Oxygen Therapy: Devices, Target Saturations and Monitoring
Oxygen is one of the most familiar treatments you will see on placement, but familiar does not mean harmless. Oxygen should be used to treat hypoxaemia, not simply breathlessness, and it should be given to achieve an appropriate target saturation rather than to make the number on the monitor as high as possible.
Safe oxygen therapy means assessing the whole patient, choosing the right delivery system, checking the prescribed or clinically indicated target range, monitoring the response and recognising when an increasing oxygen requirement is a sign of deterioration.
Start with the patient, not the oxygen device
Use an ABCDE assessment when a patient is acutely unwell. During breathing assessment, look beyond the oxygen saturation. Check respiratory rate, breathing pattern, depth, chest movement, work of breathing, ability to speak, accessory muscle use, colour, conscious level and signs of exhaustion.
A patient can have an apparently acceptable SpO₂ and still be deteriorating. For example, someone with severe pneumonia may maintain an SpO₂ of 95% only because their oxygen requirement has increased from 2 L/min to 8 L/min. The number alone does not tell you whether the patient's respiratory condition is improving or worsening.
Oxygenation and ventilation are not the same thing
Oxygenation refers to getting oxygen into the blood. Pulse oximetry gives an estimate of peripheral oxygen saturation.
Ventilation refers to moving air in and out of the lungs and removing carbon dioxide. Pulse oximetry does not measure carbon dioxide. A patient can therefore have an acceptable SpO₂ while retaining carbon dioxide or ventilating inadequately.
This is one reason why respiratory rate, conscious level, work of breathing and blood-gas results can be as important as the saturation reading.
Target oxygen saturations
For most acutely ill adults who are not at risk of hypercapnic respiratory failure, UK guidance recommends a target SpO₂ of 94 to 98%.
For many patients who are at risk of hypercapnic respiratory failure, a target of 88 to 92% is commonly used while blood-gas results are assessed, unless the patient has an individualised target. This includes many patients with chronic obstructive pulmonary disease and may also include people with other causes of chronic ventilatory failure such as some neuromuscular, chest wall or obesity-related conditions.
Do not assume that every patient with COPD should automatically be kept at 88 to 92% in every situation. Check the clinical context, oxygen prescription, previous blood-gas information, oxygen alert card if present and the authorised plan.
In life-threatening hypoxaemia, emergency treatment takes priority. Appropriately trained staff should provide oxygen according to emergency guidance while urgent senior help is obtained.
Oxygen is usually prescribed to a target range
A useful way to think about oxygen therapy is:
Target range first, then device and flow needed to achieve it.
The device and flow may need to change as the patient's condition changes, but the aim is to keep the patient within the intended saturation range while the underlying cause is assessed and treated.
Always check local policy because oxygen prescribing and emergency protocols can vary slightly between organisations and UK nations.
Pulse oximetry: what the number can and cannot tell you
Pulse oximetry is useful, but it is not infallible. Read the saturation alongside the patient and the quality of the signal.
Possible causes of an unreliable or misleading reading include:
- movement or tremor
- cold peripheries
- poor peripheral perfusion
- incorrect probe placement
- nail varnish or artificial nails in some circumstances
- an inappropriate probe size or site
- low-flow states
Pulse oximeters may also overestimate true oxygen saturation in some people with darker skin pigmentation, particularly around clinically important lower saturation ranges. If the reading does not fit the clinical picture, reassess the patient, check the equipment and escalate rather than trusting the monitor automatically.
Common oxygen delivery devices
Nasal cannulae
Nasal cannulae are commonly used for patients requiring relatively low levels of supplemental oxygen. They are generally well tolerated and allow the patient to speak, eat and drink.
The concentration of oxygen reaching the lungs is variable because it is affected by the oxygen flow, respiratory pattern, tidal volume and whether the patient breathes through the mouth or nose.
What to check: prescribed target, flow rate, tubing connection, patient comfort, nostrils, ears and surrounding skin, and whether the patient remains within target.
Simple face mask
A simple face mask provides a variable oxygen concentration. It must be used with sufficient flow to reduce rebreathing of exhaled carbon dioxide, so always follow manufacturer instructions and local policy.
It is less suitable when a precise controlled oxygen concentration is needed.
What to check: correct flow, good fit without excessive pressure, tubing and source connection, patient tolerance and skin condition.
Venturi mask
A Venturi system is useful when a more controlled and predictable inspired oxygen concentration is required. The device works by entraining room air through the Venturi valve.
The flow must match the manufacturer's instructions for the selected valve. The air-entrainment openings must remain unobstructed.
Do not rely on valve colour alone unless you have checked the actual device, because colours and specifications can vary between manufacturers.
What to check: correct valve, correct flow, unobstructed entrainment ports, prescribed target and patient response.
Reservoir mask
A reservoir mask is used when a high inspired oxygen concentration is required in critical illness. The reservoir bag should be inflated before application and should not collapse completely during inspiration.
Resuscitation Council UK guidance commonly uses a high-concentration reservoir mask at approximately 15 L/min in critically ill patients when indicated, while the patient is being urgently assessed and treated.
What to check: oxygen source, adequate flow, reservoir inflation, mask fit, immediate response and urgent escalation.
High-flow nasal oxygen, NIV and invasive ventilation
High-flow nasal oxygen, non-invasive ventilation and invasive ventilation are specialist respiratory supports. They are not simply stronger versions of routine oxygen therapy.
They require appropriate equipment, trained staff, closer monitoring and a clear treatment plan. As a student, you may help care for patients receiving these therapies, but initiation and adjustment depend on your competence, supervision and local policy.
When are blood gases important?
Blood gases can provide information that pulse oximetry cannot. Depending on the clinical situation, they can help assess:
- oxygenation
- carbon dioxide levels
- pH and acid-base disturbance
- respiratory failure
- response to oxygen or ventilatory support
They are especially important when hypercapnia is suspected, when the patient is deteriorating despite oxygen, when there is reduced consciousness or significant respiratory distress, or when the oxygen target depends on carbon dioxide status.
You can read more in ABG Interpretation for UK Student Nurses.
What if the saturation is below target?
Do not just turn the oxygen up and walk away. Reassess.
- Look at the patient. Are they more breathless, confused, drowsy, cyanosed or exhausted?
- Check the reading. Is the pulse-oximeter signal reliable?
- Check the system. Is the device fitted correctly? Is the tubing connected? Is oxygen actually flowing?
- Check the target. What saturation range is prescribed or clinically indicated?
- Escalate deterioration. An increasing oxygen requirement can be a major warning sign even before the saturation falls.
- Reassess after intervention. Do not assume the problem is solved because the number improves.
Worked example 1: pneumonia with increasing oxygen requirement
A patient admitted with pneumonia has an SpO₂ target of 94 to 98%. Earlier they were maintaining 95% on 2 L/min nasal cannulae. Four hours later their SpO₂ is still 95%, but they now require 6 L/min via a face mask, their respiratory rate has risen from 20 to 30 breaths per minute and they are using accessory muscles.
The important finding is not the 95% saturation. The patient now requires significantly more oxygen to maintain the same saturation and has increased work of breathing. This is deterioration and requires prompt reassessment and escalation.
Worked example 2: COPD exacerbation
A patient with a known history of hypercapnic respiratory failure is admitted with a COPD exacerbation. Their documented target is 88 to 92%. Their SpO₂ is 90% on controlled oxygen and they are awake and talking, but their respiratory rate is 28 and they are becoming increasingly drowsy.
Do not assume the oxygen therapy is satisfactory just because the SpO₂ is within target. Drowsiness may indicate worsening hypercapnia or another cause of deterioration. The patient needs urgent reassessment, blood-gas assessment where indicated and escalation to an experienced clinician.
Common oxygen therapy mistakes
- treating the monitor instead of assessing the patient
- aiming for 100% saturation without considering the prescribed target
- assuming every patient with COPD has the same target
- failing to record the oxygen device and flow rate with the saturation
- increasing oxygen repeatedly without investigating why the requirement is rising
- forgetting that pulse oximetry does not measure carbon dioxide
- using a Venturi valve with the wrong flow
- blocking the Venturi air-entrainment openings with bedding or clothing
- allowing a reservoir bag to collapse during inspiration
- failing to reassess after oxygen is started or changed
- ignoring new confusion, drowsiness or exhaustion because the SpO₂ looks acceptable
Documentation matters
Do not document only the saturation. Good oxygen documentation should make it possible for the next clinician to understand exactly what respiratory support the patient was receiving.
Record according to local policy, including relevant details such as:
- SpO₂
- target saturation range
- oxygen device
- oxygen flow rate or prescribed concentration where relevant
- respiratory rate and work of breathing
- changes made
- patient response
- escalation and review
Comfort, communication and safety
Explain why oxygen is being used and help the patient tolerate the device. Consider oral and nasal dryness, pressure areas around the ears and nose, communication, eating and drinking, mobility and anxiety.
Humidification is not automatically required for routine low-flow oxygen and should follow the clinical plan and local policy.
Oxygen supports combustion. It must be kept away from smoking, vaping, naked flames, sparks, oils and grease. Oxygen cylinders also need safe handling, storage and transport. Follow local medical-gas procedures and check that cylinders are functioning correctly when pipeline oxygen is unavailable.
Your role as a student nurse
Your exact role depends on your stage of training, competence, supervision, university requirements and local policy.
You may be able to:
- assess respiratory rate and work of breathing
- measure and interpret SpO₂ in context
- check the prescribed target range
- identify the oxygen device and flow
- check connections and obvious equipment problems
- recognise a rising oxygen requirement
- report deterioration promptly
- use SBAR to communicate concerns
- assist with oxygen therapy within your competence and supervision
- reassess and document the patient's response
You are not expected to independently diagnose respiratory failure or make unsupervised treatment decisions outside your competence. If the patient is deteriorating, call for help early.
The bit worth remembering
Oxygen is a treatment, not a target of 100%.
Think:
Patient → target range → device → response → reassess.
If the patient needs progressively more oxygen to stay within the same target, that matters. If their work of breathing, respiratory rate or conscious level is worsening, that matters. And if the monitor does not match the patient in front of you, trust the clinical assessment enough to check, reassess and escalate.
References
British Thoracic Society (BTS) (2017) BTS guideline for oxygen use in adults in healthcare and emergency settings. Available at: https://www.brit-thoracic.org.uk/document-library/guidelines/emergency-oxygen/bts-guideline-for-oxygen-use-in-adults-in-healthcare-and-emergency-settings/.
National Institute for Health and Care Excellence (NICE) (2018, updated) Chronic obstructive pulmonary disease in over 16s: diagnosis and management. NICE guideline NG115. Available at: https://www.nice.org.uk/guidance/ng115.
National Institute for Health and Care Excellence (NICE) (2011, updated 2023) Chronic obstructive pulmonary disease in adults: Quality statement 6, emergency oxygen during an exacerbation. Available at: https://www.nice.org.uk/guidance/qs10/chapter/quality-statement-6-emergency-oxygen-during-an-exacerbation.
Nursing and Midwifery Council (NMC) (2018) The Code: Professional standards of practice and behaviour for nurses, midwives and nursing associates. Available at: https://www.nmc.org.uk/standards/code/read-the-code-online/.
Resuscitation Council UK (2024) The ABCDE approach. Available at: https://www.resus.org.uk/library/abcde-approach.
This article is for education and placement preparation. Always follow the oxygen prescription, local oxygen policy, escalation pathway and supervision requirements used in your placement area.