The concept of valve resistance was introduced as a “stenotic index” in the 1950s,1 but it did not reach the worldwide acceptance obtained in the same years by the Gorlin formula to calculate valve area. In more recent years this relatively old concept has been restored as a method for assessing the severity of aortic stenosis.2 ,3 Aortic valve resistance is simply the pressure gradient/flow rate ratio expressed in units of dyne.s.cm⁻⁵. Aortic valve resistance is commonly calculated using cardiac catheterisation with the following equation:

(1.333 × P) ÷ (CO/HR × SEP)

where CO is cardiac output (ml/min), HR is heart rate (beats/min), SEP is systolic ejection period (s/beat), and 1.333 is the conversion factor from mm Hg to dyne.s.cm⁻⁵. Aortic valve resistance can also be accurately measured by Doppler echocardiography, using the following formula:

1.333 × 4Vmax²/ area LVOT × velocity LVOT

where V is the maximum velocity recorded across the aortic valve by continuous wave Doppler, area LVOT is the area of the left ventricular outflow tract obtained from the parasternal long axis view as 3.14 × (diameter/2) assuming a circular shape, and velocity LVOT is the maximum velocity recorded in the left ventricular outflow tract by pulsed wave Doppler.

There are two main reasons for the renewed interest in measuring aortic valve resistance: valve resistance represents a functional index of haemodynamic impairment rather than an anatomic index such as valve area; aortic valve resistance appears to remain more constant as flow varies than calculated aortic valve area—this is particularly relevant in low flow states, in which the valve area calculation may be inaccurate.2 ,3 Nevertheless, some issues remain:

• is aortic valve resistance really less flow dependent than aortic valve area?

• does aortic valve resistance really provide additional physiopathological and clinical information to valve area and gradient?

• are there reliable cut off values …