Introduction:
Valvuloarterial impedance (Z
VA
) 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). Z
VA
is traditionally determined by Transthoracic Echocardiogram (TTE) and brachial cuff pressure.
Hypothesis:
Our study sought to compare Z
VA-TTE
with Z
VA
calculated using a simultaneous Cardiac Magnetic Resonance (CMR) and Applanation Tonometry (AT) (Z
VA-CMR
) technique to determine whether TTE measurement of aortic flow velocity resulted in an underestimation of Z
VA.
Methods:
Twenty AS patients underwent a protocol of CMR/AT followed by TTE. Z
VA-CMR
was determined as the relationship of derived aortic pressure (radial) to CMR aortic flow velocity in the frequency domain. Z
VA-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.
Results:
Our study found that Z
VA-TTE
values (mean±SD, 638±381 dyne.s.cm
-3
) were consistently lower (p=0.07) than Z
VA-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.
Conclusions:
Whilst Z
VA -TTE
is commonly performed in patients with AS to assess global LV load, newer methods to assess Z
VA
using simultaneous CMR/AT likely represent a more accurate non-invasive assessment.
Non-Invasive Determination of Left Ventricular Workload in Patients with Aortic Stenosis Using Magnetic Resonance Imaging and Doppler Echocardiography
