A nasal cannula (Figure 12.4) consists of two plastic prongs that are inserted inside the anterior nares and supported on a light frame. A nasal cannula can be used when the patient requires a low concentration of oxygen (between 24% and 35%) with flow rates of 1–4 L/min. While Table 12.2 shows oxygen concentrations in relation to flow when oxygen is delivered via a nasal cannula, the actual uptake of oxygen and its subsequent effect on blood oxygen and carbon dioxide levels cannot be accurately predicted due to the variation in a patient's rate and pattern of breathing (O'Driscoll et al. [209]). Medium concentrations of oxygen up to 44% (6 L/min) can be used but are often not well tolerated due to nasal irritation of the mucous membranes.

As an alternative to a mask, the nasal cannula may seem less claustrophobic; is generally more comfortable; and does not interfere with eating, drinking or communication. There is also no risk of rebreathing carbon dioxide (Brill and Wedzicha [31]).

Simple face‐masks (Figure 12.5) are medium‐concentration masks that entrain the air from the atmosphere, delivering a variable oxygen concentration (anything from 40% to 60%; Table 12.3). These masks are useful for patients with type 1 respiratory failure who need a higher percentage of oxygen temporarily while the cause of their hypoxia is treated.

As with the nasal cannula, the actual oxygen concentration delivered is not accurate as it differs depending on the set flow rate and the patient's rate and depth of breathing (O'Driscoll et al. [209]). Using less than 5 L/min is not recommenced due to the possible build‐up and rebreathing of carbon dioxide, caused by the low flow and resistance to breathing against the mask (Brill and Wedzicha [31], Herren et al. [106]). If the patient requires more than 60% oxygen (10 L/min), expert help should be sought as the patient may require more invasive respiratory support.

Reservoir masks (Figure 12.6) are similar to simple face‐masks with the addition of a reservoir bag. They allow oxygen to be delivered at concentrations between 60% and 90% when used at a flow rate of 10–15 L/min. Oxygen flows into the bag (the bag should be inflated with oxygen prior to use) and mask during inhalation while, on exhalation, air is diverted out of the mask's side valves. A separate one‐way valve prevents expired air from flowing back into the reservoir bag and the rebreathing of carbon dioxide.

As with the nasal cannula and simple face‐mask, the actual concentration of oxygen delivered varies depending on the patient's breathing rate and pattern, as well as the mask fit (O'Driscoll et al. [209]).

Note that if the oxygen flow is less than 10 L/min, carbon dioxide can accumulate in the reservoir bag, resulting in an increase in carbon dioxide inhalation (Herren et al. [106]) and a failure to meet the patient's oxygen requirements. This device is usually used during an emergency situation and in the presence of expert nursing and medical support. It may also be used as a short‐term measure before more invasive respiratory support is instituted.

The Venturi mask (composed of a simple face‐mask and a Venturi adaptor) delivers high‐flow oxygen when the oxygen flow rate is set above the minimum rate printed on the side of the attachment (Figure 12.7). The adaptors are colour coded according to the percentage of oxygen they deliver and are available in the following concentrations: 24%, 28%, 35%, 40% and 60%. The Venturi effect ensures an accurate concentration of oxygen is delivered regardless of the proportion of air drawn into the attachment and the flow of oxygen delivered (providing it is above the minimum stated). Unlike in the devices mentioned previously, the oxygen concentration delivered is not affected by the patient's rate and depth of breathing. Because of the accuracy in oxygen delivery offered by these masks, the 24% and 28% Venturi masks are suited to patients at risk of hypercapnic respiratory failure (Brill and Wedzicha [31], O'Driscoll et al. [209]).

Venturi masks are also suitable for patients with an increased respiratory rate (more than 30 breaths per minute) who require an increased inspiratory flow. It is suggested that for such patients the flow rate is increased by 50% from the minimum printed on the side of the attachment to help overcome this demand (O'Driscoll et al. [209]). For example, increasing the oxygen flow from 2 to 4 L/min on a 24% Venturi mask doubles the total inspiratory flow from 51 L/min to 102 L/min. As shown in Table 12.4, when a high oxygen flow is used with a higher oxygen concentration adaptor, the total flow decreases dramatically, which may not be suitable for a patient in extremis. In this scenario, a different type of oxygen delivery (such as high‐flow oxygen via a nasal cannula, non‐invasive ventilation, or use of continuous positive airway pressure) may be preferred.

Tracheostomy masks (Figure 12.8) perform in a similar way to the simple face‐mask. The mask is placed over the tracheostomy tube or laryngectomy stoma. Oxygen needs to be humidified to prevent drying of airways and secretions since the patient's natural mechanisms of humidification have been bypassed (NTSP [206]). For more information, see the section below on humidification.

A summary of the oxygen devices discussed and their advantages and disadvantages can be found in Table 12.5.