Development of a closed-loop pacemaker controller regulating mixed venous oxygen saturation level

A simplified model of the CV system is developed where the heart rate (HR) is the controlled input variable, the mixed venous oxygen saturation level (SO ₂) is the model output, and the work loads are the disturbances. The model relates the SO ₂ level to the HR and the work through linear relationships, gains, and first-order time constants, and nonlinear relationships, the most important being the nonlinear relation between cardiac output (CO) and heart rate due to the optimal-heart-rate phenomenon. The critical gain is calculated and is shown to be a function of the CO due to the shortening of circulation time delay with increased CO-a stabilizing phenomenon. The step response of the system in open and closed-loop modes to various loads are simulated in the time domain. A proportional-plus-derivative controller which in the simulation increases SO ₂ from 36% with a 100-W load at constant HR to 57% is described. This is achieved at a cost of increasing HR to 62 to 96 beats/min.