A novel approach for mouse echocardiography expanding accessibility and enabling retrospective analysis in diastolic function assessment

Murine echocardiography is an essential tool in cardiovascular research for non-invasive in-vivo assessment of cardiac function. While most systolic function parameters are measured in rather simple accessible recordings of the parasternal long-axis view - brightness-mode (PSLAX - B-mode), the current gold standard for measuring key parameters of diastolic function, relies on apical four-chamber (A4C) view -  pulsed wave Doppler cine loops. However, high-quality recordings of this view demand considerable expertise, limiting researchers with less experience in conducting comprehensive function studies. This contrasts the current demand for research in diastolic (dys)function, particularly in the highly important field of heart failure with preserved ejection fraction (HFpEF). This study investigates an alternative approach to measure isovolumetric relaxation time (IVRT), a key parameter of diastolic function, based on PSLAX - B-mode cine loops and therefore circumventing the need for extensive training in advanced mouse echocardiography. 

Speckle tracking on PSLAX - B-mode cine loops allows for continuous accurate measurement of left ventricular volume (LVV). Using this, we generate an LVV-time curve and by deriving the corresponding LVV change-time curve. According to the continuity equation and under the assumption of the mitral valve cross sectional area being constant during diastole, the shape of this curve represents mitral valve flow velocity during ventricular filling, providing a surrogate for direct Doppler measurements.

We tested this new approach in comparison to gold-standard Doppler measurements in three mouse models (n=8 each) with varying levels of cardiac function: unimpaired, HFpEF, and heart failure with reduced ejection fraction (HFrEF). Comparative analyses were conducted using Pearson correlation and Bland-Altman analysis. For mice with unimpaired heart function, the correlation coefficient (r²) of the two measurement approaches was lower at 0.4743 (p=0.0589), yet results were reliable indicated by a bias of 0.5 ms (4.12%). The HFpEF group showed strong correlation (r² = 0.8940, p=0.0004) and a bias of -0.99 ms (-6.38%). In the HFrEF group high correlation (r² = 0.6861, p=0.0111) and a bias of -3.13 ms (-12.07%) suggested consistent performance even in severely impaired hearts. With data from all groups combined, the correlation was robust (r² = 0.9139, p < 0.0001), and bias minimal (-1.21 ms, -4.78%). Also, the coefficient of variation of 5.3% matched closely to the inter-observer variability for IVRT in Doppler-based measurements. 

In conclusion, this novel volume change based approach offers a reliable alternative to flow velocity based IVRT measurements. The approach may be implemented into existing protocols and will, by being based solely on PSLAX recordings, permit a reliable assessment of diastolic heart function for researchers new to murine echocardiography. Perhaps even more importantly, it also allows retrospective analysis of PSLAX B-mode cine loop data in completed studies without A4C view recordings.