An integrative cardiovascular model of the standing and reclining sheep

We have developed an integrative model of the ovine cardiovascular system that simulates the effects of posture and ventricular function to help design the circulatory and pulmonary assist devices often tested in conscious sheep that stand and recline during chronic experiments. Our present focus is the simulation itself and not the evaluation of specific assist devices. Adjusted parameters of the model provide good fits to in vivo experimental data and can describe the hemodynamic changes that follow balloon occlusion of the inferior vena cava and standing. Since circulatory assist devices are often tested in animal preparations of left heart failure, we extended the model to study the effects that standing and inferior vena cava occlusion would have in sheep with primarily systolic left heart dysfunction. We also developed an elastance-based formulation for ventricular muscular work and a sensitivity analysis of the autonomic reflexes affecting blood pressure. The model can analyze the biophysical mechanisms underlying the responses to orthostatic stress and left ventricular dysfunction and should help improve the development and testing of assist devices.