Background:The pressure half time(PHT) method has been widely used to estimate mitral valve area(MVA) in patients with mitral stenosis(MS). However, its limitation has been repeatedly recognized under different circumstances. This study sought to evaluate the effect of cardiac rhythm on the accuracy of the PHT-derived MVA calculation in relation to left atrial(LA) compliance. Methods: 41 patients with pure moderate or severe MS were enrolled. 18 patients with sinus rhythm were allocated to group 1(GrI) and 23 with chronic atrial fibrillation to group 2(GrII). MVA was obtained by the PHT method and planimetry. Net atrioventricular compliance(Cn) was calculated with the equation previously validated and was regarded as a representative of LA compliance. Results:There was no difference between the 2 groups in terms of age, gender, LV dimensions or EF and transmitral pressure gradient. LA volume and Cn were higher in GrII than in GrI(199짹118mL vs. 127짹27mL, p=0.02 for LA volume; 6.6짹1.5mL/mmHg vs. 4.7짹1.2mL/mmHg, p<0.001 for Cn). The extent of overestimation of the true MVA by the PHT method was 8.0짹19.2% for GrI and -24.9짹13.9% for GrII(p=0.002), respectively and was correlated with Cn(r2=0.71,p<0.001, Figures). Overestimation was exponentially increased in patients with Cn of <4mL/mmHg. Conclusion:Changes in cardiac rhythm with associated alterations in Cn can alter the accuracy of the PHT method for estimating MVA. Given the limitation provided here, 2D planimetry, not the PHT method, should be used as a primary tool for MVA calculation.