Oxygen Therapy and Non-Invasive Ventilation: Targets, Devices and Escalation

Introduction

Oxygen is the most frequently administered drug in hospital and one of the least carefully prescribed. British Thoracic Society guidance is unambiguous: oxygen must be prescribed on the drug chart with a target saturation range, administered by a signed-for delivery device, titrated by nursing staff to keep the patient within that range, and reviewed like any other treatment.¹

Both under-treatment and over-treatment cause harm. Hypoxia kills quickly; excessive oxygen worsens hypercapnia in susceptible patients, promotes absorption atelectasis, generates reactive oxygen species, and in myocardial infarction and stroke has been shown to offer no benefit and possible harm. The aim is therefore not to maximise oxygen but to keep the patient in a defined range.

Oxygen delivery devices

Devices divide into variable performance, where the delivered concentration depends on the patient's respiratory pattern, and fixed performance, where it does not. In a patient whose CO2 matters, use a fixed performance device.

Practical prescribing

Starting oxygen

1. Assess - respiratory rate, saturations, conscious level and work of breathing, and identify whether the patient is at risk of hypercapnia
2. Prescribe a target range, not a fixed flow rate, and record the starting device
3. In critical illness - reservoir mask at 15 L/min immediately, then titrate down once stable
4. In a patient at risk of hypercapnia - start with a 24 or 28% Venturi mask targeting 88-92%
5. Take an arterial blood gas in anyone unwell enough to need admission, recording the FiO2
6. Reassess with a repeat gas 30 to 60 minutes after any change in oxygen delivery
7. Wean and stop as the patient improves - oxygen left running after recovery is a common and avoidable error

Humidification and safety

• Humidify oxygen given at high flow, for prolonged periods, or through a tracheostomy
• Fire risk - oxygen supports combustion. Smoking near oxygen has caused deaths and serious burns, which is why current smoking contraindicates home oxygen.
• Do not use oxygen-driven nebulisers in patients at risk of hypercapnia - drive them with air and give supplementary oxygen by nasal cannulae alongside
• Check the device is connected to oxygen and not air, and that flow meters are set correctly - a startlingly common cause of unexplained desaturation

Home oxygen

• Long-term oxygen therapy (LTOT) - for chronic hypoxaemia, used at least 15 hours a day. Assess with two arterial gases at least 3 weeks apart when clinically stable: PaO2 below 7.3 kPa, or 7.3 to 8 kPa with polycythaemia, peripheral oedema, pulmonary hypertension or nocturnal hypoxaemia. This is the only oxygen prescription shown to improve survival.
• Ambulatory oxygen - for patients on LTOT who leave the house, or for exertional desaturation with demonstrated benefit
• Palliative oxygen - for refractory breathlessness at the end of life in a hypoxaemic patient. In non-hypoxaemic patients, a handheld fan and opioids are more effective than oxygen, which is worth knowing because oxygen is so often reached for first.
• Short-burst oxygen - not recommended; the evidence does not support it
• A formal home oxygen risk assessment, including smoking and fire safety, is mandatory before any prescription

CPAP and non-invasive ventilation

Non-invasive respiratory support delivers positive pressure through a mask rather than an endotracheal tube. The two modes do quite different things, and confusing them is one of the most consequential errors in acute medicine.

Indications for NIV

• COPD with respiratory acidosis - pH 7.25 to 7.35 with PaCO2 above 6 kPa, persisting after no more than one hour of optimal medical therapy. This is the strongest indication, with clear evidence of reduced intubation rates and mortality.²
• pH below 7.25 - NIV may still be used but in a higher-dependency setting with immediate access to intubation, since failure rates are high
• Chest wall deformity or neuromuscular disease with hypercapnic failure
• Obesity hypoventilation syndrome with decompensated type 2 failure
• Cardiogenic pulmonary oedema - CPAP is usually first line, with NIV where the patient is also hypercapnic
• Weaning from invasive ventilation, and post-extubation support in selected patients
• Palliative relief of breathlessness in patients not for intubation, where it improves symptoms

Contraindications

• Untreated pneumothorax - an absolute contraindication until a chest drain is in place
• Facial trauma, burns or recent facial or upper airway surgery
• Fixed upper airway obstruction
• Vomiting, or an unprotected airway with a high aspiration risk
• Reduced conscious level with an inability to protect the airway - though a degree of drowsiness from CO2 narcosis is not itself a contraindication and often improves rapidly on NIV
• Severe agitation or inability to cooperate
• Haemodynamic instability requiring vasopressor support
• Copious respiratory secretions the patient cannot clear
• Impending respiratory arrest - this patient needs intubation, not a mask
• Recent upper gastrointestinal surgery, and bowel obstruction

Setting up and monitoring

1. Explain what is happening and hold the mask to the face initially - tolerance in the first ten minutes largely determines whether NIV succeeds
2. Start at IPAP 12 to 15 cmH2O and EPAP 4 to 5 cmH2O, then increase the IPAP in steps of 2 to 5 towards 20 to 30 cmH2O as tolerated. It is raising the difference between IPAP and EPAP that clears CO2.
3. Set a backup rate in case the patient stops triggering
4. Entrain oxygen to a target of 88 to 92%
5. Repeat the blood gas at 1 hour and 4 hours, and after each significant change. Improvement in pH at 1 hour is the best predictor of success.
6. Monitor clinically - respiratory rate, conscious level, comfort, synchrony, mask leak and accessory muscle use
7. Nurse in an appropriate area with staff trained in NIV, and continue medical treatment of the underlying condition throughout

When NIV is failing

• No improvement in pH or PaCO2 at 1 to 2 hours, or continued deterioration
• Worsening conscious level, agitation or inability to tolerate the mask
• Persistent asynchrony or large leak despite mask adjustment
• Rising respiratory rate, exhaustion or haemodynamic instability
• The response is to escalate to invasive ventilation if that is within the ceiling of care, or to move to symptom control if it is not. Persisting with failing NIV in a patient for whom intubation is appropriate simply delays it and worsens the outcome.

Complications

• Pressure injury to the bridge of the nose - the commonest complication; use a protective dressing from the outset and rotate mask types
• Dry mucous membranes and eye irritation from leak - humidify and refit
• Gastric distension and aerophagia, and aspiration
• Claustrophobia and intolerance
• Hypotension from reduced venous return, particularly in the hypovolaemic
• Pneumothorax and barotrauma
• Delayed intubation - the most dangerous complication, and the reason for strict monitoring and clear failure criteria

Invasive ventilation and escalation

Intubation and mechanical ventilation are indicated where non-invasive support fails or is unsafe, and where the underlying condition is expected to be reversible.

• Indications - failure of NIV, exhaustion, a falling conscious level, an unprotected airway, severe refractory hypoxaemia, haemodynamic instability, or the need for airway protection during another procedure
• Advantages - a secure airway, complete control of ventilation, the ability to suction secretions, and delivery of high PEEP
• Disadvantages - the need for sedation, ventilator-associated pneumonia, ventilator-induced lung injury, ICU-acquired weakness, delirium and difficulty weaning
• Tracheostomy - considered where prolonged ventilation is anticipated, reducing sedation requirements and facilitating weaning
• High-flow nasal oxygen occupies a useful middle position: better tolerated than CPAP, effective in type 1 failure, and usable during procedures and while eating

Red flags

Prognosis and outcomes

Getting oxygen prescription right is one of the clearest examples in medicine of a simple process change producing measurable benefit. National audits have repeatedly found that a substantial proportion of inpatients receive oxygen without a prescription and that many spend time outside their target range, and improving this reduces both hypoxaemic and hypercapnic events.

NIV for acute hypercapnic respiratory failure in COPD reduces intubation rates, mortality and length of stay, and is one of the best-evidenced interventions in acute respiratory medicine. In-hospital mortality for these admissions is around 10 to 20%, and the outlook depends far more on the underlying disease severity and the patient's baseline function than on the ventilator itself. An admission requiring NIV is a marker of advanced disease, and it is a natural point at which to discuss future treatment preferences.

Long-term oxygen therapy remains the only treatment that improves survival in chronic hypoxaemic COPD, and only when used for at least 15 hours a day - which makes adherence, comfort and practical support around the equipment as important as the prescription itself.