Masks and oxygen delivery devices

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° bend with a 22-mm end to fit into a catheter mount or a breathing system.

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.

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.

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.

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.

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.

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.

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:

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.

8. Some designs have an extra port attached to the body of the mask allowing it to be connected to a side-stream CO₂ monitor (Fig. 6.7). This allows it to sample the exhaled CO₂ so monitoring the patient’s respiration during sedation.

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.

1. Two prongs which protrude about 1 cm into the nose.

2. These are held in place by an adjustable head strap.

1. There is entrainment of ambient air through the nostrils. The nasopharynx acts as a reservoir.

2. The FiO₂ achieved is proportional to:

3. Mouth breathing causes inspiratory air flow. This produces a Venturi effect in the posterior pharynx entraining oxygen from the nose.

4. There is increased patient compliance with nasal cannulae compared to facial oxygen masks. The patient is able to speak, eat and drink.

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

2. The proximal end of the mask consists of a Venturi device. The Venturi devices are colour-coded and marked with the recommended oxygen flow rate to provide the desired oxygen concentration (Figs 6.12 and 6.13).

3. Alternatively, a calibrated variable Venturi device can be used to deliver the desired FiO₂.

1. The Venturi mask uses the Bernoulli principle, described in 1778, in delivering a predetermined and fixed concentration of oxygen to the patient. The size of the constriction determines the final concentration of oxygen for a given gas flow. This is achieved in spite of the patient’s respiratory pattern by providing a higher gas flow than the peak inspiratory flow rate.

2. As the flow of oxygen passes through the constriction, a negative pressure is created. This causes the ambient air to be entrained and mixed with the oxygen flow (Fig. 6.14). The FiO₂ is dependent on the degree of air entrainment. Less entrainment ensures a higher FiO₂ is delivered. This can be achieved by using smaller entrainment apertures or bigger ‘windows’ to entrain ambient air. The smaller the orifice is, the greater the negative pressure generated, so the more ambient air entrained, the lower the FiO₂. The oxygen concentration can be 0.24, 0.28, 0.31, 0.35, 0.4 or 0.6.

3. The Bernoulli effect can be written as:

    where κ is the density, v is the velocity, P is the pressure.

4. The total energy during a fluid (gas or liquid) flow consists of the sum of kinetic and potential energy. The kinetic energy is related to the velocity of the flow whereas the potential energy is related to the pressure. As the flow of fresh oxygen passes through the constricted orifice into the larger chamber, the velocity of the gas increases distal to the orifice causing the kinetic energy to increase. As the total energy is constant, there is a decrease in the potential energy so a negative pressure is created. This causes the ambient air to be entrained and mixed with the oxygen flow. The FiO₂ is dependent on the degree of air entrainment. Less entrainment ensures higher FiO₂ is delivered and smaller entrainment apertures are one method of achieving this (Fig. 6.8). The devices must be driven by the correct oxygen flow rate, calibrated for the aperture size if a predictable FiO₂ is to be achieved.

4. Because of the high fresh gas flow rate, the exhaled gases are rapidly flushed from the mask, via its holes. Therefore there is no rebreathing and no increase in dead space.

5. These masks are recommended when a fixed oxygen concentration is desired in patients whose ventilation is dependent on the hypoxic drive.

6. For example, a 24% oxygen Venturi mask has an air : oxygen entrainment ratio of 25 : 1. This means an oxygen flow of 2 L/min delivers a total flow of 50 L/min, well above the peak inspiratory flow rate.

7. The mask’s side holes are used to vent the exhaled gases only (as above) in comparison to the side holes in the variable performance mask where the side holes are used to entrain inspired air in addition to expel exhaled gases.

8. The Venturi face masks are designed for both adult and paediatric use (Fig. 6.14).

9. The Venturi attachments, with a reservoir tubing, can be attached to a tracheal tube or a supraglottic airway device as part of a T-piece breathing system (Fig. 6.15). This arrangement is usually used in recovery wards to deliver oxygen-enriched air to patents.

1. These masks are recommended when a fixed oxygen concentration is desired in patients whose ventilation is dependent on their hypoxic drive, such as those with chronic obstructive pulmonary disease. However, caution should be exercised as it has been shown that the average FiO₂ delivered in such masks is up to 5% above the expected value.

2. The Venturi mask with its Venturi device and the oxygen delivery tubing is often not well tolerated by patients because it is noisy and bulky.

a) Gas flow produced should be more than 20 L/min.

b) Reducing the flow of oxygen from 12 to 8 L/min results in a reduction in oxygen concentration.

c) With a constant oxygen supply flow, widening the orifice in the Venturi device increases the oxygen concentration delivered to the patient.

d) There is rebreathing in the mask.

e) The mask is a fixed performance device.

a) Use the Venturi principle to deliver a fixed O₂ concentration to the patient.

b) The size of the constriction of the Venturi has no effect on the final O₂ concentration delivered to the patient.

c) The holes on the side of the mask are used to entrain ambient air.

d) The gas flow delivered to the patient is more than the peak inspiratory flow rate.

e) There is significant rebreathing.

a) The rubber mask is covered by carbon particles which act as an anti-static measure.

b) Masks have no effect on the apparatus dead space.

c) The mask’s cuff has to be checked and inflated before use.

d) The dental nasal masks are also known as nasal inhalers.

e) Masks have a 15-mm end to fit the catheter mount.

a) Is a fixed performance device.

b) Is a variable performance device.

c) There is a Venturi effect in the posterior pharynx.

d) An oxygen flow of 8 L/min is usually used in an adult.

e) Has increased patient compliance.

a) During slow and deep breathing, a higher FiO₂ can be achieved.

b) Ambient air is not entrained into the mask.

c) Alveolar gas rebreathing is not possible.

d) Normal inspiratory peak flow rate is 20–30 L/min for an adult.

e) Can be used safely on all patients.

a) They can offer greater patient compliance.

b) They can deliver an FiO₂ that can vary from breath to breath in the same patient.

c) The size of the medium concentration oxygen face mask has no effect on rebreathing and CO₂ elimination.

d) Capnography can be used to measure the FiO₂ delivered to the patient.

e) With a variable performance mask, the FiO₂ is higher when the face mask is a tight fit.

a) Anaesthetic breathing systems with reservoirs are fixed performance devices.

b) Distal to the constriction of a Venturi, there an increase in potential energy.

c) In a Venturi mask, the higher the entrainment ratio, the higher the FiO₂.

d) A nasal oxygen catheter is a fixed performance device.

e) Venturi masks are very well tolerated by patients.

8. Catheter mounts:

a) True. The Venturi mask is a fixed performance device. In order to achieve this, the flow delivered to the patient should be more than the peak inspiratory flow rate. A flow of more than 20 L/min is adequate.

b) True. It is the flow rate of oxygen through the orifice that determines the final FiO₂ the patient receives. With a constant orifice, the amount of air entrained remains constant. So by reducing the oxygen flow rate from 12 to 8 L/min, there will be less oxygen in the final mixture.

c) True. The wider the orifice, the less the drop in pressure across the orifice and the less the entrainment of the ambient air, hence the less the dilution of the O₂, resulting in an increase in oxygen concentration delivered to the patient. The opposite is also correct.

d) False. There is no rebreathing in the mask because of the high fresh gas flow rates causing the exhaled gases to be flushed from the mask through the side holes.

e) True. The Venturi mask is a fixed performance device that delivers a constant concentration of oxygen in spite of the patient’s respiratory pattern, by providing a higher gas flow than the peak inspiratory flow rate.

a) True. Laminar flow through a constriction causes a decrease in pressure at the constriction. This leads to entrainment of ambient air leading to mixture of fixed oxygen concentration.

b) False. It is the size of the orifice that determines the degree of decrease in pressure at the constriction. This determines the amount of ambient air being entrained, hence the final concentration of oxygen.

c) False. In such a mask, the holes are used to expel the exhaled gases. There is no entrainment of ambient air in such a mask because of the high gas flows. The holes in a variable performance mask are used to entrain ambient air.

d) True. The gas flow generated is higher than the peak inspiratory flow rate. This allows the delivery of a fixed oxygen concentration to the patient regardless of the inspiratory flow rate. It also prevents rebreathing.

e) False. There is no rebreathing because of the high flows delivered to the patient. The exhaled gases are expelled through the holes in the mask.

a) True. Carbon particles prevent the build up of static electricity. The rubber face masks and the rubber tubings used in anaesthesia are covered with carbon. With modern anaesthetic practice where no flammable drugs are used, its significance has all but disappeared.

b) False. Face masks can have a significant effect on the apparatus dead space if the wrong size is chosen. In an adult, the dead space can increase by about 200 mL. It is of more importance in paediatric practice.

c) True. The cuff of the face mask is designed to ensure a snug fit over the patient’s face and also to minimize the mask’s pressure on the face. Ensuring that the cuff is inflated before use is therefore important.

d) True. Nasal inhalers are nasal masks used during dental anaesthesia allowing good surgical access to the mouth. The Goldman nasal inhaler is an example.

e) False. The face masks have a 22-mm end to fit the angle piece or catheter mount.

a) False. It is not a fixed performance device. The final FiO₂ depends on the flow rate of oxygen, tidal volume, inspiratory flow, respiratory rate and the volume of the nasopharynx.

b) True. See above.

c) True. During mouth breathing, the inspiratory air flow produces a Venturi effect in the posterior pharynx entraining oxygen from the nose.

d) False. It is uncomfortable for the patient to have higher flows than 1–4 L/min.

e) True. Patients tolerate the nasal cannula for much longer periods than a face mask. Patients are capable of eating, drinking and speaking despite the cannula.

a) True. This allows fresh gas flow (FGF) during the expiratory pause to fill the mask ready for the following inspiration. In tachypnoea (fast and shallow breathing), the opposite occurs where there is not enough time for the FGF to fill the mask.

b) False. The maximum inspiratory flow rate is much higher than the FGF, so ambient air is entrained into the mask through the side holes.

c) False. During tachypnoea, there is not enough time for the FGF to fill the mask and expel the exhaled gases. This leads to rebreathing of the exhaled gases.

d) True.

e) False. As their performance is variable, the FiO₂ the patient is getting is uncertain. Patients who are dependent on their hypoxic drive require a fixed performance mask.

a) True. These devices are better tolerated by patients because they are more comfortable. They also offer simplicity, low cost and the ability to manipulate the FiO₂ without changing the appliance.

b) True. The FiO₂ delivered can vary from one breath to another in the same patient. This is because of changes in the inspiratory flow rate and respiratory pattern. These lead to changes in the amount of air entrained so altering the FiO₂.

c) False. Rebreathing is increased when the mask body is large. In addition, the high inspiratory resistance of the side holes increases the rebreathing. CO₂ elimination can be improved by increasing the fresh oxygen flow and is inversely related to the minute ventilation.

d) False. Oxygraphy can be used to measure the FiO₂ by measuring the end-tidal oxygen fraction in the nasopharynx.

e) True. The tighter the fit of the face mask, the higher the FiO₂. Low peak inspiratory flow rate, slow respiratory rate and a higher oxygen flow rate can also increase the FiO₂.

a) True. Anaesthetic breathing systems are fixed performance devices. The reservoir bag acts to deliver an FGF that is greater than the patient’s peak inspiratory flow rate.

b) False. The potential energy is related to the pressure, whereas the kinetic energy is related to the velocity of the flow. As the flow of fresh oxygen supply passes through the constricted orifice into the larger chamber, the velocity of the gas increases distal to the orifice causing the kinetic energy to increase. As the total energy is constant, there is a decrease in the potential energy so a negative pressure is created. This causes the ambient air to be entrained and mixed with the oxygen flow.

c) False. The higher the entrainment ratio, the lower the FiO₂ delivered. This is because of the ‘dilution’ of the 100% oxygen fresh flow by ambient air.

d) False. Nasal catheters are variable performance devices.

e) False. Venturi masks are not very well tolerated by patients because of the noise and bulkiness of the masks and the attachments.