The aortic valve gradient estimates the difference in pressures on each side of the aortic valve. There should be no pressure gradient across a normal aortic valve. When a valve is unable to let blood pass through it at a normal rate it is called “stenotic.” The pressure in the compartment full of blood waiting to pass through a stenotic valve is much higher than that the pressure on the other side (downstream) because so little blood is passing through the valve and making it to the other side. Consider a large herd of cattle walking in a tunnel that has a gate in the middle. If the gate opens wide for a reasonable period of time about half of the cows can cross to the other side of the tunnel before the gate shuts. Then the same number of cows are on each side of the gate. But if the gate were rusted and only opened a little bit for a short period of time, only a few cows would get through and then there would be difference in the number of cows on each side after the gate shuts—ie. a pressure gradient.