Brennan M. Johnson

Quantifying function in the early embryonic heart.

Congenital heart defects arise during the early stages of development, and studies have linked abnormal blood flow and irregular cardiac function to improper cardiac morphogenesis. The embryonic zebrafish offers superb optical access for live imaging of heart development. Here, we build upon previously used techniques to develop a methodology for quantifying cardiac function in the embryonic zebrafish model. Imaging was performed using bright field microscopy at 1500 frames/s at 0.76 μm/pixel. Heart function was manipulated in a wild-type zebrafish at ∼55 h post fertilization (hpf). Blood velocity and luminal diameter were measured at the atrial inlet and atrioventricular junction (AVJ) by analyzing spatiotemporal plots. Control volume analysis was used to estimate the flow rate waveform, retrograde fractions, stroke volume, and cardiac output. The diameter and flow waveforms at the inlet and AVJ are highly repeatable between heart beats. We have developed a methodology for quantifying overall heart function, which can be applied to early stages of zebrafish development.

Valveless pumping mechanics of the embryonic heart during cardiac looping: Pressure and flow through micro-PIV

Cardiovascular development is influenced by the flow-induced stress environment originating from cardiac biomechanics. To characterize the stress environment, it is necessary to quantify flow and pressure. Here, we quantify the flow field in a developing zebrafish heart during the looping stage through micro-particle imaging velocimetry and by analyzing spatiotemporal plots. We further build upon previous methods to noninvasively quantify the pressure field at a low Reynolds number using flow field data for the first time, while also comparing the impact of viscosity models. Through this method, we show that the atrium builds up pressure to ~0.25mmHg relative to the ventricle during atrial systole and that atrial expansion creates a pressure difference of ~0.15mmHg across the atrium, resulting in efficient cardiac pumping. With these techniques, it is possible to noninvasively fully characterize hemodynamics during heart development.

Mechanisms influencing retrograde flow in the atrioventricular canal during early embryonic cardiogenesis.

Normal development of the heart is regulated, in part, by mechanical influences associated with blood flow during early stages of embryogenesis. Specifically, the potential for retrograde flow at the atrioventricular canal (AVC) is particularly important in valve development. However, the mechanisms causing this retrograde flow have received little attention. In this study, a numerical analysis was performed on images of the embryonic zebrafish heart between 48 and 55hpf. During these stages, normal retrograde flow is prevalent. To manipulate this flow, zebrafish were placed in a centrifuge and subjected to a hypergravity environment to alter the cardiac preload at various six-hour intervals between 24 and 48hpf. Parameters of the pumping mechanics were then analyzed through a spatiotemporal analysis of processed image sequences. We find that the loss of retrograde flow in experimentally manipulated embryos occurs in part because of a greater resistance in the form of atrial and AVC contractile closure. Additionally, during retrograde flow, these embryos exhibit significantly greater pressure difference across the AVC based on calculations of expansive and contractile rates of the atrium and ventricle. These results elucidated that the developing heart is highly sensitive to small changes in pumping mechanics as it strives to maintain normal hemodynamic conditions necessary for later cardiac development.

Erratum to: The Transitional Cardiac Pumping Mechanics in the Embryonic Heart

This is a change to the first sentence of the discussion section: The original sentence reads ‘‘The above presented results are, to our knowledge, the first to document blood flow patterns and kinematics during the early transitional phases of the developing heart using the zebrafish model’’ We would like to drop the claim to be the first as it could be mis-interpreted and change the sentence to ‘‘The above presented results document blood flow patterns and kinematics during the early transitional phases of the developing heart using the zebrafish model’’

Effect of Blood Pressure on Closing Dynamics of Bileaflet Mechanical Heart Valves

Implantation of a bileaflet mechanical heart valve (BMHV) continues to be associated with a risk of thromboembolic complications despite anti-coagulation therapy1. This has been attributed to the structurally rigid design of the leaflets and valve mechanics combined with an intricate hinge mechanism for the rigid leaflets. The lack of a built in compliance within the valve mechanics presumably leads to sharp stress gradients within the flow as well as a violent closure of the valve often associated with the audible impact of the leaflets to the housing, and a potential for momentary cavitation of blood in the wake of leaflet impact.© 2012 ASME

Quantifying the Biomechanics of the Embryonic Zebrafish Heart

Congenital heart defects are present in 4 to 50 per 1000 live births[1]. Most of these defects begin within the first few weeks post fertilization. Ample evidence exists which shows that mechanical epigenetic factors, such as pressure and shear stress, play key roles in heart development [2–3]. It has been shown in-vitro that cardiomyocytes are able to sense and respond to the presence of pulsatile flow[4], and that shear stress can activate genetic pathways which might ultimately dictate the morphological development of the cardiac tissue[5]. When blood flow characteristics have been changed experimentally, embryonic hearts consistently develop serious malformations. In order to understand mechanical epigenetic factors and their role in heart development, it is critical to contrive techniques for quantitatively measuring the biomechanics of the embryonic heart.Copyright