Oxygen therapy

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Oxygen therapy is commonly used in hospital settings for the management of acute and chronic respiratory conditions, and increasingly in the community for patients with chronic conditions requiring home oxygen therapy. As with all treatments, oxygen therapy has side effects, and inappropriate use with inadequate monitoring can be fatal. The method of oxygen delivery, monitoring, target oxygen saturation, and indications for weaning should all be tailored to the individual patient. For discharged patients who require long-term oxygen therapy, risks should be discussed with patients and adequate monitoring should be established.

To maintain a constant supply of oxygen to the cells, a variety of physiological adaptations respond to hypoxemia and hyperoxemia. ^[2]

• Consequences of hypoxemia
  • Increased ventilation
  • Vasodilation of the coronary arteries, cerebral arteries, and, transiently, the peripheral arterioles, leading to a decline in systemic vascular resistance
  • Tachycardia and increased cardiac output
  • Hypoxic pulmonary vasoconstriction
  • Increased erythropoietin production, which, in the long term, can lead to polycythemia
• Consequences of hyperoxemia
  • Vasoconstriction of the cerebral and coronary arteries, and peripheral arterioles leading to increased systemic vascular resistance and increased blood pressure
  • A small reduction in ventilation
  • Vasodilation of the pulmonary vasculature
  • Reduced renal blood flow

Room air entrainment ^[3]

• Definition: the admixture of room air with delivered oxygen due to a negative pressure gradient generated by either the patient or the delivery device itself
• Consequence: FiO₂ does not directly correlate with the oxygen flow rate.
• Common situations where this may occur:
  • Peak inspiratory flow rate (PIFR) exceeds the flow rate of the oxygen provided (e.g., with low-flow oxygen devices)
  • High flow of oxygen through a small opening with an imperfect seal around the delivery system (e.g., Venturi mask, nonrebreather mask)

Humidified versus nonhumidified oxygen

Nonhumidified oxygen

• Description: supplied oxygen without added moisture
• Indications
  • Indefinite use: low-flow (≤ 4 L/minute) oxygen via nasal cannula or a mask ^[4]
  • Conditional use
    • High-flow oxygen (> 4 L/minute) via the upper airways: only for short-term use (up to 24 hours) ^[2]
    • Any oxygen via tracheostomy or other artificial airways: only in emergencies until humidified oxygen becomes available ^[2]
• Advantages
  • More widely available
  • Reduced risk of bacterial contamination ^[5]
• Disadvantages
  • May dry out the upper airway mucosa, leading to nose bleeds and discomfort
  • Thickens secretions, leading to difficulty clearing sputum

Humidified oxygen

• Description: combination of oxygen delivery with a humidification device
• Indications ^[2]
  • High-flow oxygen (> 4 L/minute) via the upper airways for > 24 hours
  • Any oxygen via:
    • High-flow nasal cannula (HFNC)
    • Tracheal or other artificial airways
  • If patients experience discomfort with nonhumidified oxygen
• Advantages
  • Greater patient comfort
  • Does not desiccate mucus membranes
• Disadvantages
  • Risk of bacterial contamination of humidification devices
  • Less portable

Oxygen delivery devices and flow rates should always be matched to patients' individual oxygen requirements, which can be varied and dynamic.

Nasal cannula ^[6]

• Description: a basic oxygen delivery system consisting of two nasal prongs ^[6]
• FiO₂ delivered: ∼ 24–40% (1–6 L/minute)
• Clinical applications: low oxygen saturation in patients who are not critically ill
• Advantages
  • Well tolerated by patients
  • Allows patients to eat, drink, and speak clearly while remaining on oxygen
• Disadvantages
  • Not effective in nasal obstruction
  • FiO₂ varies heavily with breathing as a result of room air entrainment; therefore, nasal cannula is not advised if a consistent FiO₂ is recommended.

Face mask

• Indications
  • The patient requires a higher FiO₂ than basic nasal cannula can provide (i.e., FiO₂ > 40%).
  • Nasal cannula is contraindicated (e.g., nasal blockage or facial injury).
  • Consistent FiO₂ desired (e.g., to prevent oxygen-induced hypercapnia in COPD)

Simple oxygen face mask ^[6]

• Description
  • Plastic face mask covering the nose and mouth that allows for oxygen to enter directly through a port at the bottom of the mask
  • Holes in the side of the mask allow for exhalation (as opposed to one-way valves)
  • No external reservoir bag
• FiO₂ delivered: ∼ 30–60% (5–10 L/minute) ^[6]
• Advantages: Less susceptible to room air entrainment than nasal prongs
• Disadvantages
  • Moderately variable FiO₂
  • Cannot be titrated down to < 5 L/minute
  • Prevents normal eating and drinking

Venturi mask

• Description
  • An oxygen delivery system that consists of a face mask, a jet nozzle, and changeable ports ^[6]
  • Utilizes room air entrainment to deliver a fixed FiO₂ ^[7]
• FiO₂ delivered: up to 60%; in increments ranging from 24% to 60% ^[8]

• Advantages
  • Consistent FiO₂ delivery
  • Easy to titrate
  • Each port provides a fixed FiO₂ concentration, reducing the risk of oxygen-induced hypercapnia.
  • Minimizes rebreathing because of the high flow of gas ^[8]
• Disadvantages
  • Noisy and can affect sleep
  • Interrupts normal eating and drinking

Nonrebreather mask (NRB) ^[6]

• Description
  • Plastic mask that covers the face and mouth and a reservoir bag that should be prefilled with oxygen
  • One-way valves that prevent rebreathing expired CO₂
• FiO2 delivery: ∼ 60–80% (at flow rates of 10–15 L/minute)
• Clinical application: first-line treatment for conditions with high oxygen requirements, e.g., critically ill patients
• Advantages
  • Prevents rebreathing of CO₂
  • Rapidly and easily applied in a variety of clinical settings
• Disadvantages
  • In patients with respiratory distress, FiO₂ can vary with breathing as a result of room air entrainment.
  • Cannot be titrated down below 10 L/minute

Nebulizer

• Description
  • A device that consists of a mask or mouthpiece, a medication reservoir, and tubing that is attached to either an air compressor or oxygen
  • Allows for administration of aerosolized medication (e.g., bronchodilators, racemic epinephrine)
• Advantages
  • Direct delivery of medication into the lungs and airways
  • Patients can receive oxygen therapy alongside medication.
• Disadvantages: Oxygen-driven nebulizers are typically limited to flow rates in the range of 6–8 L/minute. ^[11]

Advanced oxygen delivery systems are indicated for patients who remain hypoxic despite treatment with basic oxygen delivery systems, and for patients with tracheostomies.

High-flow nasal cannula (HFNC) ^[12]

• Description
  • Two broad nasal prongs and a system for heating and humidifying the oxygen
  • Not to be confused with basic nasal cannula
• FiO₂ delivery: 100% at flow rates up to 60 L/minute
• Clinical applications
  • Patients with hypoxemic respiratory failure and high oxygen demands
  • Acute pulmonary edema
  • Patients with high oxygen demands whose level of care precludes intubation and mechanical ventilation
  • Infants with bronchiolitis
• Advantages
  • Higher level of patient comfort than with noninvasive ventilation.
  • Provides some degree of positive end-expiratory pressure (PEEP), improving functional residual capacity
  • Improves breathing efficiency
  • May reduce the need for intubation in acute respiratory failure compared to conventional oxygen therapy ^[13]
• Disadvantages
  • The PEEP delivered is more variable than with CPAP. ^[14]
  • Not always available
  • Increased risk of hyperoxia and oxygen toxicity

High-flow nasal cannula cannot be replicated by using high flow rates through a basic nasal cannula!

Tracheal delivery systems

Tracheostomy mask

• Description: oxygen delivery via a small plastic dome that fits over the tracheostomy site ^[15]
• FiO₂ delivery: 30–80% (8–10 L/minute) ^[16]
• Clinical applications: hypoxia in a patient with a tracheostomy who is not mechanically ventilated
• Advantages
  • More comfortable than a T-piece
  • Easy to apply in emergency settings
• Disadvantages
  • Moisture can build up on the skin around the tracheostomy site.
  • Variable FiO₂ due to the loose fit on the neck

T-piece

• Description: a T-shaped connection that allows air to flow in from an oxygen supply and exhaled air to exit from the side of the connector
• FiO2 delivery: 30–80% (8–10 L/minute) ^[16]
• Clinical applications
  • Patients who are deteriorating despite the use of a tracheostomy mask ^[2]
  • Weaning patients from mechanical ventilation ^[17]
• Advantages
  • Less moisture collects on the skin around the tracheostomy site than with a tracheostomy collar.
  • Can deliver a higher flow rate than masks
• Disadvantages
  • Moisture can collect in the tubing, adding to the weight of the tube and causing it to drag on the site. ^[15]
  • Cumbersome and restricts the patient's movements
  • If the oxygen flow rate is too low, rebreathing will occur. ^[18]

Transtracheal oxygen therapy (TTOT) ^[19]

• Description
  • Transtracheal catheter for patients who require long-term domiciliary oxygen therapy but prefer not to use nasal cannula.
  • Hollow catheter percutaneously inserted into the trachea to deliver long-term low-flow oxygen at rates of 0.5–4 L/minute ^[20][21]
• FiO2 delivery: similar to nasal cannula
• Clinical applications: hypoxemic respiratory failure in preexisting lung disease (e.g., COPD or ILD)
• Advantages
  • Less nasal irritation compared to long-term nasal cannula use
  • More aesthetically pleasing for patients
  • More efficient, meaning that lower oxygen flow rates can be used ^[20]
  • Higher compliance than nasal oxygen
• Disadvantages
  • Can easily become clogged with mucus; patients need to clean their own catheters several times a day
  • Can cause tracheal irritation and granuloma formation
  • Requires an invasive procedure to place it

Assisted ventilation

• Oxygen can also be delivered via NIPPV or invasive mechanical ventilation (See “Mechanical ventilation” for details).
• The main advantage of these systems in terms of oxygen delivery include:
  • Near-perfect seals with negligible room air entrainment, meaning that a true FiO₂ of 100% is achievable
  • The ability to add PEEP, which enhances gas exchange and alveolar recruitment
• Disadvantages include a higher potential for oxygen toxicity, as well as multiple ventilator-related complications.

Indications

• Acute hypoxia
• Preoxygenation
• Treatment of specific conditions

Target oxygen saturation range

General recommendations for starting oxygen ^[2]

• Patients with no risk factors for hypercapnic respiratory failure
  • Critically ill patients and initial SpO₂ < 85%: Start high-flow oxygen (10–15 L/minute) via NRB and titrate down to target saturations.
  • All other patients: Start on nasal cannula or a simple face mask and titrate up to target saturations.
  • Patients with acute MI and stroke were previously treated as critically ill and given liberal oxygen therapy; because of the risk of harm, oxygen should only be started if saturations are < 90% (see “Target oxygen saturation range” table).
• Patients with risk factors for hypercapnic respiratory failure: Start on a Venturi mask (24–28%) or nasal cannula at 1–2 L/minute and obtain an ABG as soon as possible.

Pulse oximetry ^[29][30]

• Technical background
  • Oxygenated hemoglobin (O₂Hb) and deoxygenated hemoglobin (HHb) exhibit different properties of light absorption
    • O₂Hb: ↑ infrared light absorption, allows ↑ red light pass through the measurement site (e.g., fingertip)
    • HHb: ↑ red light absorption, allows ↑ infrared light pass through the measurement site
  • An oximeter uses LEDs (light-emitting diodes) emitting both red and infrared light → a photodetector is positioned on the other side of the finger, opposite the LEDs, and detects the amount of light (and whether it is red or infrared light) passing through the measurement site → a processing unit calculates the amount of O₂Hb → oximeter displays SpO₂
• Reference range: Resting oxygen saturation > 95% is generally considered normal.
  • A PaO₂ of 100 mm Hg is necessary to reach a SpO₂ level of ∼ 98%.
  • Measurement can be inaccurate in patients with: ^[2]
    • Nail polish
    • Poor perfusion, e.g., severe hypotension
    • Darker skin pigmentation and saturations of < 85%
    • Carbon monoxide exposure, including chronic low-level exposure in smokers
    • Methemoglobinemia ^[31]
• Monitoring
  • Should be performed for the majority of patients receiving oxygen therapy
  • Generally accurate to within 1–2 % of true SaO₂ until saturations drop to < 80% ^[2]
  • Patients in whom pulse oximetry is inaccurate and patients at risk of hypercapnic respiratory failure should undergo regular ABGs. ^[2]

Pulse oximetry provides falsely high values in cases of carbon monoxide poisoning, as complexes of hemoglobin and carbon monoxide are indistinguishable from oxygen-hemoglobin complexes!

Reducing and discontinuing oxygen therapy ^[2]

• Weaning
  • Titrate oxygen down if:
    • Saturations are above the target range
    • Saturations have been in the higher end of the target range for 4–8 hours
  • Recheck saturations after 5 minutes at the new oxygen flow rate.
  • Do not discontinue oxygen therapy abruptly if oxygen-induced hypercapnia occurs; this can cause a significant relapse of hypoxemia.
• Discontinuation criteria
  • The patient is clinically stable on low-flow oxygen.
  • The patient's oxygen saturations have been within the target range on two consecutive observations.
• Postdiscontinuation monitoring
  • After cessation, saturations should be checked at 5 minutes and then 1 hour.
  • If saturations remain within the target range after 1 hour, the patient can remain off oxygen and return to routine monitoring of vital signs.

Description

• Oxygen therapy may be provided on a long-term basis outside of a hospital for patients with chronic conditions.
• Nasal cannula is the most common method of delivery but alternatives may be used depending on the underlying condition.
• Home oxygen may be provided via an oxygen concentrator, compressed oxygen cylinders, or liquid oxygen, depending on patient needs and preference.

Indications

Long-term oxygen therapy ^[4]

• Description
  • The most common form of home oxygen delivery
  • Treatment is typically low flow (1–2 L/minute) oxygen via nasal cannula or TTOT.
  • Typically used in advanced lung disease if patients remain chronically hypoxic despite maximal medical therapy
  • Patients prescribed LTOT should use it for a minimum of 15 hours a day. ^[34]
• Monitoring
  • Start at a rate of 1 L/minute; titrate to SpO₂> 90% (an ABG should be performed to confirm PaO₂ is > 60 mm Hg) ^[32]
  • If there are signs of worsening hypercapnia, the patient should be assessed for noninvasive home ventilation. ^[32]
  • Patients prescribed LTOT, nocturnal, or ambulatory oxygen therapy should receive follow-up and monitoring at home after 4 weeks and after 3 months. ^[32]

Description

• Definition: intermittent treatment with 100% oxygen delivered at pressures > 1.4 atmospheres ^[35][36][37]
• Typical session ^[38]
  • Duration: 90–120 minutes
  • Frequency: repeated twice daily for up to 30 days

Indications ^[37]

• Trauma and environmental
  • Carbon monoxide poisoning
  • Decompression sickness
  • Delayed radiation injuries
  • Thermal burns
  • Acute traumatic ischemia (e.g., compartment syndrome, crush injuries)
  • Air or gas embolism
• Wounds and infections
  • Arterial insufficiency (e.g., poor wound healing)
  • Gas gangrene (clostridial myositis and myonecrosis)
  • Crush injuries
  • Necrotizing fasciitis
  • Compromised skin grafts or skin flaps
  • Refractory osteomyelitis
  • Intracranial abscess
• Other
  • Severe anemia
  • Central retinal artery occlusion
  • Sudden sensorineural hearing loss

The use of hyperbaric oxygen treatment in treating autism spectrum disorder, multiple sclerosis, cerebral palsy, and acute stroke is not supported by evidence and should be avoided. ^[39]

Risks ^[40]

• Oxygen toxicity: specifically CNS oxygen toxicity and pulmonary oxygen toxicity
• Barotrauma in any of the following locations:
  • Middle ear (most commonly reported)
  • Sinuses
  • Teeth
  • Lungs
• Hyperoxic myopia
• Cataracts
• High fire risk in oxygen-enriched environments

The two most significant complications of oxygen therapy are hyperoxemia (with associated oxygen toxicity) and oxygen-induced hypercapnia.

Hyperoxemia and oxygen toxicity ^[41]

• Definitions
  • Hyperoxemia: excessive oxygenation to tissues, quantified as a PaO₂ > 120 mm Hg
    • Mild: PaO₂ 121–200 mm Hg
    • Moderate: PaO₂ 201–300 mm Hg
    • Severe: PaO₂ > 300 mm Hg
  • Oxygen toxicity: a constellation of adverse effects resulting from hyperoxemia and affecting multiple organ systems
• Risk factors
  • Hyperoxemia: FiO₂ > 80%, PEEP
  • Oxygen toxicity
    • Duration of exposure ^[42]
    • Moderate to severe hyperoxemia ^[42][43]
    • Hyperbaric oxygen therapy
    • Critically ill patients ^[44]
• Diagnosis
  • Pulse oximetry is not an accurate measure for hyperoxemia
  • ABG should be performed to diagnose hyperoxemia and determine its severity.
  • Clinical signs of oxygen toxicity may not be readily apparent.

• Prevention
  • Patients on oxygen therapy should be monitored with pulse oximetry, and oxygen should be titrated to ensure they remain within their target saturation range.
  • Critically ill patients should receive regular arterial blood gases.

Oxygen-induced hypercapnia ^[2]

• Overview
  • PaCO₂ > 45 mm Hg or a rise of > 5 mm Hg in a chronically hypercapnic patient ^[47]
  • One of the most common and serious complications of oxygen therapy
  • Can be fatal if left untreated.
• Mechanism
  • Thought to predominantly occur via a combination of two mechanisms when supplemental oxygen is administered: ^[2][48][48]
    • ↓ Hypoxic pulmonary vasoconstriction: ↑ FiO₂ → ↑ alveolar O₂ tension → ↓ hypoxic pulmonary vasoconstriction → ↑ V/Q mismatch and hypercapnia
    • Haldane effect: ↑ FiO₂ → ↑ oxygenated Hb, which has a reduced affinity to bind CO₂ (right shift in the CO₂ dissociation curve) → CO₂ being released from Hb and RBCs → ↑ PaCO₂
• Risk factors
  • Any patient with risk factors for hypercapnic respiratory failure who is receiving supplemental oxygen
  • PaO₂ is > 75 mm Hg ^[2]
  • Acute illness or new oxygen therapy in patients with chronic hypercapnic respiratory failure ^[2]

• Management
  • Gradually titrate oxygen back to 88–92%. ^[2]
  • Noninvasive ventilation in patients with decompensated hypercapnic respiratory failure who are within target saturations.
• Prevention
  • Close monitoring for symptoms of hypercapnia.
  • Patients at risk of hypercapnic respiratory failure: ABGs should be performed if drowsiness or other symptoms of hypercapnia develop, if saturations deteriorate, or if acute breathlessness occurs. ^[2]

Sudden cessation of oxygen therapy in hypercapnic respiratory failure can cause life-threatening rebound hypoxia!

We list the most important complications. The selection is not exhaustive.