New estimate of valvuloarterial impedance in aortic valve stenosis: A cardiac magnetic resonance study

Purpose: Valvuloarterial impedance (Z_VA ), estimating left ventricle (LV) afterload, has been proposed in transthoracic echocardiography (TTE) as a predictor of mortality in aortic valve stenosis (AVS). However, its calculation differs from arterial characteristic impedance (Z_C ). Our aim was to apply the concept of Z_C calculation to estimate Z_VA from MR with carotid tonometry and to evaluate these indices through their associations with symptoms, LV diastolic function and aortic stiffness.

Materials and methods: In 40 patients with AVS (76 ± 13 years), Z_VA-TI derived from velocity time integral and E/Ea were estimated by TTE. Z_VA-INS , based on Z_C formula, calculated as the instantaneous pressure gradient to peak flow ratio and aortic compliance were estimated by using MRI at 1.5 Tesla.

Results: Both Z_VA estimates were higher in symptomatic than asymptomatic patients (707 ± 22 versus 579 ± 53 dyne.s/cm⁵ , P = 0.031 for Z_VA-INS and 4.35 ± 0.16 versus 3.33 ± 0.38 mmHg.m² /mL, P = 0.018 for Z_VA-TI ). Although they were both associated with aortic compliance (r = -0.45; P = 0.006 for Z_VA-INS and r = -0.43; P = 0.008 for Z_VA-TI ) only Z_VA-INS was associated with E/Ea (r = 0.50; P < 0.001). In multivariate analysis to identify determinants of E/Ea, a model including age, mean blood pressure, LV ejection fraction, LV mass, and aortic valve area was performed (R² = 0.41; P < 0.01). When Z_VA-INS was added to the model, its overall significance was higher R² = 0.56 (P < 0.01) and Z_VA-INS and LV mass were the only significant determinants.

Conclusion: Z_VA-INS was more strongly associated with diastolic dysfunction than usual parameters quantifying AVS severity. This new Z_VA estimate could improve LV afterload evaluation.

Level of evidence: 1 J. Magn. Reson. Imaging 2017;45:795-803.