Maximal cusp separation of less than 8 mm in a long-axis view suggests critical stenosis, whereas greater than 12 mm separation suggests noncritical disease.

Measurements of aortic valve separation can be made with M-mode techniques where a characteristic “box car” pattern is seen on M-mode display when the aortic valve is open, with leaflet separation represented by the width of the box car.

In patients with membranous subaortic stenosis, M-mode assessment may show early systolic closure of the valve (Figure 2).