Left ventricular absolute myocardial work measured by cardiovascular magnetic resonance: a proof-of-concept
Authors: Almashhadani M T , Harbo M B , Blom K , Ribic D , Russell K , Rosseland V , Storaas T , Andersen G O , Opdahl A , Klow N E , Knudsen E C , Broch K , Birkeland J A , Sjaastad I , Espe E 
Published: October 2024

Abstract

Background
Myocardial Work Index (MWI) is a load-independent and non-invasive method for assessing cardiac systolic function [1]. However, MWI does not directly measure absolute myocardial work (AMW), since it does not consider the effect of variations in left ventricular (LV) wall thickness and regional wall curvature. Therefore, a direct comparison of MWI results may be difficult. Cardiovascular magnetic resonance (CMR) is the gold standard for assessing the LV geometry and therefore can be used to directly measure the AMW by calculating the area of the stress-strain curve.

Purpose
This study aims to measure and compare CMR-based AMW in three groups with different work profiles: healthy subjects, patients with ST-elevation myocardial infarction (STEMI), and patients with severe aortic valve stenosis (AS). Additionally, it aims to compare AMW with strain to provide additional insights into remodelling.

Methods
A total of 78 participants were included in our study; 38 healthy subjects from the CENS study [2], 19 STEMI patients 3-7 days post-PCI from the ASSAIL study [3], and 21 patients with AS scheduled for operation. Collected data were not adjusted for age or sex. Stress was estimated using Laplace’s equation [4]. Intra-ventricular pressure was calculated from brachial cuff pressure as previously described [5]. Mid-ventricular circumferential strain was calculated using CMR-based tissue phase mapping [4]. AMW was calculated both as Myocardial Work per unit endocardial Surface (MWS, in J/m^2) and per unit Mass (MWM, in mJ/g), both serving as measurements of global circumferential systolic function.

Results
Mean circumferential strain did not differ significantly between the AS and the healthy groups (p=0.37), but it was significantly lower in the STEMI group compared to the healthy group (p&lt;0.001). MWS was significantly higher in the AS group (p&lt;0.001) and lower in the STEMI group (p&lt;0.001) compared the healthy group. This suggests that each unit of endocardial surface in the myocardium performed, on average, more work in the AS group and less work in the STEMI group relative to the healthy group. MWM was not significantly different between the AS and the healthy group (p=0.74), while in the STEMI group, it was significantly lower (p=0.001) than in healthy subjects.

Conclusion
Our results indicate that in patients with AS, the myocardium did more work per unit surface, while each gram of the myocardium did the same amount of work. This is likely due to concentric hypertrophy and the increase of afterload. In contrast, in patients with STEMI, the myocardium did, on average, less work per unit surface and per mass. This is expected due to the direct loss of mechanical function in the infarcted myocardium. Our results indicate that AMW enhances traditional strain analysis, improves myocardial function evaluation, and may provide deeper insight into myocardial remodeling in diverse cardiac conditions.