Ch 54 Oxygen Therapy

CHAPTER 54

OXYGEN THERAPY

Outline: The aim of treatment Indications for oxygen (O2) therapy Methods of administration Choice of correct inspired O2 concentration Side effects of administration

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Oxygen can be seen as a drug and like other drugs it has indications and undesirable side effects. Like all drugs oxygen must be prescribed in the right dose (concentration or flow rate), for a prescribed length of time. THE AIM OF THE TREATMENT To make sure there is enough oxygen in the inspired air to ensure adequate tissue oxygenation. INDICATIONS FOR OXGEN THERAPY Oxygen is indicated in states of hypoxia. Hypoxia means lack of oxygen in the tissues and is discussed in detail under Respiratory physiology in Chapter 3. Oxygen is indicated in hypoxic, anaemic and stagnant hypoxia. METHODS OF ADMINISTRATION Mask There are two types of mask: Variable performance masks which use a low flow of oxygen, e.g. 2-4 L/min. Air is entrained and a large volume of air and oxygen reaches the patient. Examples are the Hudson mask and Edinburgh mask. The Edinburgh masks used with 2 - 3 L/min of oxygen delivers 27 - 30% oxygen to the patient. The inspired concentration depends on several factors one of which is the tidal volume.

Fig 54.1 An example of a variable flow mask A variable performance mask with an oxygen reservoir can increase inspired oxygen to almost 80%.

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Fixed performance masks. These need about 6-8 litres of oxygen per minute to wash out the carbon dioxide (CO2) and provide a fixed and constant inspired oxygen concentration. The oxygen passes through a jet which entrains air in such a way that the total flow rate exceeds the peak inspiratory flow rate (PIFR) of most patients. Venturi masks are used to deliver 24% oxygen, 28%, 35% or 40% oxygen depending on the mask. A different mask must be used for each range of oxygen concentration required.

Fig 54.2 Example of a fixed performance mask with a 24% oxygen attachment Disadvantages of masks: • Eating, speaking, etc. are difficult • They are dangerous if the patient vomits • They can be uncomfortable and hot • They increase dead space. A non-rebreathing valve plus a mask enables 100% oxygen to be given. Nasal cannulae or prongs These are either PVC or plastic and consist of a tube with 2 soft tips that fit into the nostrils. They have the following advantages: • There is no dead space. • Vomiting or respiratory obstruction is easily seen. • There is no interference with eating, speaking, etc.

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Nasal catheters These have similar advantages to nasal cannulae but also have dangers: • If a nasal catheter is inserted too far then the oxygen may be delivered to the oesophagus instead of to the lungs and rupture of the oesophagus or the stomach can result. Avoid this by carefully measuring the distance between the side of the nose and the tragus of the ear and making sure the catheter is not inserted further than this. • Prolonged use may cause nasal irritation. Four to six litres of oxygen per minute gives the patient a concentration of 30 to 40%. Tent This is commonly used with young children who will not tolerate a mask or a catheter. Tents are cool, moist and comfortable but leakage is common especially while the child is being nursed or monitored. The tent is an inefficient means of providing oxygen. It also limits patient care. Incubator This provides a controlled environment for the infant. Endotracheal Tube The patient may be intubated and placed on a ventilator with a selected concentration of oxygen, or intubated and allowed to breathe spontaneously while a selected concentration of oxygen is given. Hyperbaric oxygen This is used to treat symptoms of severe tissue hypoxia. This is oxygen administered at a pressure greater than atmospheric. The oxygen dissolved in the blood increases with an increase in oxygen pressure. Hyperbaric oxygen is used to treat conditions of severe hypoxaemia that cannot be treated with normal pressure oxygen. It is used in the treatment of carbon monoxide poisoning, air embolism, gas gangrene and decompression sickness. Hyperbaric oxygen is administered in specialised units. Humidifiers Ideally when oxygen is administered for longer than 24 hours the inspired oxygen should be humidified by passing it through warmed water. Humidification of gases with warm, moist air prevents drying of the respiratory mucosa and may help prevent hypothermia. In practice humidifiers can be the source of serious hospital borne infections and must be used with meticulous care. Disposable heat and moisture exchange (HME) bacterial filters are now routinely used during anaesthesia but they are expensive and therefore not readily affordable.

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Fig 54.3 Method of humidifying oxygen The oxygen concentrator This provides oxygen from room air. It is described in detail in Chapter 13. CHOICE OF CORRECT INSPIRED CONCENTRATION OF OXYGEN (FiO2) Patients can be classified into two broad groups with regard to oxygen therapy: Those with normal respiratory control. The normal respiratory centre is stimulated by a high carbon dioxide level. As the level of carbon dioxide rises there is an increase in the rate and depth of breathing. It is also stimulated to a lesser extent by a fall in the oxygen level. These patients may require oxygen because of injury or disease of a more acute nature, e.g. chest infection, fractured ribs or polio. In these cases oxygen can be given until the cause of the hypoxia is cured. Those with abnormal respiratory control. The respiratory centre in COAD, on the other hand, may be stimulated not by the high carbon dioxide level but by the low oxygen level only. This is because a high carbon dioxide level already exists in these patients and it ceases to have an effect on the respiratory centre. The low oxygen level keeps the respiratory centre going. This is referred to as the patient's "hypoxic drive" (It is the hypoxia that "drives" the respiratory centre). These patients require careful and controlled oxygen therapy. Therefore start with a low concentration of oxygen, about 24% and observe patients very carefully for signs of depression of the respiratory centre (shallow breathing and evidence of carbon dioxide accumulation). If 24% does not depress breathing then gradually increase the concentration noting respiratory rate and oxygen saturations. Those with hypoxic drive will

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reduce their respiratory rate as the oxygen concentration is increased. Normally COAD patients with an acute exacerbation need high flow oxygen. If a high oxygen concentration is required and the patient is showing signs of respiratory depression, then the patient's respiration may have to be controlled using IPPV. Assessing the adequacy of oxygen therapy • • •

Clinical signs: colour, pulse, blood pressure, mental state The oximeter, to measure oxygen concentration in the blood Arterial blood gases

SIDE EFFECTS OF OXYGEN THERAPY Respiratory depression As mentioned above this can occur in certain types of patient, particularly those with chronic obstructive airway disease (COAD). These patients may be dependent on their "hypoxic drive" to keep them breathing. When high concentrations of oxygen are given to some patients with COAD this hypoxic drive is abolished and the respiratory centre may slow down or even stop. This condition is rare and often over-diagnosed and patients are denied adequate oxygen and allowed to remain hypoxic when they need high concentrations of oxygen and/or assisted or mechanical ventilation. Arterial blood gas measurement if available will reveal an elevated CO 2 and decreased PaO2. Atelectasis Prolonged administration of oxygen tends to result in atelectasis. This is due to the rapid absorption of oxygen into the blood stream. Substernal pain Tracheitis occurs if 70-100% oxygen is inhaled for more than a few hours. Pulmonary damage and oedema This will result if 100% oxygen is inhaled for long periods eg 1-3 days. Inspired oxygen concentration should always be kept at the lowest level that allows adequate tissue oxygenation. Retrolental fibroplasia This is the formation of a fibrovascular membrane behind the lens of the eye. It results in blindness. It occurs especially in premature babies

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who have been exposed to high concentrations of oxygen at birth. Premature babies should only receive oxygen if they are hypoxic and then preferably to maintain oxygen saturation around 92%.

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