Meeting Abstract

14.1  Jan. 4  Removal of the right-to-left shunt affects cardiac dynamics in the American alligator OWERKOWICZ, T.; BLANK, J.M.; EME, J.; GWALTHNEY, J; HICKS, J.W.*; University of California, Irvine; University of California, Irvine; University of California, Irvine; University of California, Irvine; University of California, Irvine jhicks@uci.edu

Cardiac shunting is present in all non-avian reptiles and many hypotheses have been proposed to explain why it has evolved. However, no study to-date has looked at the haemodynamic consequences of removing the ability to shunt blood on overall heart function. The morphology of the crocodilian heart, with completely separated ventricles, allows the right-to-left (R-L) shunting to be prevented by occluding the left aortic arch (LAo) at its base, proximal to the foramen of Panizza. In anaesthetized and artificially ventilated juvenile alligators, we measured blood flow and pressure in the pulmonary artery (PA), left aorta (LAo) and right ventricle (RV) before and after LAo occlusion. We used stimulation of the vagus nerve and breathholding to elicit shunting. We confirmed that right vagal stimulation leads to a pronounced bradycardia, whereas left vagal stimulation results in a smaller reduction in heart rate and a significant increase in pulmonary vascular resistance. Prior to occlusion, both vagal stimulation and breathholding resulted in a large reduction in mean PA flow and pressure and an increased forward flow in LAo, indicative of R-L shunt. Occlusion of the LAo resulted in a reduction of net LAo blood flow to zero, even during vagal stimulation. In addition, there was a transient increase in blood flow in PA. Following a stabilization period, vagal stimulation produced bradycardia, a significant rise in RV systolic pressure and reduction in PA blood flow and pressure. Reductions in PA flow and pressure were less than prior to LAo occlusion. The data suggest that the capacity for a R-L shunt acts to reduce cardiac work under conditions of increased parasympathetic tone. Supported by NSF IOB grant #04445680 to JWH.