better oxygen utilization, ischemic tolerance is better , referred to ischemic preconditioning
  • Finally technical factors may also be important in missing an episode of ischemia .The window of EST lead system may miss some of  the ST depression  , referred to as  electrical blindness .
  • Canceling  out of two opposing areas of ischemic forces  ( that keeps ST segment isolectric )is also reported. A critical LAD ischemia may get masked by a diagonally opposite tight PDA lesion.

Differential patterns of ischemia

RCA-inducible ischemia  ! LAD – non inducible ischemia ! Can such a situation occur !

Yes , any combinations can occur. RCA lesion  may limit the flow   at 6 mets ,  while LAD flow limitation may occur at 10 METS etc etc .This sort of analysis of EST data has never been done. This could be important ,  to identify the physiological impact  of each lesion. Of course, nuclear  imaging and PET scans will answer these questions more accurately.

Final message

When evaluating patients with EST we need to realize ,  the term inducible ischemia is  often  used in those patients ,  during  evaluation of chest pain for  suspected CAD.

While , in the vast majority of established clinical  CAD patients ,  some  amount of  basal  ischemia  is always there.It  gets aggravated  on stress , if there is a flow limiting lesion supplying that region. Reporting these episodes as simply as inducible ischemia convey a wrong  meaning  , as if ,  these patients  are free from  other forms of ischemia , which is often not true .

Hence , when reporting  EST , physicians are encouraged to use different reporting format.One , for  the diagnostic purposes   in patients ,  prior to development  of coronary  event  ,  and  the other for analysing ,  different facets of ischemia  , in those   population after the event happens.

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