Ambient Fluid Velocity Influences Proximal Isovelocity Surface Area Calculations

Background: Recent reports have described a simplified mass balance equation designed to predict volumetric flow rate through regurgitant and shunt orifices. The proximal isovelocity surface area (PISA) technique relies on the assumption that constant velocity surfaces (isovels) that are hemispheric in shape can be constructed proximal to the orifice, and flow passing through these surfaces must also pass through the orifice by conservation of mass. The method has been validated in simplified in vitro models, but these models have been symmetric in design and do not consist of ambient crossflows that are found in the beating heart. This study addressed the hypothesis that ambient crossflow will affect the validity of the PISA method. Methods and Results: An in vitro model was designed to test the effect of crossflowing streams on the PISA calculation while maintaining control of volumetric flow through the orifice. PISA predicted flows showed a good correlation but overestimated actual flows by 10%–110% (Y = 2.16X‐118, r = 0.96). Addition of low flow data slightly reduced the overestimation (Y = 1.89X‐55.2, r = 0.97). Only under high output state conditions (high orifice and crossing flows) was good agreement found (Y = 0.92X + 9.78, r = 0.95). Conclusions: The presence of crossflowing streams in the vicinity of PISA images will distort the converging flow and its image in such a way that the simplified PISA theory may correlate with true flow but only rarely produces true agreement. Only when competing factors fortuitously cancel does the PISA method appear to be accurate. Since PISA images are readily obtained in patients and since the method has received extensive attention in the literature, systematic derivation of correction factors accounting for crossflow is indicated. In the meantime, caution should be exercised when interpreting PISA images in the presence of crossflowing streams .