Partial reinhalation of previously exhaled carbon dioxide (CO2) may impair the efficacy of noninvasive mechanical ventilation (NIMV) in improving CO2 removal and unloading ventilatory muscles . At constant alveolar ventilation, any CO2 concentration above zero in inhaled gases causes an increase of arterial CO2 tension by an equal amount. Accordingly, significant CO2 rebreathing increases alveolar ventilation requirements to maintain desired arterial CO2 tension. This can limit the beneficial effects of inspiratory assistance provided by NIMV. In this chapter, we review general mechanisms of CO2 rebreathing and the effect of different interfaces, respiratory circuits, and ventilator settings.
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