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.
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.