CARDIOPULMONARY GEOMETRY AND BLOOD FLOW IN GROWING LAMBS

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The potential for measuring pulmonary blood velocity noninvasively with pulsed Doppler ultrasound has stimulated interest in relationships between velocity patterns and abnormal pulmonary hemodynamics. Unfortunately, clinical and experimental studies have found that characteristics reported in correlate well with abnormal pulmonary pressure and/or resistance are often observed in normal subjects. For example, the prominent backflow along the right posterior wall of the pulmonary trunk in patients with pulmonary hypertension, which has been attributed to enhanced curvature of the main pulmonary artery, has also been observed in normal lambs, dogs and humans. The similarity of animal velocity profiles in vivo to profiles obtained in pulsatile flow studies in curved tubes suggests that geometry plays a role as important as hemodynamics in determining velocity patterns. Thus, understanding the relationships 1) between normal maturation and geometry and 20 between geometry and fluid dynamics is essential if techniques for diagnosing altered fluid dynamics induced by congenital heart defects in infants and children, in whom pulmonary geometries will be changing rapidly, are to be improved. Using a series of developing lambs ranging from 1 day to 1 year in age, this study will address these issues by performing in vivo, in situ and in vitro experiments. (A) During the in vivo phase, velocity profiles will be obtained in the main and branch pulmonary arteries using 20 MHz pulsed Doppler devices. Data collected will be used to project normal -or-age 2- and 3-dimensional velocity profiles and input impedance spectra and to verify the authenticity of profiles determined in vitro in later studies. (B) Casts of the right ventricle and proximal pulmonary arteries will be made in situ under normal physiologic pressure. The casts, which reveal the importance of the subvalvar infundibular region, can be CT scanned and their structure committed to computer memory, from which 2- and 3-dimensional images can be reconstructed. Algorithms will be developed for quantifying curvature and torque of the main trunk, flow divider offset, bifurcation angles, taper, and segment dimensions. Data obtained will be used to quantitate age-related changes in geometry and intra-age variability. (C) In vitro studies in glass and silicone rubber models will be components of a sophisticated pulse duplicator system designed to match the pressure and flow characteristics of the right ventricle. Flow patterns will be visualized using a laser light source and velocity measured with a laser Doppler anemometer system and pulsed Doppler devices similar to those used in vivo. In vitro studies provide flexibility not available in vivo; flow patterns can be observed under conditions of 1) constant geometry and variable hemodynamics, 2) constant hemodynamics and variable geometry, and 3) resting hemodynamics and geometry but variable compliance. Unification of results obtained should answer clearly the questions implied: 1) how do the geometries of the right ventricle and major pulmonary arteries change from birth to adult life? and 2) how do geometric variations resulting from normal aging and/or individual variability affect flow dynamics?