Supplemental oxygen is a therapeutic intervention used to increase the fraction of inspired oxygen (FiO₂) in patients with inadequate arterial oxygenation. It is one of the most frequently prescribed treatments in acute and chronic healthcare settings and is fundamental to the management of hypoxaemia associated with respiratory, cardiovascular, neurological, and systemic disease.
The primary objective of oxygen therapy is to restore adequate tissue oxygen delivery while the underlying cause of respiratory compromise is investigated and treated. Although commonly administered, oxygen should be regarded as a prescribed medication, requiring an appropriate indication, clearly defined target oxygen saturation, and ongoing clinical monitoring.
Indications for Supplemental Oxygen
Supplemental oxygen is indicated for patients with hypoxaemia, where arterial oxygenation is insufficient to meet metabolic demand. The decision to commence oxygen therapy should be based on clinical assessment together with objective measures of oxygenation, including pulse oximetry and, where indicated, arterial blood gas analysis.
Common indications include:
- Pneumonia
- Acute exacerbations of chronic obstructive pulmonary disease (COPD)
- Acute severe asthma
- Acute respiratory distress syndrome (ARDS)
- Pulmonary embolism
- Acute heart failure
- Sepsis and septic shock
- Major trauma
- Cardiac arrest and post-resuscitation care
- Perioperative and postoperative respiratory support
Oxygen therapy should be initiated promptly in patients with clinically significant hypoxaemia while simultaneously addressing the underlying pathology.
Oxygen Delivery Systems
A variety of oxygen delivery devices are available, each capable of providing different oxygen concentrations and flow rates according to clinical requirements.
Common delivery systems include:
- Nasal cannulae
- Simple face masks
- Venturi masks
- Non-rebreather (reservoir) masks
- High-flow nasal oxygen (HFNO)
Device selection depends on the patient’s oxygen requirements, work of breathing, anticipated FiO₂ requirements, tolerance of therapy, and the degree of respiratory compromise. Controlled oxygen delivery systems, such as Venturi masks, are particularly useful where precise oxygen concentrations are required.
Target Oxygen Saturation
The objective of oxygen therapy is to achieve an appropriate target oxygen saturation rather than administering the highest possible oxygen concentration.
Recommended target ranges generally include:
- 94–98% for most acutely ill adults.
- 88–92% for patients at risk of hypercapnic respiratory failure, including many individuals with chronic obstructive pulmonary disease (COPD).
Target oxygen saturation should always be individualised according to the patient’s underlying condition, physiological status, and local clinical guidance.
Monitoring During Oxygen Therapy
Patients receiving supplemental oxygen require regular clinical reassessment to ensure that treatment remains both effective and safe.
Monitoring commonly includes:
- Peripheral oxygen saturation (SpO₂)
- Respiratory rate
- Fraction of inspired oxygen (FiO₂)
- Heart rate
- Blood pressure
- Work of breathing
- Level of consciousness
- Arterial blood gas (ABG) analysis where indicated
Ongoing assessment enables clinicians to titrate oxygen therapy appropriately and determine whether respiratory support should be escalated or gradually weaned.
Potential Risks of Oxygen Therapy
Although oxygen is an essential treatment for hypoxaemia, excessive oxygen administration may be associated with adverse effects.
Potential complications include:
- Oxygen-induced hypercapnia in susceptible patients
- Oxygen toxicity during prolonged exposure to high oxygen concentrations
- Absorption atelectasis
- Drying of the upper respiratory tract
- Oxidative lung injury
- Increased fire risk associated with oxygen-enriched environments
For these reasons, oxygen should be administered according to established prescribing principles and titrated to achieve the patient’s target saturation range rather than used indiscriminately.
Supplemental Oxygen in Clinical Assessment
Supplemental oxygen forms an integral part of respiratory assessment and is considered alongside clinical examination, physiological observations, laboratory investigations, and imaging findings.
The requirement for supplemental oxygen also contributes to several validated clinical scoring systems and decision-support tools. Oxygen therapy influences the fraction of inspired oxygen (FiO₂) used in calculations such as the PaO₂/FiO₂ (P/F) Ratio, an important measure of oxygenation impairment used in the assessment of acute respiratory distress syndrome (ARDS). In addition, oxygen supplementation forms part of the National Early Warning Score 2 (NEWS2), where the requirement for supplemental oxygen contributes to overall physiological risk assessment.
Clinical Significance
Supplemental oxygen remains one of the most important therapeutic interventions in modern clinical practice. Appropriate prescription, careful titration, and continuous monitoring improve tissue oxygenation, reduce the consequences of hypoxaemia, and support patients with a wide range of acute and chronic medical conditions. When integrated with comprehensive clinical assessment and evidence-based management, oxygen therapy plays a central role in improving patient outcomes across emergency, inpatient, perioperative, and critical care settings.