A five-parameter transfer function has been used to characterize the input impedance of the human systemic circulation. Aortic pressure and flow measurements obtained at cardiac catheterization are employed to evaluate the model parameters in individual patients. The appropriate parameters are chosen using a standard optimization technique to minimize the difference between pressures computed from the model and the measured values. Impedance parameters calculated for 19 patients showed considerable variation with no apparent correlation with the type of heart disease. The model is useful in describing the loading of the left ventricle produced by the systemic circulation. It may also have application to the study of vascular disease.

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