Potential role of Reynolds number in resolving Doppler- and catheter-based transvalvular gradient discrepancies in aortic stenosis

Background and aim of the study: Discrepancies in mean transvalvular gradient have been observed between Doppler echocardiography and catheterbased techniques in the assessment of aortic stenosis (AS). The Reynolds number (RE) has been shown to influence Doppler-derived gradients, and may be useful in resolving Doppler- and catheter-based gradient discrepancies in AS. The study aim was to assess the influence of the RE on such discrepancies. Methods: A pulsatile in-vitro heart model using a bioprosthetic aortic valve with leaflets sutured together was used to simulate AS. Simultaneous gradients were measured using Doppler echocardiography and high-fidelity catheters while the RE was varied, by testing solutions of different density and viscosity across a range of cardiac outputs. Results: The echocardiographic and catheter-derived mean gradient (MG) values were correlated (r = 0.89; p <0.0001); however, significant differences in the MG were observed across hemodynamic states. A direct linear relationship was identified between RE and the absolute difference in MG measured using the two techniques (r = 0.94, p <0.0001). Relative to catheter-based measurements, the MG was underestimated by Doppler (range: 13-16 mmHg) at low RE (median 6,999) and overestimated (7-33 mmHg) at high RE (median 34,268). However, agreement between catheter- and Doppler-derived gradients was within 5 mmHg at intermediate RE (median 17,284) (p <0.0001). Conclusion: The underestimation of Doppler-derived MGs at low RE relative to catheter-based measurements may be due to an exclusion of viscous friction from the simplified Bernoulli equation, while the overestimation of Doppler-derived MGs at high RE may be due to a pressure recovery effect. However, within an intermediate range of RE, where the effects of viscous and inertial forces are balanced, the agreement between catheter- and Doppler-derived gradients was excellent.