Respiratory analogue computer model.

An analogue computer model has been constructed to simulate the dynamic behaviour of the human cardiorespiratory system for studying the effect of changes in different system-variables--as alveolar ventilation, functional residual capacity (FRC) of the human lung and the cardiac output--on the partial pressure of carbon dioxide in the arterial and mixed venous blood (pCO2a, pCO2v). The respiratory analogue computer model (RACM) simulates reliably well-known clinical physiological as well as pathological phenomena. Reducing FRC or breath rate an oscillation of pCO2a has been developed, but the mean value has not been changed. In alveolar hypoventilation the pCO2 of blood increases in good conformity with the clinical experience. Reducing cardiac output a characteristic dissociation appeared: pCO2v and pCO2a diverged from each other by increasing pCO2v and decreasing pCO2a. Since in general clinical practice of intensive care only arterial blood gas values are checked regularly, the conclusion that the patient's condition is improving by decreasing pCO2a is a serious misinterpretation in this case, because peripheral tissues are in balance with the increasing venous carbon dioxide tension, and consequently cannot get rid of this toxic agent. The computer-model-based conclusion has led to the practice of sampling both arterial and venous carbon dioxide tensions for better state assessment of seriously ill patients.