Inaccuracy of mitral pressure half-time immediately after percutaneous mitral valvotomy. Dependence on transmitral gradient and left atrial and ventricular compliance.

Mitral pressure half-time (T1/2) is widely used as an independent measure of mitral valve area in patients undergoing percutaneous mitral valvotomy. However, fluid dynamics theory predicts T1/2 to be strongly dependent on chamber compliance and the peak transmitral gradient, which are variables that change dramatically with valvotomy. These theoretical predictions were tested in an in vitro model of the left heart where valve area, chamber compliance, and initial gradient were independently adjusted. Measured T1/2 was observed to vary inversely with orifice area and directly with net chamber compliance and the square root of the initial pressure gradient. Theoretical predictions of T1/2 agreed with observed values with r = 0.998. To test this theory in vivo, the hemodynamic tracings of 18 patients undergoing mitral valvotomy were reviewed. Predictions were made for T1/2 assuming dependence only on valve area; these showed some correlations before valvotomy (r = 0.48-0.64, p less than 0.05) but none after valvotomy (r = 0.05-0.28, p = NS). Predictions for T1/2 based on the theoretical derivation (and thus including compliance and pressure in their calculation) were much better: before valvotomy, r = 0.93-0.96, p less than 0.0001; after valvotomy, r = 0.52-0.66, p less than 0.05. These data indicate that T1/2 is not an independent inverse measure of mitral valve area but is also directly proportional to net chamber compliance and the square root of the initial transmitral gradient. These other factors render T1/2 an unreliable measure of mitral valve area in the setting of acute mitral valvotomy.