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.
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.