To assess the incidence and long-term evolution of left-to-right atrial shunting (AS) after the performance of percutaneous mitral valvuloplasty (PMV), venovenous indicator dilution curves and right heart oximetric measurements were obtained in 68 consecutive patients before and after successful PMV. The procedure increased the mitral valve area (p<0.0001) and decreased the mitral gradient (p<0.0001). No AS was detected before PMV, but it was detected immediately after PMV. Oximetry identified AS in 17 patients (25%), and dilution curves identified AS in an additional 25 (total, 62%). The ratio of mean pulmonary to systemic blood flow (Qp/Qs) was 1.31±0.2, and in six patients (9%), the ratio was 1.5 or greater. Among nine clinical, 20 hemodynamic, and six procedural variables, stepwise logistic regression analysis selected the following as independent predictors of AS: smaller increases in valve area (p =0.01) after PMV, absence of previous surgical commissurotomy (p=0.02), mitral valve calcification (p=0.02), and smaller left atria (p=0.06). Among the 33 patients recatheterized at 6 months, oximetry had detected AS in 10, and dilution curves detected AS in an additional nine (total, 58%) immediately after PMV. At 6 months, AS had decreased or disappeared in 14 of these patients (74%), had increased in three (16%), and was unchanged in two (10%). Overall, at 6 months, oximetry identified AS in three patients, and dilution curves identified AS in an additional 13 (total, 48%). AS was detected at 6 months in only three patients but was not detected immediately after PMV. Although AS is very frequent immediately after PMV, Qp/Qs is usually less than 1.5. The appearance of shunting correlates with patient characteristics and with less improvement in valve area after PMV. Atrial shunting usually persists at 6 months, but its severity almost always decreases. (Circulation 1990;81:1190-1197 P ercutaneous mitral valvuloplasty (PMV) is being increasingly used as an alternative to surgery for the treatment of mitral valve stenosis.1-6 Commissural splitting and fracture of calcified areas appears to be the mechanism that produces an increase in orifice area.4,78 PMV may result in mitral valve areas similar to those achieved after surgical commissurotomy.9'10 Recent studies suggest that the marked symptomatic improvement and the increase in mitral valve area persist at follow-up."1-'4The transvenous technique of PMV requires a transseptal approach to the left atrium followed by dilation of the interatrial septum to facilitate the passage of valvuloplasty balloon catheters.1,4-6 Incidences between 10% and 25% of residual leftto-right atrial shunts have been detected by oximetry, which was used to detect atrial shunting.6"13,15-17However, this method lacks sensitivity.18-20 In addition, the long-term evolution of atrial shunts after PMV remains completely unknown. Therefore, this prospective study was undertaken to determine the incidence and long-term evolution of left-to-right atrial shunting in a consecutive series of pati...