The myocardial function index (MFI): An integrated measure of cardiac function in AL-cardiomyopathy

Background: Amyloid light chain (AL) amyloidosis is a systemic disease that can cause restrictive cardiomyopathy (AL-CM). Current imaging techniques are not sensitive to detect myocardial dysfunction in AL-CM. We sought to evaluate role of a novel marker of myocardial dysfunction (myocardial function index, MFI) obtained using changes in left ventricular (LV) blood pool and myocardial volume in diastole and systole.

Methods: Consecutive patients diagnosed with AL-CM who had underwent cardiac MRI between 2001-2017 were identified and compared to healthy individuals. Two independent operators used cardiac MRI to perform epicardial and endocardial tracings in systole and diastole to obtain myocardial volume in diastole (MVd) and myocardial volume in systole (MVs). Changes in myocardial volumes during the cardiac cycle were measured to calculate the MFI by $\frac{\left(MVd-MVs\right)+Strokevolume}{\mathrm{MVd}+LVenddiastolicvolume}$ . Multivariable analysis was performed to evaluate predictors of all-cause mortality and survival was evaluated using Kaplan Meier analysis.

Results: Patients with AL-CM (n = 129, 61 ± 10 years, 32 % women) were older and more likely to be men compared to the normal cohort (n = 101, 39 ± 15 years, 61 % women). MFI was lower in patients with AL-CM (19 % [15; 23] vs 38 % [35; 41], p < 0.001) and MFI < 30 % discriminated between AL-CM with 92 % sensitivity and 100 % specificity (AUC 0.98, p < 0.001). Higher MFI was independently associated with survival even after adjusting for conventional prognostic biomarkers of AL-CM (HR 0.02, 95 % CI 2.23 *104 - 0.24, p < 0.05). Two independent operators demonstrated high intra and inter-rater correlation in measurements used to calculate MFI.

Conclusion: MFI is a novel metric for assessing LV function. It is abnormal in patients with AL-CM and may play a role in risk stratification.