Aortic Valve Area Calculator (Gorlin Formula)

Formula

How this is calculated

The Gorlin formula treats a stenotic valve as a hydraulic orifice: flow through it equals the product of valve area, a discharge constant, and the square root of the driving pressure gradient. Rearranging for area gives AVA = flow_rate ÷ (44.3 × √ΔPmean), where 44.3 is a semi-empirical constant encoding unit conversions and a discharge coefficient validated against post-mortem valve measurements by Gorlin and Gorlin in 1951.

The mean systolic flow rate is derived from three readily available haemodynamic measurements: cardiac output (CO, in L/min from thermodilution or the Fick method), heart rate (HR, in bpm), and systolic ejection period (SEP, the seconds per beat that the aortic valve is open). Stroke volume = CO × 1000 / HR gives the mL ejected per beat; dividing by SEP gives the mean flow rate in mL/s. Combining with the mean trans-valvular pressure gradient yields AVA in cm². Normal AVA is 2–4 cm²; mild aortic stenosis (AS) is 1.5–2.0 cm², moderate 1.0–1.5 cm², severe < 1.0 cm², and very severe (critical) < 0.6 cm².

The Gorlin formula has well-documented limitations: it systematically underestimates AVA in low-flow, low-gradient states, such as severe left-ventricular dysfunction with reduced cardiac output, leading to the "pseudo-severe AS" phenomenon. In such cases, dobutamine stress haemodynamics are used to distinguish true from pseudo-severe stenosis. The continuity equation from Doppler echocardiography is generally preferred in everyday clinical practice because it does not require invasive catheterisation. This calculator is a reference tool for educational and planning purposes.