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In-vivo analysis of the instantaneous transvalvular pressure difference-flow relationship in aortic valve stenosis: Implications of unsteady fluid-dynamics for the clinical assessment of disease severity

Abstract:

Background and aim of the study: Because the hemodynamic basis of aortic valve area (AVA) has never been validated in vivo, several alternative indices have been proposed to quantify the severity of aortic stenosis (AS). This study was designed to assess the fluid-dynamics of aortic valve stenosis in order to clarify which index best accounts for disease severity. The diagnostic implications of reversed ΔP during ejection were also investigated. Methods: Chronic valvular AS characterized by stiff leaflets without commissural fusion was created surgically in eight adult mongrel dogs; three additional animals were used as controls. At two-week intervals (three studies per dog), simultaneous micromanometer pressure and transit-time Q measurements were collected under different hemodynamic conditions. Instantaneous ΔP and Q signals were processed digitally and fitted to a modified form of the unsteady Bernoulli equation in which AS is characterized by effective valve area. Results: An unsteady Bernoulli equation accurately pbkp_redicted measured instantaneous ΔP values (R = 0.97 ± 0.06), and a quadratic correlation was observed between instantaneously fitted and Gorlin-derived AVA. Additionally, ΔP < 0 mmHg during late ejection was observed in the majority of AS datasets, with a normalized time to ΔP reversal of 93 ± 13% for AS animals versus 69 ± 36% for controls (p <0.0005). Time to ΔP reversal inversely correlated with the Strouhal number (R = -0.77), and was responsible for an overestimation of mean systolic transvalvular ΔP and Q that resulted in a significant bias in the Gorlin method. Error was highest in moderate stenosis with low transvalvular output. Conclusion: Unsteady fluid-dynamics supports AVA over other measures of AS such as aortic valve resistance. However importantly, late-ejection reversal of ΔP precludes estimating the systolic ejection period from pressure tracings, and accounts for an additional source of error when AS is quantified invasively.