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The continuity equation was used to estimate non-invasively the stenotic mitral valve area by comparison with two other echocardiographic methods (planimetry and pressure half-time) and with Gorlin's formula as the gold standard. The accuracy of the equation of continuity was determined before and 24 h after valvuloplasty in a study group of 21 patients with severe mitral stenosis. According to the equation of continuity, mitral valve area was calculated by the product of the cross-sectional area and the aortic or pulmonary annulus and the ratio of the time velocity integral of the aortic or pulmonary flow to that of the mitral stenotic jet. In pre-valvotomy basal conditions, the Doppler continuity equation demonstrated significant correlations with 2D planimetry (r = 0.72, P less than 0.01), with the pressure half-time method (r = 0.62, P less than 0.01) and with the Gorlin formula (r = 0.66, P less than 0.01). There was no significant difference between the haemodynamic data and the echocardiographic measurements. Twenty-four hours after valvotomy, the Doppler continuity equation also demonstrated significant correlations with 2D planimetry (r = 0.83, P less than 0.01), with pressure half-time (r = 0.82, P less than 0.01) and with the Gorlin formula (r = 0.69, P less than 0.01). However, the haemodynamic measurements significantly overestimated (P less than 0.01) the echographic measurements. Thus, we conclude that the continuity equation provides an accurate estimation of mitral valve area in mitral stenosis before and after balloon valvotomy.