The behaviour of natural shear waves under different loading conditions

Background Shear wave imaging (SWI) is a novel ultrasound technique based on the detection of transverse waves traveling through the myocardium using high frame rate echocardiography. These waves can be naturally induced e.g. by mitral valve closure (MVC). Their propagation velocity is dependent on the stiffness of the myocardium. Previous studies have shown the potential of SWI for the non-invasive assessment of myocardial stiffness. So far, the influence of loading on shear wave propagation velocities has not been extensively investigated. Purpose The aim of this study was to explore how loading changes affect shear wave propagation velocities after MVC. Methods Until now, 5 pigs (weight: 33.5±6.9 kg) were included. Echocardiographic images and left ventricular pressure recordings were simultaneously acquired during acute loading alterations: 1) preload was reduced by balloon occlusion of the vena cava inferior, 2) afterload was increased by balloon occlusion of the descending aorta and 3) preload was increased by intra-venous administration of 500 ml of saline. Left ventricular parasternal long-axis views were acquired with an experimental high frame rate ultrasound scanner (average frame rate: 1247±179 Hz). Shear waves were visualized on tissue acceleration maps by drawing an M-mode line along the interventricular septum. Shear wave propagation velocities after MVC were calculated by measuring the slope of the wave front on the acceleration maps (Figure A). Results The changes in left ventricular end-diastolic pressures (LV EDP) between baseline and each intervention are shown in Figure B. Preload reduction resulted in significantly reduced LV EDP (p<0.01). The shear wave propagation velocities after MVC dropped with preload reduction and increased significantly by increasing afterload as well as preload (both p<0.05) (Figure C). There was a good positive correlation between the change in LV EDP and the change in shear wave velocities (r=0.83; p<0.001) (Figure D). Conclusion The shear wave propagation velocity after MVC was significantly influenced by alterations in left ventricular loading conditions and changes in these velocities were related to changes in LV EDP. These results indicate that shear wave measurements at MVC might be a potential novel parameter for the estimation of left ventricular filling pressures. More pigs will be included in the future to further confirm these findings. Funding Acknowledgement Type of funding source: Public grant(s) – National budget only. Main funding source(s): Fonds Wetenschappelijk Onderzoek - Vlaanderen