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 ).
Fig. 6.1
A range of sizes of transparent face masks with air-filled cuffs (Smiths Medical, Ashford, Kent, UK).
Fig. 6.2
Paediatric face masks. Ambu design (left) and Rendell–Baker design (right).
- 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.
- •
Made of silicone rubber or plastic.
- •
Their design ensures a snug fit over the face of the patient.
- •
Cause an increase in dead space (up to 200 mL in adults).
- •
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.
Fig. 6.3
The Goldman nasal inhaler.
- 3.
Other designs have an inlet for delivering the inspired fresh gas flow 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.
- •
Acts as an adapter between the tracheal tube and breathing system in addition to stabilizing the tracheal tube.
- •
Can be made of rubber or plastic with different lengths.
- •
Some have a condenser humidifier built in.
- •
Its length contributes to the apparatus dead space.
- •
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.
| 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 ).
| 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 |
Components
- 1.
The plastic body of the mask has side holes on both sides.
- 2.
A port is connected to an oxygen supply.
- 3.
Elastic band(s) fix the mask to the patienťs face.
Mechanism of action
- 1.
Ambient air is entrained through the holes on both sides of the mask. The holes also allow exhaled gases to be vented out.
- 2.
During the expiratory pause, the fresh oxygen supplied helps in venting the exhaled gases through the side holes. The body of the mask (acting as a reservoir) is filled with a fresh oxygen supply and is available for the start of the next inspiration.
- 3.
The final concentration of inspired oxygen depends on:
- a.
the oxygen supply flow rate
- b.
the pattern of ventilation: If there is a pause between expiration and inspiration, the mask fills with oxygen and a high concentration is available at the start of inspiration
- c.
the patienťs inspiratory flow rate: During inspiration, oxygen is diluted by the air drawn in through the holes when the inspiratory flow rate exceeds the flow of oxygen supply. During normal tidal ventilation, the peak inspiratory flow rate is 20–30 L/min, which is higher than the oxygen supplied to the patient and the oxygen that is contained in the body of the mask, so some ambient air is inhaled to meet the demands thus diluting the fresh oxygen supply. The peak inspiratory flow rate increases further during deep inspiration and during hyperventilation
- d.
how tight the mask fits on the face.
- a.
- 4.
If there is no expiratory pause, alveolar gases may be rebreathed from the mask at the start of inspiration.
- 5.
The rebreathing of CO 2 from the body of the mask (apparatus dead space of about 100 mL) is usually of little clinical significance in adults but may be a problem in some patients who are not able to compensate by increasing their alveolar ventilation. CO 2 elimination can be improved by increasing the fresh oxygen flow and is inversely related to the minute ventilation. The rebreathing is also increased when the mask body is large and when the resistance to flow from the side holes is high (when the mask is a good fit). The patients may experience a sense of warmth and humidity, indicating significant rebreathing.
- 6.
A typical example of 4 L/min of oxygen flow delivers an FiO 2 of about 0.35–0.4 providing there is a normal respiratory pattern.
- 7.
Adding a 600–800 mL bag to the mask will act as an extra reservoir ( Fig. 6.6 ). Such masks are known as ‘partial rebreathing masks’ or ‘non-rebreather masks’. A one-way valve is fitted between mask and reservoir to prevent rebreathing to ensure a 100% O 2 in the reservoir. The inspired oxygen is derived from the continuous fresh oxygen supply, oxygen present in the reservoir and the entrained ambient air. Higher variable FiO 2 (0.6–0.8) can be achieved with such masks.
Fig. 6.6
A variable performance mask with a reservoir bag.
- 8.
Some designs have an extra port attached to the body of the mask allowing it to be connected to a side-stream CO 2 monitor ( Fig. 6.7 ). This allows it to sample the exhaled CO 2 so monitoring the patienťs respiration during sedation.
Fig. 6.7
A variable performance mask with an end-tidal CO 2 monitoring port.
- 9.
Similar masks can be used in patients with tracheostomy ( Fig. 6.8 ). As with the face mask, similar factors will affect its performance. Care must be taken to humidify the inspired dry oxygen as the gases delivered bypass the nose and its humidification.
Fig. 6.8
Variable performance tracheostomy mask.
Problems in practice and safety features
These devices are used only when delivering a fixed oxygen concentration is not critical. Patients whose ventilation is dependent on a hypoxic drive must not receive oxygen from a variable performance mask.
- •
Entrains ambient air.
- •
The inspired oxygen concentration depends on the oxygen flow rate, pattern and rate of ventilation, maximum inspiratory flow rate and how well the mask fits the patienťs face.
- •
Adding a reservoir with a one-way valve can significantly increase FiO 2 delivered.
Nasal cannulae
Nasal cannulae are ideal for patients on long-term oxygen therapy ( Fig. 6.9 ). A flow rate of 2–4 L/min delivers an FiO 2 of 0.28–0.36 respectively. Higher flow rates are uncomfortable.
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