Oxygen Therapy and Hyperoxia: Target Saturations, the Harms of Too Much, and When High Flow Is Right
Why oxygen is a drug, what hyperoxia does, the published saturation targets, CO2 retention in COPD, and the cases where high-concentration oxygen is still the treatment.
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For decades, EMS gave high-flow oxygen to almost every patient on the theory that it could only help. It turns out oxygen is a drug like any other: it has indications, a target, and harms as well as benefits. Low oxygen kills and must always be treated. But giving extra oxygen to patients whose saturation is already normal has not helped in several conditions where it was routine, and has been linked to harm in some. Current guidance is to give oxygen to patients who need it and to titrate it to a target saturation.
What hyperoxia is
Hyperoxia means more oxygen than normal in the blood and tissues. In a healthy person breathing room air, hemoglobin is already close to fully saturated. Extra oxygen beyond that point carries very little more to the tissues; mostly it raises the amount dissolved in the plasma and the oxygen pressure in the arteries. A pulse oximeter cannot see hyperoxia: a reading of 100% looks the same whether the arterial oxygen is just adequate or several times higher than normal.
How too much oxygen can do harm
- It constricts blood vessels. High oxygen levels narrow the coronary and cerebral arteries in physiological studies, which can reduce flow to the very tissue that is at risk in a heart attack or stroke.
- It increases reactive oxygen species, the free radicals that add to tissue injury when blood flow returns after an interruption, such as after a heart attack, a stroke or a cardiac arrest.
- It can raise the CO2 in some patients with chronic lung disease (see below).
- It can collapse small air spaces. Breathing very high oxygen concentrations washes out nitrogen, and alveoli filled only with oxygen can collapse as it is absorbed (absorption atelectasis).
- It can hide deterioration. A patient on high-flow oxygen can keep a normal saturation while their breathing fails and their CO2 climbs, so the oximeter is late to show the problem. Capnography shows it sooner.
- In newborns, excess oxygen is harmful: resuscitating term babies with room air rather than 100% oxygen lowers mortality, and in premature babies excess oxygen damages the developing retina. Starting concentrations are in the NRP guide.
Why routine high-flow oxygen fell out of favor
The change came from clinical trials and from guidelines that followed them. The table summarizes the main groups where routine oxygen for a patient who is not hypoxic is no longer recommended.
| Patient group | What the evidence showed | What the guidance says now |
|---|---|---|
| Suspected acute coronary syndrome or heart attack | The AVOID trial (2015) found no benefit, and signs of larger infarcts, with oxygen in STEMI patients who were not hypoxic. The DETO2X-AMI trial (2017) found no reduction in mortality at one year. | The 2025 ACC/AHA/ACEP/NAEMSP/SCAI guideline for acute coronary syndromes gives oxygen when the saturation is below 90%, and does not support it routinely above that. |
| Acute stroke | The SO2S trial (2017) found no benefit from routine oxygen in stroke patients. | The 2026 AHA/ASA Guideline for the Early Management of Patients With Acute Ischemic Stroke recommends oxygen to keep the saturation above 94% in hypoxic patients, and does not recommend supplemental oxygen routinely for patients who are not hypoxic. |
| After return of spontaneous circulation | Observational studies link high oxygen levels after cardiac arrest with worse outcomes. | AHA post-cardiac-arrest care guidance recommends avoiding low oxygen and, once the saturation can be measured reliably, titrating to a target of 92 to 98%. |
| Acutely ill adults in general | A 2018 meta-analysis of trials in acutely ill adults (the IOTA review, published in The Lancet) found higher mortality with liberal oxygen than with conservative oxygen. | The British Thoracic Society has published target ranges since its 2008 guideline, rather than giving oxygen to everyone, and other national bodies use them too. The IOTA findings are consistent with that approach. |
Target saturations, and who publishes them
Different bodies publish slightly different ranges. The figures below are attributed to the body that publishes each. Your protocol chooses which applies in your system.
| Patient | Target | Published by |
|---|---|---|
| Most acutely ill adults | 94 to 98% | British Thoracic Society guideline for oxygen use in adults (2017) |
| Adults at risk of CO2 retention: COPD, and also severe obesity, chest wall deformity, neuromuscular disease and cystic fibrosis | 88 to 92% | British Thoracic Society (2017) |
| Most acutely ill adults | 92 to 96% | Thoracic Society of Australia and New Zealand (2015) |
| Adults at risk of CO2 retention | 88 to 92% | Thoracic Society of Australia and New Zealand (2015) |
| Suspected acute coronary syndrome | Give oxygen if the saturation is below 90%; not routinely above it | ACC/AHA/ACEP/NAEMSP/SCAI acute coronary syndromes guideline (2025) |
| Acute ischemic stroke | Keep the saturation above 94%; no routine oxygen if not hypoxic | AHA and ASA acute ischemic stroke guideline (2026) |
| After return of spontaneous circulation | 92 to 98% | AHA post-cardiac-arrest care guidance |
| Carbon monoxide poisoning | High-concentration oxygen whatever the oximeter reads | British Thoracic Society, and standard toxicology practice |
| Newborn resuscitation | Room air at 35 weeks or more; a low starting concentration below that; see the NRP guide | AHA and AAP neonatal guidance (see the NRP guide) |
The acute coronary syndrome and stroke thresholds above are for the uncomplicated patient. A patient in shock, respiratory distress or pulmonary edema is the critical-illness case described under When high-concentration oxygen is the treatment.
COPD, CO2 retention and the hypoxic drive
The traditional explanation is that some patients with chronic lung disease breathe only because their oxygen is low, so giving oxygen removes the drive and they stop breathing. Research has shown that this hypoxic drive explains only a small part of the problem. Most of the rise in CO2 comes from two other effects: oxygen relaxes the narrowed blood vessels in poorly ventilated parts of the lung, sending more blood to lung that is not breathing well (worsening the mismatch between ventilation and blood flow), and oxygen-rich hemoglobin holds less CO2, releasing more into the blood (the Haldane effect).
Only some patients with COPD retain CO2, and you cannot tell which ones in the field without a blood gas. That is why a lower target range exists for patients at risk. Some at-risk patients carry an oxygen alert card that states their own target; when they do, use it.
- Signs of a rising CO2: increasing drowsiness or confusion, headache, warm or flushed skin, a bounding pulse, a flapping tremor of the hands, and a rising EtCO2 on capnography.
- If a patient has become drowsy on high oxygen, do not simply take the oxygen off. The British Thoracic Society warns that stopping it abruptly can cause a dangerous rebound fall in oxygen; step it down toward the target and support the breathing as your protocol directs.
- The British Thoracic Society advises that nebulizers for patients at risk of CO2 retention be driven by air where possible, or that the time on an oxygen-driven nebulizer be limited. Many ambulances carry only oxygen to drive a nebulizer, so follow your protocol.
When high-concentration oxygen is the treatment
- Carbon monoxide poisoning: carbon monoxide binds hemoglobin far more tightly than oxygen does. A standard pulse oximeter cannot tell hemoglobin carrying carbon monoxide from hemoglobin carrying oxygen, so it can read normal in a patient who is badly poisoned. High-concentration oxygen speeds the removal of carbon monoxide, whatever the oximeter says. Some monitors can measure carbon monoxide separately; whether hyperbaric oxygen is needed is a hospital decision.
- Cyanide poisoning, which often comes with carbon monoxide in house fires: high-concentration oxygen, with any antidote given as your protocol directs.
- Cardiac arrest: the highest available oxygen during CPR, then titration to the post-arrest target once circulation returns and the saturation can be measured reliably.
- Critical illness or shock before a reliable saturation is available, such as major trauma or anaphylaxis: the British Thoracic Society advises starting with high-concentration oxygen and titrating down once a reliable reading guides it.
- Decompression illness after diving: high-concentration oxygen is standard first aid while the patient is moved toward hyperbaric treatment.
The pulse oximeter: what it can and cannot tell you
- It estimates the percentage of hemoglobin carrying oxygen. It does not measure how much oxygen the blood carries, and it says nothing directly about ventilation or CO2.
- It reads falsely normal in carbon monoxide poisoning and is unreliable in methemoglobinemia.
- Poor perfusion, cold hands, movement and some nail polish can give no reading or a wrong one. Look at the waveform before you trust the number.
- The FDA's 2021 safety communication warned that pulse oximeters may overestimate the saturation in people with darker skin, so treat a borderline reading with caution.
- It lags behind changes in breathing, especially on supplemental oxygen. Capnography shows hypoventilation sooner.
- In anemia the saturation can be normal while the oxygen content of the blood is low.
Oxygen delivery devices
The devices below are described by the relative concentration they deliver. Each device has its own manufacturer's flow range, and your protocol sets the flow you use; this guide does not give flow rates.
| Device | What it delivers | Typical use |
|---|---|---|
| Nasal cannula | Low concentration, varying with how the patient breathes | Mild hypoxemia, fine titration, and patients who will not tolerate a mask |
| Simple face mask | Moderate concentration, variable | Moderate hypoxemia when a cannula is not enough |
| Non-rebreather mask | The highest concentration of the standard masks for a patient breathing on their own, provided the reservoir bag stays inflated | Critical illness, severe hypoxemia, carbon monoxide poisoning |
| Venturi mask | A fixed, controlled concentration set by its color-coded valve | Patients at risk of CO2 retention, where a precise concentration matters; not carried by every service |
| Bag-valve-mask with oxygen | Ventilation with high concentration when a reservoir is attached | A patient whose breathing is inadequate or absent |
| CPAP | Oxygen with continuous positive airway pressure; the concentration depends on the device | Respiratory distress where your protocol uses it, such as pulmonary edema and some COPD exacerbations |
| High-flow nasal cannula | Heated, humidified oxygen at high flow | Mostly hospital and interfacility transport |
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The short version
- Oxygen is a drug. Low oxygen kills and is always treated, but extra oxygen for a patient who is not hypoxic has shown no benefit in heart attack and stroke and may do harm.
- Hyperoxia can constrict coronary and cerebral vessels, add to reperfusion injury and hide failing ventilation behind a normal saturation.
- The British Thoracic Society targets 94 to 98% for most adults and 88 to 92% for patients at risk of CO2 retention. The AHA targets 92 to 98% after cardiac arrest. Your protocol chooses.
- The hypoxic drive is only a small part of CO2 retention in COPD. Never withhold oxygen from a hypoxic COPD patient; titrate to the target.
- Carbon monoxide poisoning is the classic case for high-concentration oxygen: the pulse oximeter reads falsely normal.
- Choose the device by the concentration the patient needs, and set the flow by your protocol and the device.