Evaluation of shunt flow volumes through a large orifice using an oval cylinder flow convergence method: An in vitro study

Colour Doppler flow mapping of the flow convergence region (FCR) has been reported to be useful for the quantitation of intracardiac convergent flow. However, most implementations of this technique are generated from theoretical models based on infinitely small regurgitant orifices. Little data exists on their applicability for evaluating accelerating flow volumes towards relatively large orifices such as atrial septal defects (ASD). We therefore applied two FCR methods to quantify flow volumes through a large oval orifice mounted in a flow model. A pulsatile flow pump connected with a flow probe and meter was used to generate 11 different flow volumes (20 - 70 ml/beat). A large oval orifice simulating an ASD (with ratio of major to minor axis 2:1, area 1.8cm²) was placed between the inlet and outlet chambers of the model. Flow accelerations were imaged at an aliasing velocity of 24 cm/s. A calibrated echo probe rotation allowed us to select and measure three axial radii (a, b and c) of the maximal FCR. The maximal FCR surface areas were calculated using (i) a standard hemielliptical geometry model and (ii) a new oval cylinder model designed for computing predominantly parallel flow velocity vectors. Shunt flow volumes were calculated using the maximum FCR areas combined with the ratio of the velocity-time integral/peak velocity obtained by pulsed wave Doppler at the centre of the orifice. Both FCR methods showed a significant correlation with reference flow volumes (r=0.95, p=0.0001) but the hemielliptical model underestimated them substantially (mean difference -28±7.7%). In contrast, the new oval cylinder flow acceleration model predicted the reference ASD shunt volumes more reliably (mean difference -4.7±11.2%).