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.

Fig. 6.4 Catheter mount.

Components

1. 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

• 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 (FiO₂). The delivery devices can be divided into variable and fixed performance devices. The former devices deliver a fluctuating FiO₂ whereas the latter devices deliver a more constant and predictable FiO₂ (Table 6.1). The FiO₂ delivered to the patient is dependent on device- and patient-related factors. The FiO₂ 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 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 FiO₂ 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 FiO₂ 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 FiO₂ in the variable performance masks

High FiO₂ delivered	Low FiO₂ 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

Fig. 6.5 (A) Adult variable performance face mask. (B) Paediatric variable performance face mask.

Components

1. The plastic body of the mask with side holes on both sides.

2. A port connected to an oxygen supply.

3. Elastic band(s) to fix the mask to the patient’s 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 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 patient’s 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’s fit is on the face.

4. If there is no expiratory pause, alveolar gases may be rebreathed from the mask at the start of inspiration.

5. The rebreathing of carbon dioxide 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. Carbon dioxide 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 FiO₂ 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’. The inspired oxygen is derived from the continuous fresh oxygen supply, oxygen present in the reservoir (a mixture of the fresh oxygen and exhaled oxygen) and ambient air. Higher variable FiO₂ can be achieved with such masks. A one-way valve is fitted between mask and reservoir to prevent rebreathing.