Echocardiographic Characterization of Myocardial Stiffness in Healthy Volunteers, Cardiac Amyloidosis, and Hypertrophic Cardiomyopathy: A Case-Control Study Using Multimodality Imaging

Background: Noninvasive tools to measure myocardial stiffness are limited. Intrinsic cardiac elastography in echocardiography relates to myocardial stiffness by measuring the propagation of the myocardial stretch generated by atrial contraction. The aims of the present study were (1) to evaluate myocardial stiffness using intrinsic cardiac elastography in healthy volunteers versus those with myocardial diseases (ie, cardiac amyloidosis [CA] and hypertrophic cardiomyopathy) and (2) to identify key factors that affect myocardial stiffness. Methods: In this prospective study, myocardial stiffness was estimated in 54 participants, including 10 hypertrophic cardiomyopathy, 28 CA, and 16 healthy volunteers. Myocardial stiffness was assessed as intrinsic velocity propagation of myocardial stretch (iVP, m/s) measured by high frame rate echocardiography (ie, above 250 frames per second). Extracellular volume was quantified by cardiac magnetic resonance in 22 participants. Amyloid burden was quantified by cardiac amyloid activity in $^{99m}$Tc-labeled pyrophosphate single-photon emission computed tomography in 10 participants. Results: Myocardial stiffness was significantly higher in the CA cohort (median iVP, 2.6 m/s; interquartile range, 1.7-3.9 m/s) than in the hypertrophic cardiomyopathy cohort (median iVP, 1.4 m/s; interquartile range, 1.0-1.8 m/s; P=0.011). In patients with CA or hypertrophic cardiomyopathy, iVP was correlated with NT-proBNP (N-terminal pro-B-type natriuretic peptide) (ρ=0.498, P=0.003), extracellular volume (ρ=0.646, P=0.004), and cardiac amyloid activity (ρ=0.891, P<0.001). In multivariate linear regression analysis, extracellular volume was independently associated with myocardial stiffness even after accounting for indexed left ventricular mass, global longitudinal strain, and E/e'. In healthy volunteers, normal myocardial stiffness was defined by the upper limit of normal of iVP at 1.7 m/s. Patients with CA and normal myocardial stiffness (iVP <1.7 m/s) were characterized by a low risk profile including lower NT-proBNP (P=0.034), lower troponin T (P=0.041), lower National Amyloidosis Center stage (P=0.031), smaller interstitial expansion (P=0.014), and smaller amyloid burden (P=0.056). Conclusions: Intrinsic cardiac elastography is a reliable noninvasive tool to measure myocardial stiffness. In this pilot study, it is related to imaging markers of interstitial expansion and amyloid burden.