Oxygen Delivery Devices — Low-Flow vs High-Flow Systems, FiO2 and Flow Rates

Oxygen therapy starts with the simplest, least invasive device and escalates according to the severity of hypoxaemia. In a low-flow system the oxygen supply is smaller than the patient's inspiratory flow, so room air is entrained and the delivered FiO₂ varies with the patient's breathing pattern (rate and tidal volume). In a high-flow system the device supplies a gas flow at or above the patient's peak inspiratory flow, so little ambient air is inhaled and FiO₂ is predictable.

Not every textbook classifies every device the same way. StatPearls calls the non-rebreather mask a high-flow system, whereas a European Respiratory Society review calls it a low-flow device with high FiO₂ — in exams, remember that it gives high but unpredictable FiO₂. The Venturi mask is the classic true high-flow, fixed-FiO₂ device.

The nasal cannula is the most commonly used technique. At 1–6 L/min it gives roughly 24–40% FiO₂, but the flow rate does not correspond to a fixed FiO₂ because the oxygen mixes with room air. The ERS review tabulates approximately 24–28% at 1–2 L/min, 30–35% at 3–4 L/min and 38–44% at 5–6 L/min. Nasal cannulae at 1–4 L/min have effects on saturation similar to 24–40% Venturi masks. Because FiO₂ cannot be predicted, cannula oxygen cannot be used for an A–a gradient calculation.

• Advantages: comfortable, patients can talk and eat, less likely to be removed than a mask, preferred by most patients (up to 88% in postoperative studies).
• Limits: nasal dryness or discomfort above 4 L/min, easily dislodged, less effective with deviated septum or polyps; may be inadequate in severe hypoxaemia.
• Mouth breathing does not usually reduce delivery and may increase inspired oxygen concentration.

A simple face mask delivers about 40–60% at 5–10 L/min. It has side exhalation ports. Flows below 5 L/min cause increased resistance and a build-up of CO₂ in the mask, so rebreathing may occur. The BTS guideline describes it as suitable for type 1 (non-hypercapnic) respiratory failure and not suitable for hypercapnic (type 2) respiratory failure, because it may deliver more than 50% oxygen.

The reservoir mask (non-rebreather mask) attaches a reservoir bag of about 1 L to a mask with one-way valves: the patient inhales from the bag, and exhaled gas cannot enter it. It delivers about 60–90% at 10–15 L/min and is suited to trauma and emergencies in patients in whom CO₂ retention is unlikely; a flow below 10 L/min can make the bag collapse during inspiration. A rebreather mask, in contrast, has no one-way valve between the mask and bag, so some expired gas is collected in the bag.

A Venturi mask gives an accurate oxygen concentration regardless of the oxygen flow rate. Oxygen flows through a narrow jet and entrains room air through side ports (the Venturi/Bernoulli effect); the proportions stay constant, so the same concentration is delivered however the flow is increased. Masks are available at 24%, 28%, 31%, 35%, 40% and 60%, colour-coded as in the figure above. The total gas flow to the face is high and usually exceeds the patient's inspiratory flow, so little air is drawn in around the mask.

• 24% and 28% masks are particularly suited to patients at risk of CO₂ retention (for example COPD). Prior to blood gases, the BTS suggests a 24% Venturi mask at 2–3 L/min or a 28% mask at 4 L/min.
• Use the minimum flow printed on the valve. In patients with a respiratory rate above 30 breaths/min, set the flow up to 50% higher — this does not raise the FiO₂ but meets the patient's demand.
• Increasing oxygen flow into a Venturi mask does not increase the oxygen concentration delivered.
• Accuracy falls if the mask is not accurately placed on the face.

High-flow nasal cannula (HFNC) therapy uses an air-oxygen blender with heated, humidified gas at up to 60 L/min, delivering up to 100% oxygen. Flow rate and FiO₂ are set independently. It improves oxygenation by washing out anatomical dead space, increasing PEEP and end-expiratory volume, and by lowering the respiratory rate. Its mechanisms include physiological dead-space washout of CO₂ — dead space is about one-third of tidal volume.

1. Nasal cannula or simple face mask for mild hypoxaemia.
2. Venturi mask when a controlled FiO₂ is needed; reservoir mask for short-term high FiO₂.
3. High-flow nasal oxygen.
4. Non-invasive positive-pressure ventilation (CPAP or BiPAP).
5. Intubation and mechanical ventilation — 100% oxygen can be delivered only by a ventilator or a tight-fitting mask.
6. Rescue strategies: prone positioning, neuromuscular blockade, inhaled pulmonary vasodilators, ECMO.

Oxygen is a drug and should be prescribed with a target saturation. The BTS guideline recommends 94–98% for most acutely ill patients and 88–92% for patients at risk of hypercapnic respiratory failure (or a patient-specific range). For hypoxaemia with COVID-19, StatPearls cites a suggested target of 92–96%, balancing hypoxaemia against hyperoxia. In practice, adjust the dose, observe for about 5 minutes if saturation is stable, and reassess.

In neonates not on a ventilator, oxygen can be given through an oxygen hood, nasal cannula, face mask or 'free-flow' oxygen. Check that equipment is calibrated and working, use humidified gas for high-flow delivery, and educate patients and families about safety when oxygen is used out of hospital.