- Supplemental oxygen is a drug and should be used rationally for specific indications, at the lowest concentration and for the shortest time possible.
- Indications for Oxygen Administration
- Cardiac or respiratory arrest
- Acute respiratory distress
- Specific conditions - pulmonary hypertension, myocardial infarction, pneumothorax
- Supplemental oxygen is not indicated for patients with breathlessness.
ComplicationsThe potential complications of supplemental oxygen are:
- Hypoventilation - due to suppression of the hypoxic respiratory drive; particularly in patients with COPD
- Absorption atelectasis (due to washout of nitrogen, which noramlly splints the airways open)
- Oxygen toxicity - damage to upper or lower airways due to oxygen radical formation
- Oxygen saturations should be used to guide supplemental oxygen administration. The target oxygen saturation for most patients should be 92-96%, while in patients with chronic respiratory failure this target should be 88-92%.
- Arterial blood gases are also used to guide oxygen therapy in certain patients.
Low Flow Oxygen
- Low flow oxygen devices deliver an amount of gas that is less than the patient’s total minute ventilation.
- The oxygen from the circuit is passively mixed with room air as the patient inspires, resulting in a variable FiO2 that depends on many factors including the respiratory rate, tidal volume and whether the patient is breathing through their nose or mouth.
- Low flow nasal prongs deliver 100% oxygen at a low flow rate, which is then entrained with room air. Nasal prongs are comfortable and allow the patient to eat, however at flow rates higher than 4 L/min can cause drying of the nasal cavity.
Flow FiO₂ 1 L/min ~0.24 2 L/min ~0.28 3 L/min ~0.32 4 L/min ~0.36
Simple Face Mask
- A simple face mask, also known as a Hudson mask, allows a larger amount of oxygen to be entrained with room air. As they have no reservoir bag, the maximum FiO2 possible is around 0.60.
- The flow into a Hudson mask should be at least 5 L/min, to reduce rebreathing of carbon dioxide.
Flow FiO₂ 5 L/min ~0.36 6 L/min ~0.42 7 L/min ~0.48 8 L/min ~0.54
- Reservoir systems include a reservoir bag, which allows for oxygen to accumulate between breaths. This allows for a higher concentration of oxygen to be inspired.
- The oxygen that enters the reservoir bag is passively mixed with room air, and therefore the amount of oxygen delivered by these systems is variable depending on the patient’s respiratory rate and tidal volume.
- These systems are used acutely in severely hypoxic patients. They should not be used for more than a few hours as they do not supply humidified air and can cause dehydration of the upper airway.
Partial Rebreather Mask
- A partial rebreather mask includes a reservoir bag, allowing for higher amounts of entrained oxygen. Unlike a non-rebreather mask, they do not contain an exhalation port and therefore the patient will rebreath some of their expired gas.
Flow FiO₂ 7 L/min ~0.65 8-10 L/min ~0.80
- A non-rebreather mask includes a reservoir bag and an exhalation port - preventing gas from being reinspired. This requires a tight seal over the patient’s face.
Flow FiO₂ 15 L/min Up to 0.90
High Flow Oxygen
- High flow devices deliver an amount of gas that is sufficient to meet the patient’s total minute ventilation, allowing for delivery of a precise amount of oxygen.
- A Venturi mask contains an entrainment device that uses the Bernoulli principle to entrain a precise amount of oxygen along with room air. This allows for a predictable FiO2 in patients where this is required.
Colour O₂ Flow FiO₂ Blue 2 L/min 0.24 White 4 L/min 0.28 Orange 6 L/min 0.31 Yellow 8 L/min 0.35 Red 10L/min 0.40 Green 15 L/min 0.60
High Flow Nasal Prongs
- High flow nasal prongs (HFNP) allow high flow of a precise amount of oxygen, and are useful in patients with hypoxic respiratory failure. The system is humidified, meaning that they can be used for longer periods of time. These systems also provide a small amount of positive end-expiratory pressure (PEEP), helping to keep the airways open.
- Non-invasive ventilation should occur in an appropriate venue with sufficiently trained staff, such as a respiratory ward, HDU or ICU.
- Continous positive airway pressure (CPAP) splints the upper airways open, and relieves upper airway obstruction. This device does not actively ventilate the patient.
- Indications for CPAP
- Obstructive sleep apnoea
- Acute pulmonary oedema
- Bilevel positive airway pressure (BiPAP) provides both expiratory positive airway pressure (EPAP), which opens the airways; and inspiratory positive airway pressure (IPAP), which ventilates the patient.
- Indications for BiPAP
- Acute hypercapnic respiratory failure - e.g. due to COPD, chest wall deformities, neuromuscular disorders
- Immunocompromised patients with acute respiratory failure
- Step-down after extubation of high-risk patients
- Contraindications to BiPAP
- Severe impairment of consciousness
- Severe facial deformity
- Inability to maintain or protect the airway
- Upper airway injury - burns, trauma, surgery
- Haemodynamic instability
- Untreated pneumothorax
- Invasive ventilation is performed following either intubation or tracheostomy.
- Indications for Invasive Ventilation
- Inability to maintain airway patency (upper airway obstruction) - soft tissue swelling, deformity, obesity, tongue displacement
- Inability to protect the airway - sedation (e.g. for procedures), upper airway bleeding, excessive secretions
- Inability to ventilate - unconsciousness, sedation, neuromuscular disease, exhaustion
- Inability to oxygenate - severe acute respiratory failure
- Cardiac arrest
- Listed below is a summary of common ventilation modes:
- Continuous mandatory ventilation (CMV) - the ventilator delivers a set number of breaths per minute at a set volume; the patient cannot trigger breaths
- Volume Assist / Control - the ventilator delivers a set number of breaths per minute at a set volume; the patient can trigger assisted breaths at the same tidal volume
- Continuous Positive Airway Pressure (CPAP) - the ventilator delivers a set positive pressure (PEEP), with no additional inspiratory pressure
- Pressure support (PS) - the ventilator supports the patient’s spontaneous breaths with a set positive inspiratory pressure, with or without positive end expiratory pressure (PEEP)
- Pressure Assist / Control - the ventilator delivers a set number of breaths per minute at a set inspiratory pressure; the patient can trigger assisted breaths at the same pressure
- Syncronised Intermittent Mandatory Ventilation (SIMV) - the ventilator waits for the patient to breath, and supports each breath with set volume or pressure. If the patient does not breath within a set timing window, the ventilator delivers a mandatory supported breath