REGULATION OF PACO2 DURING REST AND EXERCISE - A MODELING STUDY

A nonlinear mathematical model of the CO2 control system was used to e xamine a number of issues concerning the regulation of PaCO2 during re st and exercise. To gain insight to the regulatory properties of the r espiratory system, the open loop gain (GL) and closed loop sensitiviti es SI = partial derivative PaCO2/partial derivative PICO2 and SV = par tial derivative PaCO2/partial derivative VCO2 were calculated. GL indi cates the ability of a control system to regulate th controlled variab le, PaCO2 in the model. SI and SV represent the change in PaCO2 to uni t changes in PICO2 and VCO2, respectively. Model predications were obt ained for rest and various intensities of exercise for the following c hallenges to the respiratory system: (a) CO2 inhalation, (b) i.v. CO2 loading, (c) application of an external dead space, and (d) a shift in the resting operating point. Increasing exercise intensity produced a substantial decrease in GL and increase in SI consistent with the hyp othesis that exercise degrades the ability of the respiratory system t o regulate PaCO2. However, SV decreased indicating that the respirator y system would actually be better able to regulate PaCO2 if there were fluctuations in VCO2. Thus, GL does not completely describe the regul atory characteristics of the respiratory control system. It is demonst rated that the regulatory characteristics of the respiratory system as described by GL, SI, and SV are complex and depend on the nature of t he challenge. Techniques for systematically describing the regulatory properties of the CO2 control system are described.