Effect of Suspended Red Blood Cells On Pressure Gradients Across Blalock-Taussig Shunts 151

It is generally assumed that blood behaves strictly as a particle-free fluid and not as a suspension in arterial sized vessels (> 1 mm). However, we have previously demonstrated a biphasic deviation of pressure gradients(PG) of blood from a particle-free fluid of equal viscosity in straight tubes, with the blood PG being greater than the particle-free fluid at low flowrates and lower at high flowrates. Since this behavior may affect velocity-derived PG, we addressed the hypothesis that PG across a BT shunt would show significant deviation from that of a particle-free blood analog fluid (BAF). Methods: The in vitro BT shunt model consisted of two arterial sections connected by a physiologically sized shunt (D = 5 mm, L = 9 cm). PG across the shunt were obtained by measuring pressure 10 cm proximal and distal to the inlet and outlet respectively. For a given flow rate the PG across the shunt was recorded for both blood and an equal viscosity BAF(aqueous glycerin solution, 3.8 cP). PG differences were plotted vs. Reynolds number (Re = ratio of inertial to viscous forces) and divided into laminar and turbulent flow regimes, with a transition Re of 2100 (the approximate transition from laminar to turbulent flow in tubes). Differences in PG between blood and BAF were considered significant at a level of 0.01 using a paired t-test for both the laminar and turbulent regimes. Results: For Re 0.5). However, for Re > 2100, the PG for blood were consistently lower than that of BAF and the differences were significant (mean difference = 9.9 mmHg, p < 0.01). Conclusions: Blood PG across BT shunts deviate from BAF PG. This effect is especially pronounced in the turbulent regime and is consistent with our previous investigations using straight tubes, suggesting Re-dependent phenomena may account for this behavior. These findings may have important implications in the noninvasive assessment of pulmonary artery pressure in patients with BT shunts.