Ventilatory requirements during general anesthesia: the determinants of an "appropriate" response

This review addresses the ventilatory requirements for regulating arterial PCO2 in the pre-, intra-, and postoperative phases of general anesthesia with respect both to the spontaneously breathing patient and the ventilated patient. These requirements are discussed in the context of (i) the rate of pulmonary CO2 elimination (VCO2) which, in the steady state, equals the rate of metabolic CO2 production and is therefore dictated by the metabolic rate and the respiratory quotient; (ii) the "set point" at which arterial PCO2 is regulated by the ventilatory control system which, for a given VCO2, determines the level of alveolar ventilation; and (iii) the dead space fraction of the breath (ie, the ratio of the total dead space to the tidal volume) which determines the total ventilation.