Oxygen Supplementation Techniques
Room air has an inspired oxygen concentration of approximately 21% at sea level. Oxygen supplementation aims to increase this percentage. Several techniques exist, each of which have their own advantages and disadvantages (see Table 8.1). Which technique is selected depends on patient size, conformation, temperament, likely duration and available equipment.
|Oxygen supplementation technique||Suggested O2 flow rate||Approximate % oxygen obtained in inspired air*|
|Oxygen cage||10–12 l/min||40–50|
|Nasal tube||50–100 ml/kg/min||30–50|
|Endotracheal tube||Dependent on anaesthetic circuit used||100|
* Normal room air is approximately 21% oxygen at sea level.
A variety of oxygen sources can be used, and their selection will vary between individual practices, dependent on the facilities available. Most commonly, the practice will select from the following:
- Direct from an oxygen cylinder using oxygen tubing
- Direct from a piped oxygen source
- Using a breathing system attached to an anaesthetic machine.
For long-term oxygen therapy it becomes necessary to humidify the inhaled gas to prevent irritation and dessication of mucous membranes. In practice, it is recommended that any oxygen supplementation over an hour’s duration should be humidified. This is achieved by a chamber where the oxygen is passed through sterile water (see Figure 8.2) (some commercial oxygen cages have a built-in humidification system). Without humidification the dry gas causes an increase in viscosity of secretions in the respiratory tract, leading to impaired mucociliary clearance and an increase in the risk of respiratory infection. In the normal situation, inspired air is humidified in the upper airway as it passes through the nasal chambers; any supplementation technique that bypasses the upper airway (nasal catheter, transtracheal catheter) should be humidified from initiation.
Short-Term Methods of Oxygen Supplementation
When a patient in respiratory distress presents at the practice, a brief targeted examination should be performed (focusing on the respiratory, cardiovascular and neurological systems) and initial stabilisation of the patient should be carried out. During this period oxygen should generally be supplemented using non-invasive techniques, commonly via mask or flow-by techniques (see Flow-by, below). However, these techniques require a member of staff to administer them continuously in order for them to be effective, hence these are generally viewed as short-term measures and, if required, the facilities to administer oxygen for a longer period may need to be organised.
Using a mask to supplement oxygen is a straightforward and relatively effective technique, with no requirement for specialised equipment. An appropriately sized oxygen mask can be attached to any anaesthetic circuit (see Figure 8.3). Tight-fitting masks should be avoided; dyspnoeic animals will struggle and this becomes counterproductive as their oxygen demand will increase, and dangerous as they are in a fragile state. Tight-fitting masks also promote rebreathing of expired carbon dioxide and can cause hyperthermia. Suggested oxygen flow rates range from 1 l/min (for cats and small dogs) through to 10 l/min (for giant breeds). A high percentage of inspired oxygen (80–90%) can be achieved in sedated or anaesthetised healthy animals using a tightly fitting mask. In the dyspnoeic patient, using a loose-fitting mask, the actual percentage of inspired oxygen may actually be as low as 35–55%. This technique is best reserved for collapsed or weak patients who are unable to move, and are not likely to panic.
By holding the end of an anaesthetic circuit near to the nostrils or mouth of a patient, oxygen can be supplied with less stress than a mask, although the technique is inefficient; the inspired oxygen concentration is unlikely to exceed 40% (see Figure 8.4). This technique is suitable for administering oxygen to animals during examination. The equipment is readily to hand, and is less stressful for a dyspnoiec animal than having a mask placed over the muzzle. Flow rates of 2–10 l/minute are suitable, with the oxygen outlet as near to the nose or mouth as is comfortable (patients tolerate this technique if the stream of oxygen is directed perpendicular to their nostrils rather than directed up the nares).
Transtracheal Oxygen Supplementation
In patients with severe respiratory distress caused by upper airway obstruction that do not respond to flow-by oxygen administration, it is possible to place a percutaneous catheter into the trachea and administer oxygen into the respiratory tract at a level below that of the obstruction (see Figure 8.5). This technique is useful in dogs weighing over 10 kg that have a sufficiently large trachea to allow a catheter to be confidently placed (see Practical techniques at the end of the chapter).
Longer Term Oxygen Supplementation
The short-term supplementation techniques mentioned above are suitable for initial stabilisation and assessment. Following this period it is likely a dyspnoeic patient will need ongoing supplementation; ideally, this supplementation can be carried out while the animal is calming down in a cage or kennel. Suitable longer term means of oxygen supplementation include the following.
Oxygen can be supplied directly into the respiratory tract using a nasal catheter placed in one or both nostrils and then connected to an oxygen source (see Figure 8.6). Silicone feeding tubes are usually most suitable for this use as their soft pliable nature is best tolerated (5 French catheter for cats, and 8–10 French in larger dogs). A single nasal catheter can increase inspired oxygen levels to 40–50%; placing a second catheter in the other nostril may increase levels to 60–70%. These levels assume the animal is not mouth breathing or panting; if this is the case, then mixing of air in the pharynx occurs and reduces the efficiency of this technique (see Practical techniques at the end of the chapter).