CO2 homeostasis during periodic breathing: predictions from a computer model.

The Bohr/Riley model of CO2 homeostasis describes the relationship between CO2 production, ventilation, and arterial PCO2 and assumes that ventilation and CO2 delivery to the lung are both anatomically and temporally well matched. In contrast to normal breathing, periodic patterns of ventilation show temporal mismatch of ventilation to CO2 delivery. We developed a computer model of lung CO2 clearance that uses CO2 transfer equations to generate iterative solutions for PCO2 in multiple body compartments as a function of time. During continuous ventilatory patterns our model predicts steady-state arterial PCO2 identical to that of the Bohr model. During periodic ventilation, we predict mean PCO2 will be elevated unless mean ventilation is increased above that required by the Bohr model. Waxing and waning tidal volumes, low functional residual capacity, and low capillary blood volume potentiate the hypercapnia. However, if cardiac output oscillates in phase with breathing, hypercapnia is minimized. This analysis suggests a new mechanism for the development of sustained hypercapnia, separate from absolute hypoventilation or the presence of lung disease.