An in situ perfused crocodile (Crocodylus porosus) heart preparation was developed to investigate the effects of input and output pressure on cardiac dynamics and to determine the conditions that lead to a right-to-left cardiac shunt. The pericardium was kept intact, both the left and right atria were perfused and all three outflow tracts (right aortic, left aortic and pulmonary) were cannulated, enabling pressures and flows to be monitored. The perfused heart preparation had an intrinsic heart rate of 34 beats min-1 and generated a physiological power output. Both the left and right sides of the heart were sensitive to filling pressure. Increasing the filling pressure to both atria resulted in an increase in stroke volume and cardiac output (Frank­Starling effect). Increasing the filling pressure to the right atrium also had a positive chronotropic effect. Large right ventricular stroke volumes initiated a right-to-left shunt, despite the left aorta having a pressure 1.5 kPa higher than the pulmonary output pressure. The left ventricle was able to maintain its output and stroke volume up to an output pressure of approximately 8 kPa. However, the right ventricle was significantly weaker. Right ventricular output and stroke volume showed a marked decrease when the output pressure was increased above 5 kPa. A right-to-left shunt occurred when pulmonary output pressure was increased. Surprisingly, a shunt occurred into the left aorta before the pressure in the pulmonary artery became greater than that in the left aorta. Once the pressure in the pulmonary artery exceeded the left aortic pressure, pulmonary artery flow ceased and right ventricular output was solely via the left aorta. A right-to-left shunt could also be initiated by increasing the filling pressure to the left atrium.

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