Valvuloarterial impedance (Zva): Global left ventricular afterload in aortic stenosis
Valvuloarterial impedance (Zva) is an echocardiographic measure of global left ventricular afterload in aortic stenosis . It accounts for the effect of both aortic stenosis and systemic arterial compliance.
Zva (mmHg/ml/m2) = (systolic blood pressure + mean transvalvular pressure gradient)/stroke volume index
Here the systemic arterial pressure and mean transvalvar gradient are taken beyond the region of pressure recovery in aortic stenosis. This mean transvalvar gradient has been called the net mean gradient.
In a study of 208 patients with at least moderate aortic stenosis, Briand M and colleagues found that Zva was the only hemodynamic variable independently associated with left ventricular dysfunction .
Ito H et al showed that post operative Zva was significantly associated with regression of left ventricular mass after aortic valve replacement . Left ventricular global afterload has to be maintained low for enhancing reverse remodelling of the left ventricle. This would include optimal control of blood pressure in a hypertensive. It has been documented earlier that in patients with mild – moderate aortic stenosis, hypertension was associated with more abnormal left ventricular structure and increased adverse cardiovascular events .
Patients with moderate aortic stenosis and concomitant hypertension may have a global hemodynamic load equivalent to or more than those with severe aortic stenosis without hypertension . This can lead to development of left ventricular dysfunction and symptoms.
Left ventricular ejection fraction may underestimate the myocardial systolic impairment in asymptomatic patients with severe aortic stenosis. Myocardial dysfunction has been demonstrated by two dimensional speckle tracking study in those with increased global left ventricular afterload, especially in those with low flow aortic stenosis . Circumferential myocardial deformation was shown to be significantly affected.
High global left ventricular load was defined as Zva >5 mm Hg/ml/m2. It was found in 18% of the 252 patients in the study by Rieck AE et al . High global left ventricular load was associated with female gender, higher age, hypertension, more severe aortic stenosis and lower ejection fraction. Multivariate Cox regression analyses showed that high global left ventricular load predicted higher rate of major cardiovascular and aortic valve events. But it failed to predict mortality .