Acute Oxygen Treatment (page 2)

Issue 12 (2000) Article 3: Page 2 of 3

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Acute Oxygen Treatment (Continued)

Oxygen delivery systems

Oxygen can be delivered to the patient using different devices. There are two main types of devices; fixed and variable performance masks.

Fixed performance masks ensure that the patient receives a constant inspired oxygen concentration (FiO2) despite of any changes in minute ventilation. These include:

Closed or semi-closed anaesthetic breathing systems with a reservoir bag, attached to anaesthetic machine with pressurised gas supply.
Head boxes for neonates - oxygen is piped into the box at a constant inspired oxygen concentration. Sufficient gas flow is needed to flush CO2 out.
HAFOE High Air Flow Oxygen Enrichment Devices e.g Ventimask

HAFOE masks (figure 2) are colour coded and each mask states the flow of oxygen in litres per minute required to achieve a specific inspired oxygen concentration. There are holes which allow entrainment of room air by the Venturi principle. Relatively high flows of oxygen are needed: e.g. 8 l/min to ensure an inspired oxygen concentration of 40% and 15 l/min to ensure an inspired oxygen concentration of 60%. The flows of 2, 4 and 6 l/min will provide 24, 28 and 31% oxygen respectively. The patient breathes a fixed concentration of oxygen enriched air because the gas flow is greater than the peak inspiratory flow rate of the patient. Thus there is minimal dilution from atmospheric air. The high gas flow flushes expired gas from the mask preventing rebreathing.

Teaching Point

HAFOE masks use the Bernoulli effect to draw in or entrain a second gas via a side arm. This is the Venturi principle. Gas flowing through a tube is passed through a constriction or narrowing formed in the tube. The gas increases speed to pass through the narrowing, and therefore gains kinetic energy because of the increased velocity. The total energy of the system must remain the same, thus there has to be a fall in potential energy. The potential energy of a gas is the pressure it exerts. Therefore, if there is a fall in potential energy there will be a fall in pressure at that point. A second gas can be sucked in or entrained through a side arm into this area of low pressure (figure 3).

Variable performance masks/devices. The second type of oxygen delivery system includes those which deliver a variable concentration of oxygen. The oxygen concentration delivered depends on patient minute ventilation, peak inspiratory flow rate and oxygen flow rate. For example, when a patient is breathing with a low minute ventilation and is given a high oxygen flow, oxygen concentration will be relatively high. If the patient breathes more without an increase in oxygen flow, there will be a fall in inspired oxygen concentration. Using these masks the oxygen concentration is not fixed or accurate, but in most situations a flow rate of 2l/min provides 25-30% O2 and 4 l/min provides 30-40% O2. Examples of these devices include:

Nasal cannula. These do not increase dead space. Inspiratory oxygen concentration depends on the flow rate. No rebreathing occurs.
Nasal catheters, 8FG, can be inserted into the nose as far as the pharynx, so that they can just be observed behind the soft palate. A gas flow of 150ml/kg/min gives an inspired oxygen concentration of 50% in children less than 2 years. No rebreathing occurs.
The same concept can be used in adults and the cannula may be fashioned from any soft tipped fine catheter (a fine nasogatric tube or urinary catheter may be used in emergencies).
When using nasal cathers they must be taped securely in place so that they cannot migrate down into the oesophagus.
Plastic oxygen masks (figure 4) have a small dead space. The effect of the dead space depends on the patient's minute ventilation and oxygen flow. There is usually a small amount of rebreathing.

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