Assessment of an Indicator-Dilution Technic for Quantitating Aortic Regurgitation by Electromagnetic Flowmeter

The square-wave electromagnetic flowmeter was used to assess the upstream sampling dye-dilution technic for quantitating aortic regurgitation by simultaneous determinations of the regurgitant fraction by flowmeter, as given by the ratio of regurgitant volume to total stroke volume (RV/TSV) and the identical ratio of regurgitant flow to total forward flow (Q R /Q F ), and the regurgitant fraction by dye curve, as given by the ratio of the areas of the immediately appearing portion of the left ventricular curve to the area of the femoral artery dye curve (LV A /FA A ), after injection of indicator approximately 1 cm downstream to the aortic valve. A good correlation (r = 0.91) was found between these two independent methods which can be represented by the regression equation Q R /Q F = 0.82 (LV A /FA A ). This correlation indicates that the concept that the regurgitant fraction of indicator (LV A /FA A ) is proportionate to the regurgitant fraction of blood (RV/TSV) is valid. The observed systematic difference between flow-meter and dye-curve regurgitant fractions may be due to the fact that time-average rather than volume-average concentrations of the indicator are obtained by sampling of blood through eatheters. Two different and independent calibration technics were used to establish a calibration factor for converting recorded flowmeter deflections to volumetric units for measurement of rates and volumes of blood flow in the ascending aorta. The first technic involved simultaneous recording of dye-dilution curves and velocity pulses in the ascending aorta by the flowmeter before the production of aortic regurgitation. The second involved bidirectional perfusion of the isolated aorta at known rates of blood flow with simultaneous flowmeter recordings at the termination of each experiment. The data obtained validated the use of the flowmeter for measurements of total stroke volume and regurgitant stroke volume. The close correspondence between flowmeter recordings of the regurgitant flow and hydraulic estimates of regurgitant flow, based on the size of the defect in the aortic valve measured at necropsy and continuous records of the pressure gradient across the valve, also lent strong support to the validity of these measurements. The relationship Q R = Q S (Q R /Q F )/(1 - Q R /Q F ) can be used to estimate regurgitant flow from values of the regurgitant fraction (Q R /Q F ) and the systemic flow (Q S ). The mathematical relationship of the regurgitant fraction to regurgitant flow, determined by the dilution-curve technic, plus the variability obtained between simultaneous flowmeter and dilution-curve estimates of regurgitation, indicate that the error in the estimation of regurgitant flow by the dye-dilution technic is least for small-to-moderate degrees of regurgitation and increases progressively with the severe degrees of regurgitation associated with regurgitant fractions of greater than 0.6. On the basis of 231 simultaneous determinations by the indicator-dilution and flowmeter technics carried out in nine dogs, and assuming that the flowmeter values were uniformly correct, it was estimated that the standard deviation of the errors in measurement of regurgitant flow by the upstream sampling dye-dilution method in these experiments was 0.15 L per minute for regurgitant fractions below 0.6. and that this variability increased so that the standard deviation was 1.3 L per minute at regurgitant fraction levels of 0.9 to 1.0. The results of determinations carried out over a range of heart rates from 39 to 216 beats per minute indicate that the indicator-dilution method is applicable over a wide range of heart rates. The observed changes in aortic regurgitation with variation in heart rate correlated closely with the changes estimated by hydraulic equations based on the hydrodynamics of steady flow through an orifice. The increase in regurgitant stroke volume with increased duration of diastole at slow heart rates was disproportionately greater than the increase in total stroke volume, so that the effective forward (systemic) flow fell to extremely low levels at slow heart rates, particularly in dogs with large defects in the aortic valve. The applicability of the upstream sampling indicator-dilution technic to the detection and estimation of the severity of aortic regurgitation in human beings merits further study.