Milad Samaee School of Mechanical and Aerospace Engineering, Oklahoma State University, 201 General Academic Building, Stillwater, OK 74078 Nicholas H. Nelsen School of Mechanical and Aerospace Engineering, Oklahoma State University, 201 General Academic Building, Stillwater, OK 74078 Manikantam G. Gaddam School of Mechanical and Aerospace Engineering, Oklahoma State University, 201 General Academic Building, Stillwater, OK 74078 Arvind Santhanakrishnan 1 School of Mechanical and Aerospace Engineering, Oklahoma State University, 201 General Academic Building, Stillwater, OK 74078 e-mail: askrish@okstate.edu Diastolic Vortex Alterations With Reducing Left Ventricular Volume: An In Vitro Study Despite the large number of studies of intraventricular filling dynamics for potential clin- ical applications, little is known as to how the diastolic vortex ring properties are altered with reduction in internal volume of the cardiac left ventricle (LV). The latter is of partic- ular importance in LV diastolic dysfunction (LVDD) and in congenital diseases such as hypertrophic cardiomyopathy (HCM), where LV hypertrophy (LVH) can reduce LV inter- nal volume. We hypothesized that peak circulation and the rate of decay of circulation of the diastolic vortex would be altered with reducing end diastolic volume (EDV) due to increasing confinement. We tested this hypothesis on physical models of normal LV and HCM geometries, under identical prescribed inflow profiles and for multiple EDVs, using time-resolved particle image velocimetry (TR-PIV) measurements on a left heart simula- tor. Formation and pinch-off of the vortex ring were nearly unaffected with changes to geometry and EDV. Pinch-off occurred before the end of early filling (E-wave) in all test conditions. Peak circulation of the vortex core near the LV outflow tract (LVOT) increased with lowering EDV and was lowest for the HCM model. The rate of decay of normalized circulation in dimensionless formation time (T * ) increased with decreasing EDV. When using a modified version of T * that included average LV cross-sectional area and EDV, normalized circulation of all tested EDVs collapsed closely in the normal LV model (10% maximum difference between EDVs). Collectively, our results show that LV shape and internal volume play a critical role in diastolic vortex ring dynamics. [DOI: 10.1115/1.4047663] Keywords: left heart simulator, intraventricular flow, diastolic vortex, end diastolic volume, hypertrophic cardiomyopathy 1 Introduction A large number of studies have examined the fluid dynamics of filling (diastole) in the human cardiac left ventricle (LV) [1–17]. Vortex ring formation is observed as blood enters the LV from the left atrium via the mitral valve. Diastolic filling vortex properties have been observed to change between normal cardiac function and impaired pumping (LV systolic dysfunction) [2,4,9], as well as in impaired filling or LV diastolic dysfunction (LVDD) [5,7,18–20]. Pathological remodeling of the LV can occur in LVDD on account of pressure overload. The LV wall subse- quently thickens with simultaneous reduction in LV internal volume, leading to concentric LV hypertrophy (LVH) [21]. In addition to factors such as hypertension and coronary artery dis- ease, congenital diseases such as hypertrophic cardiomyopathy (HCM) can also result in LVDD with reduction in LV internal volume [22]. Recent evidence shows that healthy hearts show strong coupling between the volumes of the filling vortex ring and the LV [15], suggesting an elevated importance of LV shape. However, a mechanistic understanding of how diastolic vortex properties are altered with reduction in internal volume of the LV (as in concentric LVH) remains unavailable. As multiple patho- logical conditions resulting in LVDD are typically accompanied by reduced internal volume of the LV, such a fundamental under- standing can help in determining whether changes in diastolic vor- tex properties can be potentially useful in monitoring progression of LVDD. The vortex formation time (VFT) is the most commonly eval- uated index [23–25] based on the diastolic vortex. VFT signifies the dimensionless time instant when a vortex ring stops growing, reaching both maximum volume and strength (circulation), and pinches-off from the inflow jet. VFT was originally formulated by considering the propagation of a vortex ring in an unconfined (semi-infinite) surrounding domain [26]. The entire duration of early filling (E-wave) portion of diastole is typically used in car- diac VFT calculation [2]. However, a recent study employing human subject data [27] showed that vortex ring pinch-off occurs well before the completion of E-wave, questioning the appropri- ateness of VFT definition. This study also observed that VFT, when recalculated by only accounting for pinch-off time and not the entire E-wave, was not affected by diastolic impairment [27]. The main reason for this discrepancy was noted to be the assump- tion of an unconfined domain in VFT formulation [27,28]. In LVDD, reduction in LV internal volume (due to LVH and/or impaired LV relaxation) would further increase the confinement imposed by the LV cavity on the filling vortex. A previous study [29] showed that confined vortex rings experience an increased rate of decay of circulation, when compared to vortex rings that propagate in an unconfined domain. Collectively, these findings suggest that confinement from the LV cavity needs to be consid- ered when examining filling vortex dynamics in LVDD. Exclud- ing dilated cardiomyopathy where systolic dysfunction with eccentric LVH are observed (increased LV internal volume) [9], no study has systematically examined how decreasing LV internal volume impacts circulation of the diastolic vortex. The purpose of this study was to comparatively examine how diastolic vortex properties are altered with reduction in LV inter- nal volume. We hypothesized that increasing confinement imposed on the diastolic vortex ring, by decreasing LV internal volume, would alter peak circulation and increase the rate of cir- culation decay following pinch-off. We used an in vitro approach and examined diastolic vortex dynamics within a flexible-walled 1 Corresponding author. Manuscript received December 8, 2019; final manuscript received June 18, 2020; published online September 8, 2020. Assoc. Editor: Keefe B. Manning. 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