An Optimal Control Approach to Modeling the Cardiovascular-Respiratory System: An Application to Orthostatic Stress

Abstract

This paper introduces a model designed to study the cardiovascular-respiratory system and its control features. It has been previously applied to study several physiological situations and in this paper it will be applied to the simulation of the phenomenon of orthostatic stress. Orthostatic stress refers to stress placed on the cardiovascular system when the body is in the upright position as compared to the supine position. The model consists of cardiovascular and respiratory components and includes cardiovascular autoregulation, ventilation control, and the baroreflex loop. Instead of an explicit formula for calculating the control response from the pressures and blood gases, we use an optimal control. Steady state and dynamic model simulation is compared with experimental data we have collected using head up tilt (HUT) experiments. The simulations fit the measured data well and represent reasonable physiological values. This work also examines an important issue related to orthostatic stress experiments. The head up tilt experiment (where gravity creates extra pressure stress on the lower body) is to be distinguished from the lower body negative pressure (LBNP) experiment where the lower body is subject to reduced exterior air pressure. Both tests create blood volume shifts to the lower body but the two physiological conditions are not entirely equivalent. We discuss some of these issues and examine several aspects of implementation of orthostatically induced pressures.