The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [363]
Figure 10.3 Nasal cannulas.
Table 10.1 Oxygen flow rates for nasal cannulas
Oxygen flow rate (L/min) % Oxygen delivered
1 24
2 28
3 32
4 36
5 40
6 44
Simple semi-rigid plastic masks
Simple semi-rigid plastic masks (Figure 10.4) are low-flow masks which entrain the air from the atmosphere and therefore are able to deliver a variable oxygen percentage (anything from 21% to 60%) (Table 10.2). Large discrepancies between the delivered FiO2 and the actual amount received by the patient will occur, dependent on the patient’s rate and depth of breathing. These masks are useful for patients who need a higher percentage of oxygen temporarily whilst the cause for their hypoxia is treated. This type of mask may be worn for hours or several days, but they should be used in conjunction with a humidifier if used for more than 12 hours. If the patient requires 60% added oxygen or more, this is the threshold for requiring more invasive respiratory support and expert help should be sought (NICE 2007).
Figure 10.4 Simple semi-rigid plastic mask.
Table 10.2 Approximate oxygen concentration related to flow rates of semi-rigid masks
Oxygen flow rate (L/min) % Oxygen delivered
2 24
4 35
6 50
8 55
10 60
12 65
15 70
Partial rebreathing masks
These are similar to the simple semi-rigid plastic masks with the addition of a reservoir bag, which allows the oxygen delivered to increase beyond 60%. During inspiration, the patient draws air and oxygen from the mask, bag and through the holes in the side of the mask. When the patient expires, the initial one-third of the expired gases will flow back into the reservoir bag. The expired gas is rich in oxygen and contains very little carbon dioxide. The patient is able to breathe the previously expired gas along with the oxygen from the source (Ball 2000).
Note: if the oxygen flow is too low, the carbon dioxide can accumulate in the reservoir bag and fail to meet the patient’s requirements, resulting in an increase in carbon dioxide (Pierce 1995). This device should only be used in the presence of expert nursing and medical support and usually during emergency intervention or before more invasive ventilatory therapy is instituted.
Non-rebreathing mask
The semi-rigid mask (Figure 10.5) has the addition of a reservoir bag with a one-way valve between the reservoir bag and mask, preventing accumulation of expired gases in the reservoir bag and retention of carbon dioxide (Pierce 1995). Oxygen delivery of greater than 80% can be achieved (Ball 2000).
Figure 10.5 Non-rebreathing mask.
Fixed performance masks or high-flow masks (Venturi-type masks)
With fixed performance masks (Figure 10.6) it is possible to achieve an unvarying mixture of gases and a known concentration of oxygen using the high air flow oxygen enrichment principle (Table 10.3). These masks derive their name from the Venturi barrel in which a relatively low flow rate of oxygen is forced through a narrow jet. There are side holes in the barrel and this jet allows the air to be drawn in at a high rate. As the mixture of gas created is at a flow rate above that of inspiration, the mixture will be constant (Foss 1990). There are many Venturi-type masks available, but the larger-capacity masks are the most accurate and therefore the safest when a known concentration of oxygen is required or when efficient elimination of carbon dioxide is essential, for example, to provide respiratory therapy for the patient with chronic respiratory disease (Fell and Boehm 1998).
Figure 10.6 High-flow mask with Venturi barrel.
Table 10.3 Fixed performance mask oxygen flow rates
Oxygen flow rate (L/min) % Oxygen delivered
2 24
6 31
8 35
10 40
15 60
Tracheostomy mask
Tracheostomy