# Evolution of cardiac tissue and flow mechanics in developing Japanese Medaka

## TL;DR

This study explores how the heart of Japanese Medaka fish develops by measuring pressure and strain during early stages of development.

## Contribution

The paper introduces a novel metric called Endocardial Work to assess ventricle filling during heart development.

## Key findings

- Peak pressure drops at the atrioventricular canal and outflow tract increase steadily with age in medaka fish.
- A novel metric, Endocardial Work, differentiates developmental heart stages based on pressure and strain.
- Retrograde flows in the heart regions are associated with negative pressure drops.

## Abstract

The effects of pressure drop across cardiac valve cushion regions and endocardial wall strain in the early developmental stages of a teleost species heart are poorly understood. In the presented work, we utilize microscale particle image velocimetry (μPIV) flow measurements of developing medaka hearts from 3 to 14 dpf (n = 5 at each dpf) to quantify the pressure field and endocardial wall strain. Peak pressure drop at the atrioventricular canal (ΔPAVC) and outflow tract (ΔPOFT) show a steady increase with fish age progression. Pressure drops when non-dimensionalized with blood viscosity and heart rate at each dpf are comparable with measurements in zebrafish hearts. Retrograde flows captured at these regions display a negative pressure drop. A novel metric, Endocardial Work (EW), is introduced by analyzing the ΔPAVC-strain curves, which is a non-invasive measure of work required for ventricle filling. EW is a metric that can differentiate between the linear heart stage (< 100 Pa-%), cardiac looped chamber stage (< 300 Pa-%), and the fully formed chamber stage (> 300 Pa-%).