Application of proximal isovelocity surface area method to determine prosthetic mitral valve area

Hemielliptic proximal isovelocity surface area method modified for clinical application: more accurate quantification of mitral regurgitation in Doppler echocardiography

The proximal isovelocity surface area (PISA) method is one of the various methods used for quantitatively estimating mitral regurgitation. The PISA shape is hemielliptic rather than hemispheric on a slit-like orifice, and the hemielliptic method is more accurate than the hemispheric method for in vitro studies. Nevertheless, the hemispheric method is used clinically because of its simplicity, whereas the hemielliptic method is difficult to approach from 3 orthogonal directions. The present study tries to establish a modified hemielliptic method for use in clinical applications. A closed-circuit, constant flow system was designed to simulate PISA, and various types of slit-like orifices were selected. Three orthogonal PISA radii were measured and flow rates were calculated using the original hemielliptic formula from the 3 orthogonal radii. Flow rates were also calculated indirectly using a linear regression formula, and PISA radii from a bird's eye approach and lateral approaches (modified hemielliptic method) were compared. Flow rates that were determined using the original hemielliptic method correlated significantly with actual flow rates (r = 0.92, p < 0.0001; y = 1.1x - 13; SEE = 13.63 ml/s). Similarly, flow rates calculated using the modified hemielliptic method correlated significantly with actual flow rates (r = 0.90, p < 0.001; y = 0.94x - 0.78; SEE = 14.13 ml/s). The study's results imply that the modified hemielliptic method can be used to accurately quantify mitral regurgitation and could be applied for clinical examinations.

Can a linear electrical analog model of a mechanical valve predict flow by using a pressure gradient?

The objective was to determine whether a previously developed technique for biological aortic valves could predict flow through a mechanical valve. An electrical analog model of the aortic valve that includes compliance, resistance, and inertance parameters, and corresponding second order differential equations was used to predict flow given a pressure gradient, as previously reported. Simulated pressures and flow were recorded by using a pulse duplicator system. The heart rate was varied from 60 to 180 bpm, and the stroke volume was varied from 22 to 67 cc. Resistance, inertance, and compliance parameters of the governing differential equation were estimated by using a least-squares fit to the measured flow at 120 bpm and 50 cc stroke volume. By using these parameter estimates, flow was calculated for other heart rates and stroke volumes. To achieve a better flow prediction, a nonlinear filter (third order polynomial range calibration equation) was applied to the output of the linear model (flow). The mean error, full-scale error, and spectral error in magnitude and phase between measured and predicted flow were compared. Error in mean flow ranged from 3% at medium flow rates to 90% at low flow rates. The maximum and minimum full scale errors were 12% and 5%, respectively. Error in the harmonics of measured and calculated flow ranged from 0% to 55%. Larger errors were usually present at the higher harmonics. The agreement between measured and calculated flow was better at normal and high flows but rather poor at low flows. The nonlinear filter (range calibration equation) was unable to account for the discrepancies between the measured and calculated flow over all flow ranges. It seems that this linear model and nonlinear filter have limited application, and an alternate nonlinear approach may produce better results.

True shape and area of proximal isovelocity surface area (PISA) when flow convergence is hemispherical in valvular regurgitation

The proximal isovelocity surface area (PISA) method for quantifying valvular regurgitation uses an echocardiographic image with superimposed colour Doppler mapping to visualise the contours of velocity in the blood travelling towards the regurgitant orifice. The flux of blood through the regurgitant orifice is obtained as the product of the area of one of these (presumed hemispherical) contours and the speed of the blood passing through it. However, colour Doppler mapping measures the velocity component towards the echo probe (v cos theta;) rather than speed (v), so that the contours of equal Doppler velocity (isodoppler velocity contours) differ from isospeed contours. We derive the shape of the isodoppler contour surface obtainable by colour Doppler mapping, and show that its area is much less than that of the hemispherical isospeed contour. When regurgitant flux is derived from an appropriate single measure of contour dimension, an appropriate result may be obtained. However, if the true echocardiographic surface area is measured directly, the regurgitant flux will be substantially underestimated. Indeed, the conditions necessary for isodoppler velocity contours to be hemispherical are extraordinary. We should not therefore make deductions from the apparent shape for the convergence zone without considering the principles by which the image is generated. The discrepancy will assume practical significance when increased resolution of colour Doppler technology makes measurement of apparent surface area feasible. Assuming the flow contours are indeed hemispherical, a 'correction' factor of 1.45 would be required.