Volumetric characterisation of contracting cardiac trabeculae

Abstract

Experimental studies performed on isolated cardiac muscle are crucial for understanding the mechanisms of heart contraction. Not only do they reveal the sub-cellular processes that occur during a normal contraction, but also the changes that occur with disease. An understanding of these processes is crucial for the prevention and treatment of heart diseases. Experimental studies are usually performed on samples of different shapes and sizes, requiring that volume dependent measurements such as force and heat-rate be normalised to crosssectional area or volume in order to compare data between samples. However, many research groups are limited to estimating the shape and size of trabeculae by assuming an elliptic cylindrical geometry of the muscle. This simplification falls short when the cross sections of the muscle cannot be appropriately modelled by an ellipse. The accuracy also varies considerably depending on how the muscle is mounted. This thesis addresses the problem of normalisation by using optical coherence tomography (OCT) to image the 3D volume of the trabecula during an experiment, and by quantifying the cross-sectional areas and volume of the tissue using an image processing pipeline. An additional challenge arises from the non-uniform strain commonly developed during contraction, which is accompanied by irregular deformation of the volume. To measure this deformation, the OCT was extended to image a contracting trabecula, by synchronising the capture of cross-sectional images with the stimulation of the muscle. This approach allowed the collection of the first volumetric quantification of trabecular surface geometry during contraction. To further analyse the strain field developed during contraction, transmission images of the muscle were captured, and muscle deformation quantified using digital image correlation (DIC). Measurements of volume and strain during contraction were combined with measurements of force, length, heat-rate, and sarcomere length in a single experiment in order to demonstrate the ability to collect and synchronise measurements across all modalities. The instrumentation and analysis methods developed in this thesis promise to provide greater insight into the mechanical and energetic properties and function of living samples of cardiac muscle tissue, in both health and disease.