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If ischemia is a powerful trigger for ventricular arrhythmias ,Why cardiac arrhytmias are less common in NSTEMI ?

Posted May 10 2009 10:43pm

Acute coronary syndrome is the commonest cardiac emergency. STEMI and NSTEMI are the two clinical limbs of ACS. Generally they have distinct clinical, ECG, angiographic features.(Ofcourse,  with some degree of overlap) . It is  a  mystery , both clinical presentations differ so much inspite of the common denominator  , namely ,  an injured plaque with add on thrombus  within the coronary artery.

The mystery is since  decoded , the primary difference between these two entities is STEMI the occlusion occurs sudden and complete and in NSTEMI it occurs slow and incomplete

In STEMI ,  most of the clinical features and , need for emergent treatment , response to thrombolysis /PCI are dictated by the time dependent risk to myocardial loss .

Cardiac arrhythmias in ACS

It is a  much published  factoid   for  many decades  only one third of STEMI patients  reach the hospital alive ! The reason being , STEMI  is very much prone for primary VF.

Contrary  to this ,  almost all patients with NSTEMI reach the hospital alive ! How ?

Both are ACS, if ischemia is a powerful trigger for dangerous ventricular  arrhythmia’s ,  NSTEMI should also behave  similarly .

So what protects against arrhythmias in NSTEMI ?

  • We realise ,  by observational experience (Not EBM !)  It is the suddenness and totality of ischemia that trigger dangerous form of arrhythmia  .
  • Further, a balanced  ischemia in two contralateral segments (or global  ischemia) some how protects against development of ventricular  fibrillation .This may be due to preservation  of  electrical homogeneity  , and the spherical VT spiral waves are not sustainable.
  • In contrast , STEMI has a sudden  focal , ischemic  zone that initiates the VT and    ischemia free  contralateral segment  welcoming  and sustaining the  reentrant wavelet.
  • The observation of primarily single vessel disese in STEMI and multivessel disease in NSTEMI also give credence to this concept.
  • Further , ischemic preconditioning can exert an important anti arrhythmic  effect in NSTEMI as  patients with unstable angina have   slow, repetitive episodes of ischemia prior to the index event .
  • Post MI scar mediated VT/VF is independent of degree of overall ischemia
  • It is also established ,  a sub group of  STEMI pateints  who  had  preinfarction angina(  ie . a brief  period of UA/NSTEMI) have very low risk of SCD  supporting the concept of sensitising the myocardium against ventricular arrhythmias.

Final message

Even though , there is a convincing concept  of   ischemia induced  cardiac arrhythmia in literature , in real patients it is very difficult to link the two.

UA/NSTEMI is the most common  acute ischemic event but the incidence of VT/VF here,  is far less than one would expect.

In ACS , focal , total  ischemia is more likely to precipitate a VT/VF than multifocal and global ischemia.

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