Which delivery device’s FiO2 cannot be predicted by the simple FiO2 equation (flow x 4 + 20)?

Which delivery device’s FiO2 cannot be predicted by the simple FiO2 equation (flow x 4 + 20)?

• A. Nasal cannula
• B. Venturi mask
• C. Oxymizer
• D. Simple mask

Answer: (C)

The concept here is that the simple FiO2 equation (FiO2 ≈ 20% + 4% per liter per minute of flow) is a rough guide for devices where oxygen blends with room air in a relatively predictable, flow-driven way. It works reasonably well for basic, low‑flow setups where the patient isn’t pulling in dramatically different breaths from one moment to the next. The device with an oxygen reservoir—the Oxymizer—breaks that predictability. It has a built‑in reservoir that stores oxygen during exhalation and releases it during inhalation. Because the delivered FiO2 then depends on how much air the patient inhales (tidal volume and inspiratory flow) in that breath, the actual FiO2 can vary from breath to breath and isn’t reliably predicted by just the flow setting. That breath-to-breath variability is why you can’t rely on the simple flow×4 + 20 calculation for this device. In contrast, other common devices either provide FiO2 in a more fixed or straightforward way (for example, venturi masks deliver a set FiO2 defined by the adapter, not by flow; nasal cannulas and simple masks have FiO2 that roughly tracks with flow, at least as a rough estimate). But the reservoir-based Oxymizer is the one whose FiO2 can’t be predicted with that simple equation.

The concept here is that the simple FiO2 equation (FiO2 ≈ 20% + 4% per liter per minute of flow) is a rough guide for devices where oxygen blends with room air in a relatively predictable, flow-driven way. It works reasonably well for basic, low‑flow setups where the patient isn’t pulling in dramatically different breaths from one moment to the next.

The device with an oxygen reservoir—the Oxymizer—breaks that predictability. It has a built‑in reservoir that stores oxygen during exhalation and releases it during inhalation. Because the delivered FiO2 then depends on how much air the patient inhales (tidal volume and inspiratory flow) in that breath, the actual FiO2 can vary from breath to breath and isn’t reliably predicted by just the flow setting. That breath-to-breath variability is why you can’t rely on the simple flow×4 + 20 calculation for this device.

In contrast, other common devices either provide FiO2 in a more fixed or straightforward way (for example, venturi masks deliver a set FiO2 defined by the adapter, not by flow; nasal cannulas and simple masks have FiO2 that roughly tracks with flow, at least as a rough estimate). But the reservoir-based Oxymizer is the one whose FiO2 can’t be predicted with that simple equation.