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382 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

with chronic obstructive pulmonary disease (COPD). ● patient factors: inspiratory flow rate, respiratory rate,
These patients require close monitoring of PaCO 2 levels tidal volume, respiratory pause
when oxygen therapy is instituted or increased. Although ● oxygen device factors: oxygen flow rate, volume of
COPD patients frequently may have a lower baseline mask/reservoir, air vent size, tightness of fit
SpO 2 (88–94% compared to 96–100% in patients with Normal inspiratory flow in a healthy adult ranges between
no lung pathology), treatment of hypoxia is still essential, 25 and 35 L/min. Patients with respiratory failure tend to
and oxygen should not be withheld or withdrawn while increase their flow demand from 50 up to 300 L/min.
hypoxia remains, even if hypercapnia worsens. 2,3
Patients in respiratory distress are characterised by high
respiratory rates and low tidal volumes that can signifi-
4,5
cantly decrease the FiO 2 available via an oxygen delivery
Practice tip device, depending on the type in use.
Oxygen should not be withheld or withdrawn while hypoxia All oxygen delivery devices use some type of reservoir to
remains, even if hypercapnia worsens. support oxygen delivery and prevent CO 2 rebreathing. In
the case of a face mask, the reservoir is the mask; for nasal
cannulae it is the patient’s pharynx. Patients with high
Oxygen Toxicity inspiratory flow and tidal volume will deplete the reser-
voir faster than it can be replenished, resulting in air
Administration of high concentrations of oxygen may entrainment and dilution of the oxygen concentration.
lead to oxygen toxicity; symptoms include non-productive
cough, substernal pain, reduced lung compliance, inter- VARIABLE FLOW DEVICES
stitial oedema, and pulmonary capillary haemorrhage. A range of low or variable flow oxygen delivery devices
These symptoms may be mistakenly attributed to the are available to meet a patient’s physiological needs.
underlying illness, especially in a sedated and ventilated These devices range from nasal cannulae and oxygen
patient. Many of the symptoms abate once the percentage masks with different features, through to bag–mask
or fraction of inspired oxygen (FiO 2 ) is reduced, although ventilation.
irreversible pulmonary fibrosis may occur (see Box 15.1).
The concentration and duration of oxygen exposure that Low-flow Nasal Cannulae
4
induces oxygen toxicity varies between patients; the Traditional low-flow nasal cannulae sit at the external
lowest possible FiO 2 should therefore be used to achieve nares and deliver 3–4 L/min of oxygen. Higher flows may
the target PaO 2 or SpO 2 .
cause discomfort and damage from the drying effect on
OXYGEN ADMINISTRATION DEVICES respiratory mucosa. They are generally well-tolerated by
the patient. Increased flow demand with respiratory dis-
Initial management of hypoxia in a spontaneously- tress dilutes the oxygen, reducing the FiO 2 to the alveoli.
breathing patient with an intact airway is low-flow oxygen Mouth-breathing and talking can also render nasal can-
via nasal cannulae (up to 6 L/min) or face mask (up to nulae ineffective.
15 L/min). Although oxygen devices have traditionally
had FiO 2 ascribed to specific flow rates, the FiO 2 delivered High-flow Nasal Cannulae
to the alveoli is influenced by: High-flow nasal cannulae (HFNC) have slightly larger
prongs that facilitate oxygen flow of up to 60 L/min
leading to less air entrainment effect than with other
5,6
oxygen delivery systems. HFNC generate low levels of
BOX 15.1 Signs and symptoms of oxygen end-expiratory pressure and can therefore reduce tachy-
7,8
toxicity pnoea and work of breathing. The high gas flow may
flush CO 2 from the anatomical dead space preventing
Central nervous system: CO 2 rebreathing and thereby decreasing PaCO 2 , although
● nausea and vomiting this is not well supported by the literature. 9,10 These
● anxiety systems are also generally well-tolerated by the patient,
● visual changes but must be used with heated humidification to avoid
● hallucinations drying the respiratory mucosa. HFNC are now used more
8
● tinnitus frequently in clinical practice to avoid more invasive ther-
● vertigo apies but there is limited high-quality evidence on their
● hiccups use in adults.
● seizures
Pulmonary:
● dry cough Practice tip
● substernal chest pain
● shortness of breath Patients using any type of nasal cannulae should avoid mouth-
● pulmonary oedema breathing and talking to minimise diluting oxygen delivery to
● pulmonary fibrosis the lungs.