Oxygen therapy nursing: devices, flow rates, FiO2 targets

Oxygen therapy is among the most commonly administered treatments in acute care, yet it is also among the most frequently misapplied. Nurses administer supplemental oxygen to correct hypoxemia – an insufficient partial pressure of oxygen in arterial blood – and to protect tissues from the downstream consequences of oxygen deprivation: organ dysfunction, cardiac dysrhythmias, altered mental status, and death. The decision of when to start oxygen, which device to use, how much to give, and when to wean it requires clinical reasoning every time. A nasal cannula delivering 2 L/min is appropriate for a patient with mild community-acquired pneumonia; the same device is wholly inadequate for a patient in anaphylactic shock. Getting this wrong exposes the patient to either unresolved hypoxemia or the toxicity of excess oxygen. Both are dangerous.

This guide covers oxygen delivery systems, flow rate selection, SpO2 targets by condition, the hypoxic drive controversy, pulse oximetry limitations, oxygen toxicity, and the NCLEX-focused pearls most likely to appear on licensing exams.

Oxygen delivery devices: flow rates and FiO2

FiO2 – the fraction of inspired oxygen – is what matters clinically. Room air is 21% oxygen (FiO2 0.21). Supplemental oxygen raises FiO2 above this baseline. The device selected determines both the achievable FiO2 range and the precision of delivery.

FiO2 estimation for nasal cannula: The textbook rule of thumb is FiO2 (%) ≈ 20 + (4 × L/min): 1 L/min ≈ 24%, 2 L/min ≈ 28%, 4 L/min ≈ 36%, 6 L/min ≈ 44%. Treat these as exam approximations. The BTS guideline notes that the delivered FiO2 from nasal cannulae cannot be predicted, gives a range of roughly 24% to 50% across 1–6 L/min, and states that 1–4 L/min has effects on saturation roughly equivalent to 24–40% from a Venturi mask. That is why flow is titrated to the SpO2 target rather than to a calculated FiO2.

Low-flow vs high-flow (variable vs fixed performance): Nasal cannulae, simple masks, and reservoir masks are low-flow devices. They supply less gas than the patient inhales, so room air makes up the difference and the delivered FiO2 varies with breathing pattern. Venturi masks and HFNC are high-flow devices, which is why their FiO2 stays stable.

Oxygen administration: step-by-step procedure

Step 1: assess the patient

Before selecting a device, establish the clinical picture. Obtain SpO2, respiratory rate, work of breathing (use of accessory muscles, nasal flaring, paradoxical breathing), and mental status. Know the baseline – a patient with COPD who normally runs SpO2 88–90% is different from a post-surgical patient whose baseline is 98%. Mouth breathing is a common worry with nasal cannulae, but most studies reviewed by the BTS found it delivers the same or a higher inspired oxygen concentration, so a dyspneic mouth-breather can still use a cannula if SpO2 reaches target. Check whether the patient is at risk of hypercapnic respiratory failure (COPD, obesity hypoventilation, neuromuscular or chest wall disease), since this sets the target range.

Review the physician or provider order. In most settings, nurses can initiate emergency oxygen at their clinical judgment, but standing orders or protocols should guide routine administration.

Step 2: select the device

Match the device to the clinical need:

• SpO2 below target, not critically ill – nasal cannula at 2–6 L/min or simple face mask at 5–10 L/min (BTS 2017 starting choices)
• SpO2 below 85%, shock, sepsis, major trauma, or other critical illness – reservoir (non-rebreather) mask at 15 L/min, then titrate down once stable
• At risk of hypercapnic respiratory failure – 24% Venturi mask at 2–3 L/min, 28% Venturi at 4 L/min, or nasal cannula at 1–2 L/min, aiming for 88–92% pending blood gases
• Persistent high oxygen requirement in respiratory failure without hypercapnia – consider HFNC

Step 3: apply and initiate

Confirm correct flow rate is set before applying the device to the patient. For a non-rebreather mask, inflate the reservoir bag before placing it on the patient’s face. Ensure proper fit – mask should cover nose and mouth fully; nasal cannula prongs should curve into the nares, not press against the philtrum.

Step 4: titrate to target

Oxygen is a drug – titrate it. Reassess SpO2 at 5 minutes after initiation. Adjust flow to meet the target SpO2 for this patient’s condition (see Clinical decision-making section below). The goal is the lowest FiO2 that achieves the target SpO2.

Step 5: monitor continuously

Reassess work of breathing, mental status, respiratory rate, and SpO2 at regular intervals. Worsening despite supplemental oxygen is an escalation signal that calls for a different device or level of support. A patient on a non-rebreather mask whose SpO2 remains 84% needs escalation to HFNC, BiPAP, or intubation; turning the NRM up to 16 L/min will not fix it.

Step 6: wean when appropriate

Wean oxygen proactively once the underlying cause is resolving and the patient maintains target SpO2 on a lower FiO2. Wean the flow rate incrementally – reduce by 1–2 L/min, reassess SpO2 at 15–30 minutes, continue if SpO2 holds. Step down devices as tolerated (reservoir mask → simple face mask → nasal cannula, or to a 24% or 28% Venturi mask in a patient at risk of hypercapnia). Where a weaning order or standing protocol exists, wean without waiting for a new order – prolonged unnecessary oxygen carries its own risks. Never stop oxygen abruptly in a patient who has become hypercapnic on high-concentration oxygen: the BTS guideline warns that sudden withdrawal can cause rebound hypoxemia, with PaO2 falling below its pre-oxygen level, so step the concentration down while monitoring saturation continuously.

Clinical decision-making: SpO2 targets by condition

SpO2 targets are not universal. The British Thoracic Society (BTS) guidelines and supporting evidence establish condition-specific ranges:

General inpatient (most conditions): SpO2 94–98%. Most post-surgical patients, pneumonia, pulmonary embolism, sepsis, and similar conditions target this range. Avoid routinely pushing SpO2 to the top of the scale: the IOTA meta-analysis of 25 trials (16,037 acutely ill adults) found that liberal oxygen increased in-hospital mortality compared with a conservative strategy (relative risk 1.21).

COPD and other risks of hypercapnic respiratory failure: SpO2 88–92%. This applies to COPD and to other at-risk groups such as obesity hypoventilation and neuromuscular or chest wall disease. The strongest evidence is Austin et al.’s prehospital randomized controlled trial, in which oxygen titrated to 88–92% reduced mortality by 58% overall (4% versus 9%) and by 78% in confirmed COPD (2% versus 9%) compared with high-flow oxygen. Bardsley et al. (2018) supports the related practice of air-driven rather than oxygen-driven nebulization to avoid inadvertently exceeding the target. Use a Venturi mask to hit the target precisely. (Do not confuse this with the DECAF score, which predicts in-hospital mortality in COPD exacerbation and says nothing about oxygen targets.)

Acute MI and acute coronary syndromes: Oxygen is for hypoxemia only. The 2025 ACC/AHA ACS guideline reserves supplemental oxygen for patients with SpO2 below 90% and does not support routine oxygen in patients whose saturation is 90% or above. The BTS guideline uses its usual 94–98% target range for these patients. Two trials drive this. AVOID (oxygen 8 L/min in STEMI patients with SpO2 ≥94%) found a larger infarct by creatine kinase, though peak troponin did not differ significantly. DETO2X-AMI (6,629 patients with SpO2 ≥90%) found no difference in 1-year mortality (5.0% versus 5.1%) or rehospitalization for MI. Routine oxygen in a normoxic MI patient therefore offers no benefit and may increase myocardial injury.

Neonates and preterm infants: Targets vary by gestational age and condition. Preterm infants generally target SpO2 91–95% to balance retinopathy of prematurity risk with hypoxemia risk. Neonatal resuscitation begins with room air in term infants. This is a specialized area – defer to NICU protocols.

Critical illness and after cardiac arrest: Start with a reservoir mask at 15 L/min in shock, sepsis, major trauma, or critical illness, then titrate to 94–98% once reliable oximetry is available. After return of spontaneous circulation, avoid both hypoxemia and very high PaO2. In a registry of 6,326 post-arrest patients (Kilgannon 2010), a PaO2 of 300 mmHg or more was independently associated with higher in-hospital mortality. The ERC-ESICM 2025 post-resuscitation guideline gives 100% (or maximum available) oxygen until saturation or blood gases can be measured reliably, then titrates to SpO2 94–98% or PaO2 75–100 mmHg. Follow local protocol for ventilated patients.

Carbon monoxide poisoning: 100% FiO2 via NRM regardless of SpO2 reading. SpO2 is unreliable in CO poisoning (see Pulse oximetry limitations). High FiO2 accelerates CO displacement from hemoglobin.

When to escalate device

Escalate when the patient fails to achieve target SpO2 on the current device despite adequate flow, or when clinical status is deteriorating despite supplemental oxygen. Escalation sequence:

NC → Simple mask → NRM → HFNC or BiPAP/CPAP → intubation and mechanical ventilation

In practice you skip steps: a critically ill or severely hypoxemic patient starts on a reservoir mask, and a patient at risk of hypercapnia moves from controlled oxygen to NIV if a respiratory acidosis develops.

HFNC evidence and monitoring: In the FLORALI trial (310 patients with non-hypercapnic hypoxemic respiratory failure), intubation rates did not differ significantly between HFNC, standard oxygen, and NIV (38%, 47%, and 50%), but HFNC gave more ventilator-free days and lower 90-day mortality. HFNC can also delay a needed intubation, so response must be tracked. The ROX index, (SpO2/FiO2) ÷ respiratory rate, helps: in Roca’s validation cohort of patients with pneumonia, a ROX of 4.88 or more at 2, 6, or 12 hours was associated with a lower risk of intubation, and values below 2.85, 3.47, and 3.85 at those time points predicted HFNC failure. After extubation in patients at low risk of reintubation, HFNC reduced 72-hour reintubation compared with conventional oxygen (4.9% versus 12.2%) in Hernández’s 2016 JAMA trial.

Call the provider when: SpO2 fails to reach target on NRM at 15 L/min, respiratory rate >30 and rising, increasing work of breathing, altered mental status, or the patient looks like they are tiring. Do not wait for the SpO2 to crash – respiratory arrest is preceded by a period of compensated distress that is the intervention window. For a deeper look at escalation decision-making in deteriorating patients, see acute respiratory failure nursing.

The hypoxic drive question

The hypoxic drive theory holds that patients with COPD breathe only because of low oxygen, so giving oxygen removes the stimulus and they stop breathing. It is one of the most persistently taught and misapplied ideas in clinical practice. Oxygen-induced hypercapnia is real; the hypoxic drive explanation for it is mostly wrong.

What is real: Uncontrolled high-concentration oxygen raises PaCO2 and can cause respiratory acidosis, drowsiness, and coma in some patients with COPD and other causes of chronic hypercapnia. Not every COPD patient is affected, which is why the BTS guideline asks you to identify patients at risk rather than treat all COPD the same way.

Why it happens: The BTS 2017 guideline ranks the mechanisms in order of likely importance:

1. Worsened ventilation-perfusion (V/Q) mismatch – the main factor. Poorly ventilated lung units are normally poorly perfused because of hypoxic pulmonary vasoconstriction. High-concentration oxygen reverses that vasoconstriction, sending more blood through units that still have a high CO2, and a patient with limited respiratory reserve cannot raise ventilation enough to compensate.
2. Reduced ventilatory drive – a small contribution. Ventilation falls only modestly as PaO2 rises toward 100 mmHg, and PaCO2 keeps rising above that level, where drive is no longer changing. Even Campbell, often credited with the hypoxic drive idea, wrote in 1967 that he doubted it was “the whole story.”
3. The Haldane effect – oxygenated hemoglobin carries less CO2, so more CO2 is released into plasma.
4. Smaller contributions from absorption atelectasis, the higher density of oxygen, and CO2 rebreathing from face masks run at low flows.

What is overstated: The blanket fear of oxygen in any patient with COPD is dangerous. Withholding oxygen from a COPD patient with SpO2 78% is harmful, because hypoxemia kills faster than hypercapnia. Austin et al.’s prehospital trial showed that the answer is titration: oxygen titrated to 88–92% reduced mortality compared with uncontrolled high-flow oxygen.

Clinical application: Titrate oxygen in COPD and other at-risk patients to SpO2 88–92%, using a 24% or 28% Venturi mask or nasal cannula at 1–2 L/min. Obtain a blood gas, and repeat it 30–60 minutes after any change. The BTS 2017 guideline advises starting NIV, with targeted oxygen, if the patient is hypercapnic (PaCO2 above 45 mmHg) and acidotic (pH below 7.35) and the respiratory acidosis persists for more than 30 minutes after standard medical management. If PaCO2 is raised but pH is normal and bicarbonate is high, the hypercapnia is probably long-standing: keep the 88–92% target and recheck the gas. The aim is controlled titration to a target. NCLEX items may still use the phrase “hypoxic drive”; the action they reward (controlled, titrated oxygen) is the same.

For COPD-specific nursing considerations, see COPD nursing. For ARDS and higher-acuity oxygen failure, see ARDS nursing.

Pulse oximetry limitations

SpO2 is not the same as SaO2 (arterial oxygen saturation). Pulse oximetry estimates saturation by comparing absorption of red and infrared light – it assumes hemoglobin is either fully oxygenated or fully deoxygenated. This assumption fails in several clinically important scenarios:

Carbon monoxide poisoning: Carboxyhemoglobin (COHb) absorbs red light similarly to oxyhemoglobin, causing the oximeter to read COHb as oxygenated hemoglobin. A patient with 40% carboxyhemoglobin may show SpO2 of 98% while being severely hypoxic at the tissue level. SpO2 is unreliable in CO poisoning – use co-oximetry (arterial blood gas with co-oximeter).

Methemoglobinemia: Methemoglobin (MetHb) causes SpO2 to trend toward 85% regardless of actual saturation – it does not reflect the true oxygen-carrying capacity. Suspect in patients exposed to nitrites, dapsone, or local anesthetics (benzocaine) who appear cyanotic with a “normal” or fixed SpO2. Treat with methylene blue.

Peripheral hypoperfusion: Poor peripheral perfusion – from shock, vasoconstriction, hypothermia, or peripheral vascular disease – reduces the pulsatile signal that the oximeter relies on. Readings become unreliable, intermittent, or unobtainable. Move the probe to the forehead or earlobe if finger readings are poor.

Nail polish and artificial nails: Dark nail polish, particularly blue and black, can falsely lower SpO2 readings by blocking light transmission. Remove polish or reposition to a bare digit, earlobe, or forehead. Acrylic nails can also interfere.

Anemia: Severe anemia does not directly cause false SpO2 readings – what hemoglobin remains is still measured. However, SpO2 96% with a hemoglobin of 5 g/dL represents severely impaired oxygen delivery (DO2 = CO × CaO2) despite a reassuring saturation number. Do not rely on SpO2 alone in anemic patients.

Skin tone and SpO2 accuracy: Multiple studies – including the 2020 analysis by Sjoding et al. in the New England Journal of Medicine – demonstrated that pulse oximeters overestimate true arterial oxygen saturation in patients with darker skin tones. The disparity is clinically meaningful: Black patients were approximately three times more likely than white patients to have occult hypoxemia (SaO2 <88% on blood gas) with a SpO2 reading of 92–96%. This is an unresolved design limitation of current oximetry technology. In patients with darker skin tones or in any patient where clinical status does not match the SpO2 reading, obtain an arterial blood gas for definitive measurement.

Motion artifact: Movement causes false low readings or signal loss. Ensure the patient is still during the reading; some devices have motion-filtering algorithms but accuracy is reduced.

Oxygen toxicity

Supplemental oxygen is not benign at high concentrations or prolonged exposure. Two main mechanisms drive oxygen toxicity:

Absorption atelectasis: When breathing high FiO2, nitrogen – the dominant gas in room air that normally splints alveoli open – is washed out and replaced by oxygen. Oxygen is rapidly absorbed into pulmonary capillaries, collapsing alveoli and creating atelectasis. This paradoxically worsens oxygenation and can impair lung mechanics. This effect is seen even at FiO2 of 0.5–0.6 over time.

Free radical (reactive oxygen species) damage: High PaO2 generates superoxide radicals and hydrogen peroxide within lung tissue. Prolonged exposure – generally FiO2 >0.6 for more than 24–48 hours – causes oxidative injury to alveolar epithelium, type II pneumocytes, and pulmonary endothelium, resulting in diffuse alveolar damage that is histologically indistinguishable from ARDS. This is sometimes called pulmonary oxygen toxicity or hyperoxic lung injury.

Clinical threshold: Avoid FiO2 >60% for extended periods unless there is no alternative. In mechanically ventilated patients, the goal is to reduce FiO2 to ≤0.6 as soon as PEEP and other strategies permit. In spontaneously breathing patients on NRM, transition to HFNC or NIV if FiO2 requirements remain high. For the mechanics of ventilator management in these patients, see mechanical ventilation nursing.

Retinopathy of prematurity: In neonates, hyperoxia causes abnormal retinal vascularization and can lead to blindness. This is a distinct mechanism from adult oxygen toxicity and underscores why SpO2 targets in preterm infants are intentionally lower than in adults.

Common mistakes

Using nasal cannula when flow requirements exceed device capability. Standard nasal cannulae are used up to 6 L/min (an estimated 24–50% FiO2). A patient with SpO2 of 82% on 6 L/min NC needs device escalation, because pushing a standard cannula to 8 or 10 L/min exceeds what it is designed for. NCLEX scenarios frequently test this: recognizing when to change devices instead of increasing flow.

Not titrating – setting and forgetting. Oxygen is prescribed to reach a target saturation. If SpO2 rises to 100% on 6 L/min NC in a patient with a COPD exacerbation, the correct response is to reduce flow to the target range (88–92%), not to leave it unchanged.

Running a simple face mask below 5 L/min. Below 5 L/min, resistance to breathing rises and exhaled CO2 can build up in the mask and be rebreathed (BTS 2017). If the patient needs less oxygen than a mask at 5 L/min delivers, switch to a nasal cannula. Some nursing textbooks give 6 L/min as the floor; either way, never run a mask at cannula-level flows.

Assuming a normal SpO2 rules out a problem. In CO poisoning, methemoglobinemia, or severe anemia, SpO2 can be falsely reassuring. A patient who looks sick despite SpO2 of 96% warrants investigation.

Applying supplemental oxygen to a normoxic MI patient. The reflex to “put everyone on oxygen” in acute chest pain offers no benefit when saturation is normal and may increase myocardial injury (AVOID, DETO2X-AMI). The 2025 ACC/AHA ACS guideline reserves oxygen for SpO2 below 90%; under BTS, hypoxemic patients are titrated to 94–98%.

Neglecting to inflate the reservoir bag before placing an NRM. If the bag is not pre-inflated, the patient’s first breath collapses it and they inhale against resistance. Pre-inflate before application.

Not reassessing after device escalation. A patient placed on NRM at 15 L/min with SpO2 of 86% who does not improve to ≥90% within 10–15 minutes needs the next escalation step.

NCLEX tips

• A simple face mask needs at least 5 L/min (many textbooks say 6) to prevent CO2 rebreathing. Reservoir masks need enough flow to keep the bag inflated – 15 L/min for a non-rebreather.
• Nasal cannula delivers approximately 4% FiO2 per liter: 1 L/min = 24%, 2 L/min = 28%, 3 L/min = 32%, 4 L/min = 36%, 5 L/min = 40%, 6 L/min = 44%. This is an exam approximation; real FiO2 varies with breathing pattern, so titrate to SpO2.
• The non-rebreather mask provides the highest FiO2 of the low-flow devices – about 60–90% at 15 L/min, depending on fit and breathing pattern. It is the first choice in critical illness and severe hypoxemia.
• COPD patients target SpO2 88–92%, not 94–98%. The Venturi mask is the preferred device for controlled, precise oxygen delivery in COPD.
• The Venturi mask is a fixed-performance (high-flow) device: it delivers a set FiO2 (24, 28, 31, 35, 40, or 60%), and turning up the oxygen flow does not raise the concentration.
• SpO2 is falsely normal in carbon monoxide poisoning – always check carboxyhemoglobin via co-oximetry, and give 100% oxygen via NRM regardless of SpO2.
• SpO2 trends toward 85% in methemoglobinemia regardless of actual hemoglobin saturation.
• Oxygen toxicity risk increases with FiO2 >60% maintained for extended periods – mechanisms include absorption atelectasis and free radical lung damage.
• High-flow nasal cannula provides flow rates up to 60 L/min, heated humidification, and titrable FiO2 from 21–100%. It generates a small amount of positive airway pressure and is evidence-based for hypoxemic respiratory failure.
• Never withhold oxygen from an acutely hypoxic patient regardless of diagnosis. The concern in COPD is uncontrolled high-concentration oxygen raising PaCO2, mainly by worsening V/Q matching rather than by removing a hypoxic drive. Titrate to 88–92%; do not avoid.
• Pulse oximetry overestimates true saturation in patients with darker skin tones – a clinically significant disparity documented in peer-reviewed literature. Obtain an ABG when clinical assessment doesn’t match the SpO2 reading.
• Device escalation sequence: nasal cannula → simple face mask → non-rebreather → HFNC/NIV → mechanical ventilation. A critically ill patient starts on the non-rebreather.
• Reservoir bag on an NRM must never fully collapse – if it does, the flow rate is too low for the patient’s demand. Increase the flow and check the valves and fit.
• Oxygen is titrated to effect – the target is the lowest FiO2 that achieves the condition-appropriate SpO2 range. Hyperoxia (SpO2 consistently >98% on supplemental O2) is not a clinical goal.

For related content, see asthma nursing and ARDS nursing.

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