Abstract
Objective: In healthy adults, the right atrium (RA) serves as a reservoir for the systemic flow return from the superior vena cava (SVC) and inferior vena cava (IVC), preparing the two flows to be transferred to the right ventricle (RV) and pulmonary circulation. This study aims to quantify the haemodynamics of the RA and the associated SVC and IVC inflows, which have not been fully understood to date. Approach: Eighteen adults with structurally normal hearts underwent 4D flow magnetic resonance imaging. The cardiac cycle was resolved to 20 temporal phases with a spatial resolution of 3 × 3 × 3 mm3. Analysis included objective visualisation of the flow structures in the RA identified by three different vortex identification criteria, kinetic energy (KE), enstrophy and dissipation. KE and helicity flux were also assessed in both caval veins. Main results: Vortex identification methods confirmed that in the majority of participants the blood flow from the caval veins filling the RA during ventricular systole is not chaotic, but rather forms an organised pattern of a single coherent forward turning vortex structure. Thirteen participants displayed a single vortex flow structure, four showed multiple vortices and one had a helical flow pattern without a clear vortex structure. A strong positive correlation exists between the flow KE and enstrophy density. Significance: This suggests that flow energy in the RA is mainly rotational, part of which is convected by the highly helical SVC and IVC inflows. Multiple vortices tend to be associated with higher dissipation rates in the central RA region due to turbulence. The rotational nature of the flow in the RA maintains KE better than non-rotational flow. RA flow characteristics are highly related to the helicity content in the caval veins, as well as the KE flux intensity. Lower caval helicity or IVC KE flux dominance tends to favour single vortex formation while the opposite tends to lead to multiple vortices or the rare helical flow patterns. Atria lacking single vortex flow are inclined to have a larger energy input from atrial contraction.
Highlights
The right atrium (RA) forms the interface for deoxygenated blood to pass from the superior and inferior vena cava (SVC and IVC) to the right ventricle (RV) for circulation through the pulmonary system
IVC dominant kinetic energy (KE) flux tends to result in a single organised forward rotating vortex commonly observed in the RA during ventricular systole (Kilner et al 2000), while the lack of IVC flux dominance tends to lead to the heterogeneous multiple vortices or the rarer helical flow pattern
Almost all of the KE in the blood flow originating from the two caval veins is in the form of rotational energy which is quantified by enstrophy and is reflected by the correlation of the KE and the enstrophy (figure 7(a))
Summary
The right atrium (RA) forms the interface for deoxygenated blood to pass from the superior and inferior vena cava (SVC and IVC) to the right ventricle (RV) for circulation through the pulmonary system. Evaluation of the fluid dynamics within the RA mainly has been qualitative (Parikh et al 2017, Kilner et al 2000, Suwa et al 2014) This is useful for examining the generic flow regimes within the RA, there is very little as yet which quantitatively characterises flow in the RA. Kilner et al (2000) theorised that the looped shape of the heart aids in the conservation of fluid momentum, improving its efficiency They found that in the RA incoming flows from the IVC and SVC did not collide with each other, as one might expect, but instead interacted with one another in a circular motion. It was hypothesised that this helps the flow maintain its momentum and redirect itself towards the RV, requiring less external work to be done on the fluid This investigation used streamlines and flow visualisation to show this phenomenon, without quantitative analysis
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