291 Assessing Valvuloarterial Impedance in Aortic Stenosis: A Comparison of Echocardiographic- and Cardiac Magnetic Resonance-derived Methods

Abstract

Valvuloarterial impedance (ZVA) represents the valvular and arterial factors that oppose Left Ventricular (LV) ejection and is recognised as an important index to assess global LV load in patients with Aortic Stenosis (AS). ZVA is traditionally determined by Transthoracic Echocardiogram (TTE) and brachial cuff pressure. Our study sought to compare ZVA-TTE with ZVA calculated using a simultaneous Cardiac Magnetic Resonance (CMR) and Applanation Tonometry (AT) (ZVA-CMR) technique. Twenty AS patients underwent a protocol of CMR/AT followed by TTE. ZVA-CMR was determined as the relationship of derived aortic pressure (radial) to CMR aortic flow velocity in the frequency domain. ZVA-TTE was determined from digitised flow velocity within the left ventricular outflow tract (LVOT) on pulsed-wave Doppler and derived central pressure waveforms. Systemic vascular resistance (SVR) was calculated from mean pressure and flow. Values from both methods were compared. Our study found that ZVA-TTE values (mean±SD, 638±381 dyne.s.cm-3) were consistently lower (p=0.07) than ZVA-CMR values (946±318 dyne.s.cm-3), and attribute this to an overestimation of LVOT flow velocity on TTE. SVR calculated by CMR/AT (2215±616 dyne.s.cm-5) was almost four times higher than TTE (618±245 dyne.s.cm-5) (p<0.001). This is due to more robust axi-symmetrical sampling of aortic flow across the entire cross-section of the ascending aorta (well above the stenotic jet) during CMR, than using operator-dependent TTE. Whilst ZVA -TTE is commonly performed in patients with AS to assess global LV load, newer methods to assess ZVA using simultaneous CMR/AT likely represent a more accurate non-invasive assessment.