🩺 Oxygen Therapy Administration

Oxygen therapy is the administration of oxygen at concentrations higher than room air (21%) to treat or prevent hypoxaemia. In the UK, oxygen is treated as a drug and must be prescribed by specific target saturation ranges rather than a fixed flow rate. Management is guided by the British Thoracic Society (BTS) guidelines, ensuring that patients receive enough oxygen to prevent tissue hypoxia while avoiding the risks of hypercapnia in susceptible individuals. Proper delivery involves choosing the correct interface (cannulae, Venturi, or non-rebreathe) based on the clinical context.

Oxygen therapy is indicated for the treatment of documented hypoxaemia (low SpO2) and to relieve the symptoms of respiratory distress. In the UK, British Thoracic Society (BTS) guidelines state that oxygen should be used to achieve a target saturation range. For most acutely ill patients, the target is 94-98%. For those at risk of hypercapnic respiratory failure (e.g., COPD, morbid obesity, or neuromuscular disease), the target is 88-92% until blood gas results are available to guide further titration.

There are no absolute contraindications to oxygen in an emergency (e.g., cardiac arrest or anaphylaxis), where 100% oxygen is always indicated. However, relative contraindications include patients with known paraquat poisoning or those with previous bleomycin lung toxicity, where high-dose oxygen can accelerate pulmonary fibrosis. In patients at risk of hypercapnic respiratory failure, uncontrolled high-flow oxygen is contraindicated, and therapy must be carefully titrated to specific saturation targets.

Equipment includes an oxygen source (wall outlet or cylinder) and a flow meter. Delivery devices vary: Nasal Cannulae (for low-flow, 24-44% FiO2), Simple Face Masks (5-10L/min), and Venturi Masks (providing fixed, accurate FiO2 using colour-coded valves). For emergencies, a Non-Rebreathe Mask with a reservoir bag (delivering 60-90% FiO2) is used. Humidification systems may be added for long-term therapy. A pulse oximeter is essential for monitoring saturation (SpO2) and adjusting therapy.

Initially, assess the patient's airway and breathing, and measure baseline SpO2. Choose the appropriate delivery device based on the patient's clinical need and risk of hypercapnia. Connect the device to the oxygen flow meter and set the required flow rate (ensure the reservoir bag is inflated if using a non-rebreathe mask). Fit the mask or cannulae comfortably to the patient's face, ensuring a good seal. Re-assess the patient's saturations and respiratory rate after 5-10 minutes and titrate the flow to reach the prescribed target range. Monitor the patient continuously using the NEWS2 framework.

Interpretation of oxygen therapy efficacy is based on pulse oximetry and Arterial Blood Gas (ABG) analysis. Success is achieving the target SpO2 range without a significant rise in carbon dioxide (PaCO2) or a drop in pH (acidosis). If a patient requires increasing amounts of oxygen to maintain saturations, this must be interpreted as clinical deterioration, necessitating an urgent medical review (NEWS2 score escalation). The 'Venturi' system is specifically interpreted by the colour of the valve, which dictates the exact percentage of oxygen delivered regardless of the patient's respiratory rate.

A critical error is 'over-oxygenating' patients with Type 2 Respiratory Failure (e.g., COPD), which can suppress their hypoxic drive and lead to hypercapnia and respiratory acidosis. Another error is failing to ensure the reservoir bag on a non-rebreathe mask is inflated before application, which effectively limits the fraction of inspired oxygen (FiO2). Using a standard nasal cannula at flow rates above 6L/min is ineffective and causes significant mucosal dryness and discomfort. Lastly, students often forget to prescribe oxygen, which is a legal requirement in the UK as it is classified as a drug.

In an OSCE, always check the patient's saturation before and after starting oxygen. Double-check if the patient has a 'Blue Card' or a history of COPD to determine their target range. When using a non-rebreathe mask, put your finger over the valve to fill the bag before putting it on the patient. Ensure you mention that you would prescribe the oxygen on the drug chart, including the device, flow rate, and target saturation range. Check the skin behind the ears for pressure sores from the tubing.

Oxygen therapy is a core topic for the MLA 'Prescribing' and 'Acute Medicine' modules. Students must know the target ranges (94-98% vs 88-92%) and which mask corresponds to which scenario. Venturi masks are high-yield for exams because they deliver a fixed FiO2 despite varying minute ventilation. Understanding the NEWS2 (National Early Warning Score) system, where oxygen use adds 2 points to the score, is essential for UK clinical practice.