Insights into catheter/Doppler discrepancies in congenital aortic stenosis

Despite inherent discrepancies between Doppler and catheter gradients in aortic stenosis, the simplified Bernoulli equation is still the accepted noninvasive technique to quantitate severity. The Reynolds number is a dimensionless parameter that characterizes the nature of flow as being viscous, turbulent, or transitional. Recently, in vivo and animal studies have successfully used a Reynolds number-based approach to reconcile Doppler-estimated and catheter-measured discrepancies. At the midrange of Reynolds number, pressure recovery effects are most evident, resulting in "overestimation" of catheter gradients by Doppler. At the lower range of the Reynolds number viscous effects are important, whereas at a higher range, turbulent factors are dominant; both result in a tendency toward agreement. We recorded 18 peak instantaneous gradients from dual left ventricular catheters (15 to 95 mm Hg), while simultaneously recording Doppler velocities before and after intervention in 11 pediatric patients (ages 0.5 to 16 years, mean 4.5). Doppler correlated but overestimated catheter-measured peak instantaneous gradients (y = 0.84x + 18.4, r = 0.8, SEE +/- 15.2 mm Hg, mean percent difference 29.9 +/- 36) over the range of catheter gradients measured. Accounting for the Reynolds number successfully collapsed data onto a single curve. Our study confirms in a clinical setting the importance of applying fluid dynamic principles such as the Reynolds number to explain apparent discrepancies between catheter and Doppler gradients. These principles provide a foundation for developing clinically appropriate correction factors.