Sonomicrometry-Based Analysis of Post-Myocardial Infarction Regional Mechanics

The dynamic shape changes of the tongue base during respiration, chewing and swallowing

This study aimed to analyze dimensional deformations of the tongue base during respiration, chewing, and swallowing. Eight 7-8-month-old Yucatan minipigs were used. Under deep sedation, eight 2mm ultrasonic piezoelectric (SONO) crystals were implanted in the tongue base forming a cubic-shaped configuration, representing right/left dorsal and ventral lengths, anterior/posterior dorsal and ventral widths, and right/left anterior and posterior thicknesses. Next, 8 pairs of electromyographic (EMG) microelectrodes were inserted into the tongue, jaw, hyoid, pharyngeal, and palatal muscles. SONO and EMG signals during respiration were recorded. Then, minipigs were allowed to wake-up for unrestrained feeding. The feeding sessions were recorded with synchronized EMG and videofluoroscopy to confirm the phases of jaw movement in chewing, and swallowing episodes. Amplitudes, durations, and timings for each dimension of the SONO crystal-circumscribed region were measured from the start of the jaw opening. Findings during respiration showed elongated lengths, anterior widths and anterior thickness (p<0.05). For chewing, the width elongated up to 17% while the length and thickness shortened (12-33% and 10-32% respectively, p<0.05). Onsets of deformational changes in length and thickness occurred 10-30% earlier than in width. The cycle duration was 0.55 ± 0.11seconds chewing, and 0.69 ± 0.16seconds swallowing. During swallowing, the dorsal length (5-12%) and posterior width (10-14%) elongated whereas the posterior thickness (9-15%) and ventral length (4-10%) shortened. Explicit 3D-kinematic patterns in relation to specific functions characterize the tongue base deformation. The findings of this analysis will contribute to a better understanding of the oropharyngeal biomechanics upon abnormal conditions.

Influence of Supraphysiologic Biomaterial Stiffness on Ventricular Mechanics and Myocardial Infarct Reinforcement

Injectable intramyocardial biomaterials have promise to limit adverse ventricular remodeling through mechanical and biologic mechanisms. While some success has been observed by injecting materials to regenerate new tissue, optimal biomaterial stiffness to thicken and stiffen infarcted myocardium to limit adverse remodeling has not been determined. In this work, we present an in-vivo study of the impact of biomaterial stiffness over a wide range of stiffness moduli on ventricular mechanics. We utilized injectable methacrylated polyethylene glycol (PEG) hydrogels fabricated at 3 different mechanical moduli: 5 kPa (low), 25 kPa (medium/myocardium), and 250 kPa (high/supraphysiologic). We demonstrate that the supraphysiological high stiffness favorably alters post-infarct ventricular mechanics and prevents negative tissue remodeling. Lower stiffness materials do not alter mechanics and thus to be effective, must instead target biological reparative mechanisms. These results may influence rationale design criteria for biomaterials developed for infarct reinforcement therapy. STATEMENT OF SIGNIFICANCE: Acellular biomaterials for cardiac application can provide benefit via mechanical and biological mechanisms post myocardial infarction. We study the role of biomaterial mechanical characteristics on ventricular mechanics in myocardial infarcts. Previous studies have not measured the influence of injected biomaterials on ventricular mechanics, and consequently rational design criteria is unknown. By utilizing an in-vivo assessment of ventricular mechanics, we demonstrate that low stiffness biomaterial do not alter pathologic ventricular mechanics. Thus, to be effective, low stiffness biomaterials must target biological reparative mechanisms. Physiologic and supra-physiologic biomaterials favorably alter post-infarct mechanics and prevents adverse ventricular remodeling.

Regional and temporal changes in left ventricular strain and stiffness in a porcine model of myocardial infarction

The aim of the present study was to serially track how myocardial infarction (MI) impacts regional myocardial strain and mechanical properties of the left ventricle (LV) in a large animal model. Post-MI remodeling has distinct regional effects throughout the LV myocardium. Regional quantification of LV biomechanical behavior could help explain changes in global function and thus advance clinical assessment of post-MI remodeling. The present study is based on a porcine MI model to characterize LV biomechanics over 28 days post-MI via speckle-tracking echocardiography (STE). Regional myocardial strain and strain rate were recorded in the circumferential, radial, and longitudinal directions at baseline and at 3, 14, and 28 days post-MI. Regional myocardial wall stress was calculated using standard echocardiographic metrics of geometry and Doppler-derived hemodynamic measurements. Regional diastolic myocardial stiffness was calculated from the resultant stress-strain relations. Peak strain and phasic strain rates were nonuniformly reduced throughout the myocardium post-MI, whereas time to peak strain was increased to a similar degree in the MI region and border zone by 28 days post-MI. Elevations in diastolic myocardial stiffness in the MI region plateaued at 14 days post-MI, after which a significant reduction in MI regional stiffness in the longitudinal direction occurred between 14 and 28 days post-MI. Post-MI biomechanical changes in the LV myocardium were initially limited to the MI region but nonuniformly extended into the neighboring border zone and remote myocardium over 28 days post-MI. STE enabled quantification of regional and temporal differences in myocardial strain and diastolic stiffness, underscoring the potential of this technique for clinical assessment of post-MI remodeling. NEW & NOTEWORTHY For the first time, speckle-tracking echocardiography was used to serially track regional biomechanical behavior and mechanical properties postmyocardial infarction (post-MI). We found that changes initially confined to the MI region extended throughout the myocardium in a nonuniform fashion over 28 days post-MI. Speckle-tracking echocardiography-based evaluation of regional changes in left ventricular biomechanics could advance both clinical assessment of left ventricular remodeling and therapeutic strategies that target aberrant biomechanical behavior post-MI.