Essentially the question one has to ask when considering oxygen therapy for your patient is what are the patient’s oxygen saturations? If they are low, you need to consider the fact that they may be at risk of hypercapnic respiratory failure, possibly because of some underlying disease such as COPD.
As you can see from the British Thoracic Society algorithm above, if there is a risk of hypercapnia then you should target oxygen saturations between 88 to 92%. If there is not a risk of hypercapnia then you should aim for oxygen saturations of 94 to 98%.
So if the patient’s saturations below the target range then this would be an indication for oxygen therapy.
Complications of oxygen therapy can vary from fairly simple complications such as a dry or bloody nose, skin irritation from the nasal cannula or face mask, or fatigue and morning headaches.
The complications can be more serious however. There may be adverse effects such as absorptive atelectasis, which is a collapse of parts of the lung making gas exchange more difficult. There may also be a reduction of hypoxic vasoconstriction, which the body implements when oxygen levels are low. This is a protective mechanism which ensures that where there are low levels of oxygen in the lungs the venous supply is reduced, thereby reducing any shunt effect. It is believed that it is this effect that can lead to hypercapnia in the COPD patient for example rather than the hypoxic drive as previously thought.
Where a patient is exposed for a prolonged period to high levels of oxygen there is a serious risk of oxygen toxicity occurring. This can affect the central nervous system, the pulmonary system and the eyes.
Early signs of central nervous system involvement will include twitching of the small muscles of the hand. Later signs may include vertigo, nausea, altered behaviour, clumsiness and finally convolutions.
Longer exposure to 100% oxygen can produce tissue injury within the lungs. There will be intense, carinal irritation on deep inspiration which will eventually lead to uncontrolled coughing, and finally chest pain and dyspnoea.
Within the eyes there may be a constriction of the peripheral field of vision and delayed cataract formation can occur.
Methods of oxygen delivery.
There are a number of ways of delivering oxygen to the patient, ranging from the quite simple requiring only the most basic of equipment, to the much more complicated eventually leading to the patient on the ventilator.
One of the simplest ways of delivering oxygen to the patient is via a nasal cannula. This is simply plastic tubing which has nasal prongs that sit in the vestibule of each nostril. For reasons of patient comfort and because the oxygen delivered can drive the mucosal membranes quite quickly it is not appropriate to deliver more than 3 to 4 L of oxygen via the nasal cannula. Consequently this method of delivery will only treat mild hypoxaemia. However if it is only mild hypoxaemia that is being treated then patients tend to tolerate this method of delivery well and it also enables them to eat and drink freely.
Next there are the simple semirigid plastic masks. Flow rates as high as 15 L per minute can be delivered via these masks but there is only a small reservoir this being the plastic mask itself. One needs to be aware that at low flow rates there is a risk of carbon dioxide retention with this type of mask. One can achieve a maximum oxygen concentration of between 60 and 70% depending upon the patient’s respiratory rate. As the patient breathes quicker so the concentration will fall. Also, unlike the nasal cannula, it is not possible for the patient to eat and drink without removing the mask, and sometimes speaking becomes difficult also. This sometimes means that the patient is less tolerant of this type of mask.
Another type of mask is the venturi mask. This relies on a well understood physical principle whereby oxygen passes through a small hole which entrains air to a predictable dilution. These masks come with a number of different coloured valves which can be changed according to the percentage of oxygen which needs to be delivered. These valves tend to be of a different colour with the percentage and the consequent flow rates clearly marked on them. It is possible due to the venturi effect to deliver quite high flow rates to the patient using this mask.
If the patient has become very hypoxic then it may be appropriate to use the non-re-breathe mask. This is similar to the Hudson type mask or semirigid plastic mask, with the addition of a bag underneath which forms a reservoir. Between the bag and the oxygen mask is a one-way valve which ensures that when the patient exhales the carbon dioxide will leave the mask and when they inhale the oxygen which is in the reservoir bag will be the only thing they breathe in. As a result quite high concentrations of oxygen can be delivered via this method.
The non-re-breathe mask would be the mask of choice when one is concerned about the patient’s oxygen levels. In such a circumstance one should turn the flow rates up to 15 L per minute, occlude the valve with a finger or thumb, thereby allowing the back to fill and only then placing the mask over the patients mouth and nose.
Whilst it is hard to be clear what the minimal moisture levels need to be to maintain normal ciliary function and mucus clearance it is known that the function of the cilia are impaired by many disease processes which affect the lungs. High inspired oxygen concentrations can also cause problems so the aim of humidification is to try to maintain a physiological level of moisture within the airways which will aid clearance.
Ideal humidification should have the following requirements:
- the inspired gas should be at a temperature between 30 to to 36°C with a water content of 30 to 43 g/m³
- the temperature should remain constant
- ranges of fresh gas flow should not affect it
- the device should be simple
- there should be safety mechanisms
- the increased resistance, compliance and dead space do not adversely affect breathing.
When considering the non-ventilated patient there are two types of humidifiers, cold water and hot water humidifiers.
Cold water humidifiers are the cheaper option but are not necessarily efficient reducing a water content of only around 9 g/m³.
Hot water humidifiers are an improvement on this however by their very nature they are more complicated and therefore more costly. The inspired gas can either be passed over or through a heated water reservoir. As the hot water humidifier produces micro droplets there is a potential source of infection.
Three other factors may play a part in the patient’s ability to breathe effectively; patient positioning, deep breathing exercises, and an effective cough.
In order to improve the patients ventilation it is important that they can sit as upright as they can tolerate. Indeed many of the patients with a physiological deficit choose to sit bolt upright e.g. the patient with COPD or heart failure. Compared to the supine or lying down position the upright position has a number of advantages. It will increase lung volumes, improve lung compliance, and reduce possible airway closure. By airway closure in this instance we mean when the small airways within the lungs will start to collapse because of changes in pressure and volume within the lungs. The consequences of all of these means that their work of breathing becomes easier and they can mobilise their secretions more effectively therefore clearing them which also helps their respiratory function to improve.
If the patient as an element of respiratory compromise it may be that they begin to take shallow breaths. The consequence of this is that the smaller airways and alveoli will begin to collapse as there is a reduced movement of air in and out of the lungs. This tends to manifest itself in the base of the lungs. One way to ensure that this is minimised is to encourage the patient to take deep breaths on regular occasions. This will help reinflate those airways which have a tendency to collapse.
Finally in order to clear any excess secretions or those that have built up because of shallow breathing it is also important to encourage the patient to cough effectively. An effective cough is one which clears the secretions to the back of the throat. These secretions can then either be swallowed or spat out. If these secretions are not cleared they tend to become a nice growth medium for any bacteria. If the patient is already unwell then they are prone to infections. This is when a good physiotherapy service will come into its own.