DOPPLER ECHOCARDIOGRAPHY Continuous - wave Doppler velocities and gradients across fixed tunnel obstructions : studies in vitro and in vivo

The simplified Bernoulli relationship appears to be quite accurate for predicting gradients across discrete valvular obstructions. Controversy exists about how accurately it predicts the severity of disease in longer segment obstructions. In this study we constructed a pulsatile model of subvalvular pulmonary stensosis in vitro to study nine custom-made subvalvular tunnels 2, 4, and 7 mm in length with flow cross sections of 0.5 to 1.5 cm2 and with the stenotic segment proximal to a nonstenotic bioprosthetic valve, and a pulsatile model in vitro of a 16 mm long tunnel-like ventricular septal defect (VSD) of varying cross-sectional area (0.20 to 0.64 cm2). We also compared the observations in vitro with those in an open-chest dog preparation with a tunnel-like interventricular communication. In the subpulmonic stenosis model, for each individual tunnel, 10 instantaneous peak gradients between 15 to 105 mm Hg were available. The pressure gradients across the tunnel alone, measured in the subvalvular area, were consistently higher than the measured gradients across the tunnel plus valve, suggesting some relaminarization of flow (i.e., a decrease in velocity) and pressure recovery (i.e., an increase in pressure) distal to the obstruction. Continuous-wave Doppler velocities across the 4 and 7 mm tunnels for the highest gradients were slightly lower than for the 2 mm tunnel at the same gradients, and it was only for the 0.5 cm2 cross section, 4 and 7 mm tunnels that there was a suggestion of minor viscous energy loss. For all the subvalvular tunnels studied, the Bernoulli relationship accurately predicted the results of the pressure drop across the tunnel only, while the gradient across tunnel plus valve was consistently lower. For the VSD tunnel model in vitro, the Doppler-derived gradients were approximately 40% higher than the measured gradients. The findings for the subvalvular and VSD tunnels in vitro and similar findings in the open-chest dogs with VSD suggest that relaminarization of flow and recovery ofpressure occurred distal to the tunnel orifice, whereas continous-wave Doppler findings correlate with the highest instantaneous gradients measured in the lowest pressure areas at the vena contracta of the tunnel. Circulation 76, No. 3, 657-666, 1987. ONE OF THE UNQUESTIONED contributions of the Doppler technique to clinical cardiology has been its use as a noninvasive method for estimating gradients across discrete stenotic orifices by use of a combination of the Doppler measurement of the maximal velocity and a simplification of the Bernoulli relationship. 1 2 Since Holen et al.' first proposed and validated this method, numerous investigators in the United States and Europe have revalidated their results in many forms From the Cardiovascular Fluid Dynamics Laboratory, School ofChemical Engineering, Georgia Institute of Technology, Atlanta, and the Division of Pediatric Cardiology, University of California, San Diego. Supported by the American Heart Association, and by a grant from the Whitaker Foundation. Address for correspondence: David J. Sahn, M.D., UCSD Medical Center, Division of Pediatric Cardiology, 225 West Dickinson St. H814A, San Diego, CA 92103. Received April 1, 1986; revision accepted May 14, 1987. Vol. 76, No. 3, September 1987 of valvular and vessel stenoses.36 The controversies that have arisen regarding the validity of this equation have been mostly related to the need for angle correction for correct velocity determination and more fundamentally, the applicability of the Bernoulli relationship itself in complex flow systems like dual or irregular orifices, or very small long tunnel-like obstructions. Subvalvular pulmonary stenosis is a common form of congenital heart disease found either alone or, most commonly, in combination with ventricular septal defects (VSDs), pulmonary valve stenosis, tetralogy of Fallot, and subaortic stenosis.7 Subpulmonary obstructions can be either dynamic or fixed since they can result from hypertrophy of normal right ventricular bands, from fibrous membranes immediately below the pulmonary valve, accessory tricuspid valve tissue, aneurysms of the ventricular septum, or from congenital 657 by gest on Sptem er 6, 2017 http://ciajournals.org/ D ow nladed from