Traditionally, nasal cannulae (also known as nasal prongs or specs) deliver unhumidified oxygen to the nasopharynx (Fig. 7.2). They are more comfortable and less claustrophobic than facemasks, allowing talking, eating and drinking, and making them the most suitable means of delivering long-term oxygen therapy for chronic conditions, especially at home. The main drawback is that an unhumidified oxygen flow greater than 2 L min⁻¹ can cause discomfort and drying of the nasal mucosa. Thus unhumidified oxygen therapy in association with the tiny capacity of the nasal cannulae leads to variable F_IO₂. Clinical and laboratory studies in this context have shown an enormous variability with respiratory pattern, F_IO₂ ranging between 0.26 and 0.90.^2,³ A further design is the nasal sponge (tipped) catheter, which is lodged in one nostril and again effectively uses the nasopharynx as a small reservoir during the respiratory pause (Fig. 7.3). Devices permitting humidified oxygen therapy via nasal cannulae are now available (Fig. 7.4), allowing oxygen flow rates up to 60 L min⁻¹ to be tolerated for long periods. As a result, consistently higher, more reliable F_IO₂ can be achieved with this form of high flow humidified oxygen therapy.⁴ However, warm humidified gas is a good medium for bacterial growth; therefore, it is mandatory that any disinfection and infection control procedures deemed necessary by manufacturers and regulatory authorities are adhered to. The other point to note is that delivering high flows into the nasopharynx generates a small amount of positive end expiratory pressure (PEEP) depending on the flow used and if the mouth is open or closed. Pharyngeal pressures of up to 4 cm H₂O have been recorded with a flow of 60 L min⁻¹ with the mouth open, increasing to nearly 10 cm H₂O with the mouth closed.⁵ As such, high flow humidified oxygen therapy delivered via nasal cannulae should be regarded as a medium dependency device (see below). Its role in the management of acutely breathless patients is becoming increasingly appreciated.

Figure 7.2 Nasal cannulae.

Figure 7.3 Nasal sponge tipped catheter.

Figure 7.4 Nasal cannulae for supplying humidified oxygen.

With permission of Fisher & Paykel Healthcare.

Low capacity oxygen delivery devices (capacity <100 ml)

Facemasks for children, or tracheostomy masks (Fig. 7.5) have a volume of approximately 70–100 ml. Tracheostomy masks, especially, do not provide humidification without extra equipment, and so are best suited for short-term use. In long-term use, oxygen may be humidified by using a ‘Swedish nose’ device (Fig. 7.6) (see also Chapter 11, Humidifiers). This is a small, light heat and moisture exchanger that connects to the tracheostomy tube by means of a 15 mm International Organization for Standards (ISO) connector. Supplemental oxygen can be administered into this via a clip-on device. A ‘Swedish nose’ may also provide 2.5–5 cm H₂O of PEEP.

Figure 7.5 Tracheostomy mask.

Figure 7.6 A ‘Swedish nose’ device for supplying supplemental oxygen on a long-term basis via a tracheostomy tube.

Medium capacity oxygen delivery devices (capacity 100–250 ml)

Standard adult facemasks are designed to cover the nose and mouth and have a capacity of approximately 175–200 ml (Fig. 7.1); there are a multitude of different designs. They are, almost without exception, made of transparent soft plastic with an elastic strap to secure the mask in place. Some incorporate a short deformable metal bar allowing the mask to be shaped around the nose to achieve a closer fit. Oxygen flow rates of 2–15 l min⁻¹ result in a highly variable and unpredictable F_IO₂. Rebreathing of CO₂ can occur with O₂ flow rates of less than 2 L O₂ min⁻¹ or if minute ventilation is very high.⁶ A more recent design (Intersurgical, UK) fits filters in the expiratory ports, in an attempt to reduce infection risk from aerosolized pathogens emitted from the patient’s airway.

High capacity oxygen delivery devices (capacity 250–2500 ml)

Air is only entrained into a facemask when the patient’s inspiratory flow rate exceeds the oxygen supply rate. To achieve a F_IO₂ near 1.0 (100% oxygen) by providing sufficient oxygen flow alone, rates in excess of 60 l min⁻¹ would be required (in order to exceed the patient’s peak inspiratory flow rate) which is clearly impractical. Increasing mask size simply increases the risk of rebreathing CO₂. However, a great deal of oxygen delivered to the facemask is lost to the atmosphere whenever the patient’s inspiratory flow rate is (a) less than the oxygen supply rate and (b) during exhalation. High-capacity masks are designed to store some of this wasted oxygen in a reservoir bag (usually of >1 litre volume) attached to the facemask, thus providing a higher F_IO₂ (Fig. 7.7). With these devices, oxygen flows directly into the reservoir bag, which fills during exhalation and whenever the patient’s inspiratory flow rate is less than the flow rate of delivered oxygen. The patient inhales oxygen preferentially from the reservoir bag: provided the mask makes a reasonable seal to the face. Expiratory flap valves are incorporated on the vents or in piping below the mask to prevent air entrainment from these sites. However, ambient air will still be entrained if there are leaks between the mask and the face.⁷ Some manufacturers include a one-way valve between mask and reservoir bag, reducing rebreathing of CO₂. However, higher oxygen supply rates are required as oxygen in the exhaled dead space of the mask, pharynx and upper trachea is prevented from entering the reservoir bag and is wasted. A valveless device is, therefore, most suitable when oxygen supplies are limited, but a valve is desirable if rebreathing of CO₂ would be detrimental, for example after head injury.

Figure 7.7 High-capacity oxygen delivery device: Facemask with reservoir bag. Note expiratory flap valve visible on body of mask.

One example of a high-capacity mask (Fig. 7.8) uses a sequential gas delivery system incorporated in a small circuit below the mask with a one-way valve to prevent re-breathing, an expiratory valve and a third valve to allow entrainment of room air should the reservoir bag become exhausted. This design is reputed to allow F_IO₂ close to 1.0 to be given, although again attention to the seal between the mask and the face and the oxygen flow rate is required to achieve this.⁸ A more recent high capacity mask (Intersurgical, UK) again incorporates a one-way valve from the reservoir bag but designed such that it also allows entrainment of room air if the bag is emptied (Fig. 7.9). This and the expiratory one way valve are neatly arranged within the front of the mask itself. Additionally this device is made of non-PVC materials, in an attempt to reduce the impact on the environment during disposal.

Figure 7.8 Hi-Ox Mask, Viasys Healthcare. The blue tube is an additional nebulisation port.

Figure 7.9 A. High-capacity mask (Intersurgical, UK). B. Position of valves in exhalation. C. In inhalation with oxygen flowing. D. In inhalation in the event of oxygen supply failure.

Generally, it is accepted that facemasks with reservoir bags probably deliver an F_IO₂ between 0.6 and 0.8;^9,¹⁰ however, in reality there is likely to be considerable variation around these values due to leak between the mask and face combined with variable respiratory patterns. Best results will be achieved by using an O₂ flow rate adequate for the patient’s needs such that the reservoir bag empties by no more than a third during inspiration and by achieving the best seal possible between the mask and the face. Some masks incorporate additional features, for example a small chimney containing a red polystyrene ball that moves up and down with respiration so that a patient’s respiratory effort can be visualized and respiratory rate can easily be measured (Fig. 7.10).¹¹