6 Oxygen supplementation
Oxygenation may be impaired via a number of mechanisms (ventilation–perfusion mismatch, hypoventilation, diffusion impairment, intrapulmonary or cardiovascular shunt); more than one mechanism can, and often does, occur in the same patient at the same time. Oxygenation may be compromised in a large variety of disorders affecting the respiratory, cardiovascular and neuromuscular systems in particular. Oxygen supplementation is extremely unlikely to cause any harm (unless high concentrations are provided for a sustained period of at least 24 hours), and the author would strongly encourage a liberal approach to its use (bearing potential financial constraints in mind).
In essence, a positive response to oxygen supplementation confirms its requirement (i.e. trial therapy). However, animals in need of oxygen supplementation may be identified and monitored more objectively through the use of pulse oximetry (or preferably arterial blood gas analysis if available). Pulse oximetry measures saturation of arterial haemoglobin with oxygen (SpO2) which should be more than 95% in normal dogs and cats on room air.
A number of oxygen supplementation techniques have been widely described that differ in terms of their cost, technical ease and efficiency. The method used is determined by both patient-related (level and duration of supplementation required, compliance, size) and nonpatient-related (available facilities, clinical expertise, financial constraints) factors. Methods of supplementation include:
Flow-by oxygen supplementation is extremely easy to provide and is typically the first technique used in emergency patients while major body system examination is performed, an intravenous catheter is placed and other urgent interventions carried out. However, it is noteworthy that some animals, cats in particular, do better if placed in an oxygen cage first instead (depends on underlying disorder). Flow-by oxygen supplementation is usually only used short-term as it is wasteful. Some animals are intolerant of flow-by oxygen supplementation, even with minimal restraint, and it is clearly important not to stress these patients further.
Oxygen supplementation using a mask is another readily available technique that allows access to the patient. This technique is typically only used short-term and many of the same comments apply as for flow-by supplementation. In particular, some animals are very intolerant of having a mask placed over their muzzle and this should never be forced upon them. In addition, it must be remembered that panting is the major means of cooling in dogs. Placing a mask over a panting dog’s mouth can result in a marked increase in humidity and lead to severe hyperthermia with potentially disastrous consequences. Although it reduces the inhaled oxygen concentration that can be achieved, masks may therefore need to be only loosely applied in some cases.