Accurate quantification of LV diastolic function is paramount in the diagnosis, classification, and treatment of heart failure, particularly in patients with HFpEF. Pulsed-wave Doppler echocardiography is a relatively quick, low-cost, and widely accessible tool often used routinely in clinical practice to evaluate mitral inflow peak velocities and LV filling pressures [7, 8]. The quality of images obtained from echocardiography is largely dependent on operator expertise, and the consistency of findings is hampered by operator variability [3]. Other limitations of echocardiography include Doppler misalignment, limited acoustic windows, limited accuracy and reproducibility, and inferior image resolution relative to CMR [9].

This case report demonstrates the challenges associated with echocardiography in the assessment of mitral inflow peak velocities where AR is present. AR describes a valvular pathology where the aortic valve fails to close adequately [10]. During LV diastolic filling, both mitral inflow and regurgitant blood from the incompetent aortic valve contribute to the LV volume. A recent study investigated the flow dynamics in the LV of patients AR [11]. It demonstrated that as the severity of regurgitation increased, the “diastolic vortex“ generated from the aortic regurgitant jet interacted with the “vortex“ originating from true mitral inflow, competing for space in the LV cavity. As the regurgitation worsens, the mitral inflow “vortex” becomes confined to the LV wall while the regurgitant jet dominates the center of the LV chamber. It is therefore possible that the Doppler probe incorrectly detected mitral inflow, which mostly comprised blood flow from the aortic regurgitant jet. This could explain the anomalous velocity tracing depicted in this case report, which is uncharacteristic of typical mitral inflow.

Other studies have postulated a number of different mechanisms as to why this occurs in Doppler echocardiography. Enlargement of the mitral valve leaflet and left ventricle in response to chronic aortic regurgitation [12] could alter the hemodynamics of transmitral blood flow, although the exact mechanism for this remains unknown. A functional mitral stenosis may arise as a result of the aortic regurgitant jet forcing closed the mitral valve leaflets prematurely [12] or the regurgitant jet causing a “kinematic obstruction” between the mitral valve and LV apex [13], hampering transmitral inflow and the ultrasonographic detection of peak velocities across the mitral valve. These changes in the flow physics of the LV, mitral valve morphology, and function as a result of interference from the regurgitant jet could explain the challenges in accurately distinguishing peak E-wave and A-wave velocities using pulsed-wave Doppler echocardiography.

Phase-contrast 4D flow CMR is an imaging technique used to assess and visualize multidirectional blood flow in three dimensions (3D) resolved in time [4, 14, 15]. Four-dimensional flow CMR offers a promising alternative to echocardiography in LV diastolic function assessment, which we have discovered circumvents its limitations in AR. Its routine use has increased over the past few decades in the assessment of cardiac morphology, contractility, and myocardial perfusion. Studies have shown reproducibility and accuracy equal or superior to echocardiography in the assessment of mitral inflow velocities [5, 16, 17]. Four-dimensional flow CMR is less prone to operator-dependent variability and can provide greater imaging detail than standard echocardiography, but its widespread clinical use is hampered by its long scan and postprocessing times [18]. A study by Dyvorne et al. [19] demonstrated an accelerated 4D flow MRI technique using a combination of spiral sampling and dynamic compressed sensing to significantly reduce scan times for the quantification of blood flow in abdominal vasculature. Further investigation into how this method can translate into intracardiac blood flow quantification would be beneficial to circumvent the long scan times that are inherent to 4D flow CMR. Additionally, future study into possible technological advancements that allow further automation of 4D flow CMR analysis could reduce postprocessing times and user interference.

Here, 4D flow CMR with valve tracking and automated 3D streamline capabilities allowed better feasibility in the detection of peak mitral inflow velocities by restricting the streamline assessment to the length of the mitral valve leaflets. This capability to manually isolate mitral inflow streamlines during 4D flow mapping is unique to 4D flow CMR [20]. This allowed clearer depiction of E-wave and A-wave transvalvular peak velocities (panel F) by limiting the interference of aortic regurgitant velocities on the acquired mitral velocity measurements. This case illustrates the feasibility of 4D flow CMR in identifying mitral inflow peak velocities and LV diastolic function in patients with aortic regurgitation.

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