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Face masks and oxygen delivery devices
Face masks and angle pieces
The face mask is designed to fit the face anatomically and comes in different sizes to fit patients of different age groups (from neonates to adults). It is connected to the breathing system via the angle piece.
Components
- 1.
The body of the mask, which rests on an air-filled cuff ( Fig. 6.1 ). Some paediatric designs do not have a cuff, e.g. Rendell–Baker ( Fig. 6.2 ).
- 2.
The proximal end of the mask has a 22-mm inlet connection to the angle piece.
- 3.
Some designs have clamps for a harness to be attached.
- 4.
The angle piece has a 90-degree bend with a 22-mm end to fit into a catheter mount or a breathing system.
Mechanism of action
- 1.
They are made of transparent plastic. Previously, masks made of silicon rubber were used. The transparent plastic allows the detection of vomitus or secretions. It is also more acceptable to the patient during inhalational induction. Some masks are ‘flavoured’, e.g. strawberry flavour.
- 2.
The cuff helps to ensure a snug fit over the face, covering the mouth and nose. It also helps to minimize the mask’s pressure on the face. Cuffs can be either air-filled or made from a soft material.
- 3.
The design of the interior of the mask determines the size of its contribution to apparatus dead space. The dead space may increase by up to 200 mL in adults. Paediatric masks are designed to reduce the dead space as much as possible.
Problems in practice and safety features
- 1.
Excessive pressure by the mask may cause injury to the branches of the trigeminal or facial nerves.
- 2.
Sometimes it is difficult to achieve an air-tight seal over the face. Edentulous patients and those with nasogastric tubes pose particular problems.
- 3.
Imprecise application of the mask on the face can cause trauma to the eyes.
Face masks
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Made of silicone rubber or plastic.
-
Their design ensures a snug fit over the face of the patient.
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Cause an increase in dead space (up to 200 mL in adults).
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Can cause trauma to the eyes and facial nerves.
Nasal Masks (Inhalers)
- 1.
These masks are used during dental chair anaesthesia.
- 2.
An example is the Goldman inhaler ( Fig. 6.3 ), which has an inflatable cuff to fit the face and an adjustable pressure limiting (APL) valve at the proximal end. The mask is connected to tubing, which delivers the fresh gas flow (FGF).
- 3.
Other designs have an inlet for delivering the inspired FGF and an outlet connected to tubing with a unidirectional valve for expired gases.
Catheter mount
This is the flexible link between the breathing system tubing and the tracheal tube, face mask, supraglottic airway device or tracheostomy tube ( Fig. 6.4 ). The length of the catheter mount varies from 45 to 170 mm.
Components
- 1.
It has a corrugated disposable plastic tubing. Some catheter mounts have a concertina design, allowing their length to be adjusted.
- 2.
The distal end is connected to either a 15-mm standard tracheal tube connector, usually in the shape of an angle piece, or a 22-mm mask fitting.
- 3.
The proximal end has a 22-mm connector for attachment to the breathing system.
- 4.
Some designs have a condenser humidifier built into them.
- 5.
A gas sampling port is found in some designs.
Mechanism of action
- 1.
The mount minimizes the transmission of accidental movements of the breathing system to the tracheal tube. Repeated movements of the tracheal tube can cause injury to the tracheal mucosa.
- 2.
Some designs allow for suction or the introduction of a fibreoptic bronchoscope. This is done via a special port.
Problems in practice and safety features
- 1.
The catheter mount contributes to the apparatus dead space. This is of particular importance in paediatric anaesthesia. The concertina design allows adjustment of the dead space from 25 to 60 mL.
- 2.
Foreign bodies can lodge inside the catheter mount causing an unnoticed blockage of the breathing system. To minimize this risk, the catheter mount should remain wrapped in its sterile packaging until needed.
Catheter mount
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Acts as an adapter between the tracheal tube and breathing system in addition to stabilizing the tracheal tube.
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It is made of plastic with different lengths available.
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Some have a condenser humidifier built in.
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Its length contributes to the apparatus dead space.
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Can be blocked by a foreign body.
Oxygen delivery devices
Currently, a variety of delivery devices are used. These devices differ in their ability to deliver a set fractional inspired oxygen concentration (FiO 2 ). The delivery devices can be divided into variable and fixed performance devices. The former devices deliver a fluctuating FiO 2 whereas the latter devices deliver a more constant and predictable FiO 2 ( Table 6.1 ). The FiO 2 delivered to the patient is dependent on device- and patient-related factors. The FiO 2 delivered can be calculated by measuring the end-tidal oxygen fraction in the nasopharynx using oxygraphy.
Table 6.1
Classification of the oxygen delivery systems
| Variable performance devices | Fixed performance devices |
|---|---|
| Hudson face masks and partial rebreathing masks Nasal cannulae (prongs or spectacles) Nasal catheters |
Venturi-operated devices Anaesthetic breathing systems with a suitably large reservoir |
Variable performance masks (medium concentration [MC])
These masks are used to deliver oxygen-enriched air to the patient ( Fig. 6.5 ). They are also called low-flow delivery devices. They are widely used in the hospital because of greater patient comfort, low cost, simplicity and the ability to manipulate the FiO 2 without changing the appliance. Their performance varies between patients and from breath to breath within the same patient. These systems have a limited reservoir capacity, so in order to function appropriately, the patient must inhale some ambient air to meet the inspiratory demands. The FiO 2 is determined by the oxygen flow rate, the size of the oxygen reservoir and the respiratory pattern ( Table 6.2 ).
Table 6.2
Factors that affect the delivered FiO 2 in the variable performance masks
| High FiO 2 delivered | Low FiO 2 delivered |
|---|---|
| Low peak inspiratory flow rate | High peak inspiratory flow rate |
| Slow respiratory rate | Fast respiratory rate |
| High fresh oxygen flow rate | Low fresh oxygen flow rate |
| Tightly fitting face mask | Less tightly fitting face mask |
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