Patient Specific Modeling of Head-Up Tilt

Short term cardiovascular responses to head-up tilt (HUT) experiments involve complex cardiovascular regulation in order to maintain blood pressure at homeostatic levels. This manuscript presents a patient specific compartmental model developed to predict dynamic changes in heart rate and arterial blood pressure. The model contains five compartments representing arteries and veins in the upper and lower body of the systemic circulation, as well as the left ventricle facilitating pumping of the heart. A physiologically based sub-model describes gravitational effects on pooling of blood during the HUT, and baroreflex control mechanisms are included regulating cardiac contractility, peripheral vascular resistance, and vascular tone. Nominal parameters are computed from subject specific data as well as literature estimates. The model uses heart rate as an input and predicts arterial blood pressure as an output. The model is rendered patient specific via the use of parameter estimation techniques. This process involves sensitivity analysis, prediction of a subset of identifiable parameters, as well as nonlinear optimization used for estimating the identifiable parameters. Results show that it is possible to estimate a subset of model parameters that allows prediction of observed changes in arterial blood pressure. Furthermore, the model adequately predicts arterial and venous blood pressures, as well as cardiac output in compartments for which data are not available.