Fig. 2

Georgopolous model of breathing control during weaning. A Reflects health, were the brain and ventilation curve are synonymous, thereby eupneic and actual PaCO₂ coincide. Possible contributors to the position of the brain curve, ventilation curve and metabolic hyperbola, and the varying slope of the latter (larger ∆VE required to achieve a given ∆PaCO₂ when ventilation is higher) are also shown. B Represents a possible SGF weaning trial off: as the CO₂ cleared by the membrane lung (V’CO_2ML) is reduced the metabolic hyperbola shifts upwards and to the right, while the brain and ventilation curve shift in parallel to the left to maintain the eupneic PaCO₂. C Represents a possible FdO₂ or ECBF weaning trial off: as the oxygen provided by the membrane lung (V’O_2ML) is reduced during weaning any hypoxaemia would shifts the position and slope of the brain curve a new eupneic PaCO₂. This new equilibration point will drive non-metabolic CO₂ from body stores into the bloodstream possibly shifting the metabolic hyperbola upward and to the right. D Depicts possible weaning-induced changes in the relative position of the brain and ventilation curves (maintained synonymous for simplicity in all other Panels): note that any deviation between the two curves induces a difference between the actual and eupneic CO₂. MV: mechanical ventilation, PaCO₂: the partial pressure of arterial carbon dioxide, V’CO_2NL: the total CO₂ to be cleared by the natural lung, Vd/Vt the dead space fraction of the tidal volume, V’CO_2ML: the CO₂ cleared by the membrane lung