Using CMRI to Make the Unpredictable Less Hazy

The suddenness and unpredictability of sudden cardiac death (SCD) event magnifies the challenge faced by the clinician who endeavors to protect a high risk patient from this devastating clinical event. The issues of sudden death and its consequences are more tragic in the Indian context where younger population in their productive ages are involved and resources to protect them from this catastrophe are scarce.^{1, 2} Though implantable defibrillators (ICD) are easily available, implantation of these devices in many developing countries even for secondary prevention is dismal. Diagnostic tools and technology that can enhance the ability of current stratification algorithms to recognize patients with potentially high event rates are invaluable in this part of the world. Though left ventricular ejection fraction (LVEF) has been the gold standard parameter used for risk stratification of SCD, endorsed by all the official guidelines globally, the predictive capabilities of this parameter have not met the valid expectations of clinicians. Numerous noninvasive markers have been employed to refine the predictions gathered from applying LVEF to different subsets but none have individually found to be effective. ³ Hence LV dysfunction continues to be relied upon for decision-making for ICD implantation. It is recognized beyond doubt that it is not the LVEF number but an underlying scar in the myocardium resulting from interstitial and replacement fibrosis that forms the pathological substrate that creates the milieu for perpetuation of malignant re-entrant arrhythmias that result in SCD. Electroanatomic mapping during ablative therapy for these arrhythmias elegantly delineates the scars that anchor the ventricular tachycardia. Cardiac magnetic resonance imaging (CMRI) has evolved and established as a reliable tool capable of objectively defining this electropathological substrate. There is currently very compelling data on the capability of late gadolinium enhancement (LGE) by CMRI to predict occurrence of cardiovascular (CV) events including SCD.^4-6 The predictive capabilities of risk stratification algorithms have undoubtedly been enhanced by inclusion of this tool in assessment of different pathophysiological substrates. The publication of DANISH trial results brought out a global debate on the wisdom of using the LVEF numbers in prescription of ICD. ⁷ Though subsequent analysis of data did show that benefits of ICD were still perceived in many subgroups of non-ischemic cardiomyopathy (NICM) patients, yet the vulnerability of LVEF as a sole factor to assess SCD was clearly exposed. Identification and quantification of LGE in the ventricles of NICM patients have identified patients with moderate LV dysfunction who may be at risk of SCD while giving a clean chit to some patients with severe LV dysfunction. Use of CMRI in heart failure (HF) patients has been very useful in further stratifying patients with LV dysfunction. We used a cut-off of 5% volume of LGE to demonstrate the value of CMRI in predicting CV events in 133 HF patients with LVEF ≤ 40%. In this cohort, 44/46 CV events were seen in patients who had LGE over the cut off level. ⁸ Patients presenting to ER with resuscitated cardiac arrest are a difficult substrate as many do not have easily identifiable or modifiable substrates. Data with CMI has shown that identification of scar in these patients can help to optimally use the defibrillators. ⁹ Hypertrophic cardiomyopathy (HCM) is another substrate where lot of gray areas exist, particularly in patients who present with syncope. Knowing that reflex vasovagal is the commonest etiology of syncope even in patients with structural heart disease; it is a clinician’s dilemma to identify patients at a risk of SCD. CMRI has come to rescue in these difficult decision-making situations. LGE can delineate a scar as well as contribute to identification of aneurysms and assessment of septal thickness. ¹⁰ Limitation of technology in using LGE has been circumvented by using the extra cellular volume (ECV) assessment by T1 mapping which can identify diffuse fibrosis and in addition provide the opportunity to be used in patients with renal failure. ¹¹ CMRI is proving to be a highly effective modality capable of prognostication of SCD and decision-making in its management. It has the worth and potential to be included in the clinical guidelines.