Aortic stenosis severity will be assessed at baseline and every 6 months by echocardiography performed by a single, dedicated sonographer to maximize reproducibility. Severity will be assessed using the peak and mean aortic valve pressure gradients (this technique is used to monitor aortic stenosis severity; the higher the pressure gradient across the aortic valve the more severe the narrowing). We will also calculate the aortic valve area and calcification score.