Anaesthesia General 32616cbb In a variable performance face mask A High patient inspiratory flow rate reduces oxygen concentration B Oxygen concentration decreases during an expiratory pause C Carbon dioxide concentration decreases during an expiratory pause D Side holes allow venting of expired gases and entrainment of air. High-Yield Explanation During expiratory pause, there is no tidal flow from the patient, oxygen accumulates in the mask, thereby washing out carbon dioxide.