Rational monitoring of respiratory function during mechanical ventilation of infants and children.

Oxygen and carbon dioxide homeostasis (effective pulmonary gas exchange) requires that lung exchange matches tissue exchange. The movement of gas in and out of the lung is determined by the mechanical properties of the system especially resistance and compliance and the work needed to provide this movement is supplied either by the patient's muscles or by the mechanical ventilator. If the latter is required, its optimal use demands that the user understands the relationship between the mechanical device and the mechanical properties of the patient's respiratory system so that optimal gas exchange can be provided. The following discussion provides the minimum physiological information needed for successful use of a mechanical ventilator and suggests that this requires measurement of at least four parameters: alveolar partial pressure of CO2 (PACO2), arterial oxygen saturation (SaO2), the mechanical RC time constant (RC) and functional residual capacity (FRC).