Prescribing Oxygen

Question 1

Oxygen should be regarded as a drug that is prescribed for patients with hypoxaemia (low blood oxygen concentration).

Answer 1

Oxygen should be regarded as a drug that is prescribed for patients with hypoxaemia (low blood oxygen concentration).

Question 2

When prescribing and administering oxygen there are several things to consider:

Answer 2

Does the patient need oxygen?
What are the target saturations?
How should oxygen be delivered?
What is the cause of hypoxaemia?
What is the ongoing monitoring?

Question 3

The headline point is that oxygen should be used to treat hypoxaemia and maintain a patients’ saturations in the target range. This should be …

Answer 3

This should be 94-98% or 88-92% in patients at risk of type 2 respiratory failure.

Question 4

Hypoxia and hypoxaemia are two different terms but often used synonymously in clinical practice.

Answer 4

In clinical practice, the term ‘hypoxic’ is commonly used instead of ‘hypoxaemic’ to refer to a patient with low oxygen saturations as measured on pulse oximetry. Although the terms hypoxia and hypoxaemia are used interchangeably, they mean two different things:

Hypoxia: failure of tissue oxygenation
Hypoxaemia: low arterial oxygen concentration (i.e. low partial pressure of blood oxygen)

Question 5

The five causes of tissue hypoxia include:

Answer 5

Hypoxaemic hypoxia: due to hypoventilation, ventilation/perfusion (V/Q) mismatch, or pulmonary shunts
Circulatory hypoxia: due to inadequate cardiac output
Anaemic hypoxia
Histotoxic hypoxia: inability of the tissue to use oxygen (e.g. cyanide poisoning)
Oxygen affinity hypoxia: decreased oxygen delivery to tissue (i.e. haemoglobin holds onto oxygen)

Question 6

The sole indication for oxygen is hypoxaemia.

Answer 6

Patients with hypoxaemia require oxygen to prevent hypoxia. This is because hypoxia impairs cellular aerobic metabolism. When aerobic metabolism is impaired cells undergo anaerobic respiration, but this is a short-term inefficient system that cannot sustain life for prolonged periods of time. By-products of anaerobic respiration include substances such as lactic acid that can lead to metabolic acidosis and reduced cellular function.

Oxygen is prescribed for two main groups with hypoxaemia:

Acutely unwell: typically administered in a hospital or healthcare setting.
Chronic hypoxaemia: oxygen may be prescribed for long-term use in patients with chronic lung disease. This is known as long-term oxygen therapy (LTOT). Specific criteria exist for its use.

Question 7

Acute indication - oxygen

Answer 7

Hypoxaemia is a common presentation in critically ill patients. Often, patients may be so unwell that oxygen is administered at high flow rates while waiting for a reliable oximetry reading. Once obtained, oxygen can be titrated to the appropriate target saturations.

Oxygen should always be administered to achieve a normal or near-normal oxygen saturation based on the target saturation set for the patient (see below).

Question 8

Patients with, or at risk of, type 2 respiratory failure should have a lower target saturation at …

Answer 8

Patients with, or at risk of, type 2 respiratory failure should have a lower target saturation at 88-92%.

Question 9

There are two major target saturations for patients being treated with oxygen:

Answer 9

94-98%: patients not at risk of type 2 respiratory failure

88-92%: patients with, or at risk of, type 2 respiratory failure

Question 10

Type 2 respiratory failure

Answer 10

Type 2 respiratory failure (T2RF) is characterised by hypoxaemia (PaO2 < 8 kPa) and hypercapnia (PaCO2 > 6.5 kPa). It is also referred to as hypercapnic respiratory failure. It can be either acute or chronic depending on its speed of onset and presence of compensatory mechanisms.

Question 11

Patients at risk of T2RF include:

Answer 11

Moderate-to-severe chronic obstructive pulmonary disease (COPD): may be undiagnosed
Cystic fibrosis
Severe obesity (i.e. obesity hypoventilation syndrome)
Neuromuscular disease (e.g. Motor neurone disease)
Severe chest wall deformity (e.g. kyphoscoliosis)
Previous episode of T2RF

Question 12

In patients with, or at risk of, T2RF higher levels of oxygen can induce or worsen hypercapnia due to a combination of ventilation/perfusion mismatch and increased physiological deadspace. Therefore, we can get oxygen-induced …

Answer 12

In patients with, or at risk of, T2RF higher levels of oxygen can induce or worsen hypercapnia due to a combination of ventilation/perfusion mismatch and increased physiological deadspace. Therefore, we can get oxygen-induced hypercapnia.

Question 13

In a patient without a known diagnosis that increases the risk of T2RF such as COPD, it may be difficult to determine whether they should have a lower target saturation at 88-92%.

Features that can suggest a patient is at risk of T2RF:

Answer 13

Heavy smoker
Severe emphysematous changes on imaging
Compensated respiratory acidosis (i.e. raised PaCO2 & raised bicarbonate)
History of obstructive sleep apnoea
Large neck or very obese
Rising PaCO2 with oxygen therapy

If in doubt, always ask a senior for advice on target saturations.

Question 14

Nasal cannulae - oxygen delivery

Answer 14

Oxygen delivered (%): 24-44%
Variable oxygen concentration: affected by respiratory rate and amount delivered via nose.
Flow rate (L/min): 1-6 litres

Question 15

Nasal cannulae

Oxygen delivered (%): 24-44%
Variable oxygen concentration: affected by respiratory rate and amount delivered via nose.
Flow rate (L/min): 1-6 litres

In general, more than … litres is uncomfortable for patients and causes dryness of the nasal passages.

Answer 15

In general, more than 4 litres is uncomfortable for patients and causes dryness of the nasal passages.

Question 16

Venturi mask

Answer 16

Oxygen delivered (%): 24-60%
Fixed oxygen concentration: designed to entrain a set amount of oxygen and air giving a fixed concentration
Flow rate (L/min): coloured coded (2-3 Blue / 4-6 White / 8-12 Yellow / 10-15 Red / 12-15 Green)

Question 17

Simple face mask - oxygen delivery

Answer 17

Oxygen delivered (%): 30-60%
Variable oxygen concentration: affected by respiratory rate
Flow rate (L/min): 6-10 litres

Question 18

Non-rebreathe mask

A reservoir bag is connected to the mask with a one-way flap valve.

Answer 18

Oxygen delivered (%): ~60-85%
Variable oxygen delivery: affected by respiratory rate
Flow rate (L/min): 10-15 litres (if lower rates used the reservoir bag can deflate during inspiration)

Question 19

Humidifed oxygen

Answer 19

May be required for flow rates > 4 L/min due to upper airway dryness and discomfort. A humidifier attachment can be added to some devices

Question 20

Oxygen titration

Answer 20

If a patient is becoming more hypoxaemic, oxygen should be titrated according to target saturations. Ensure ongoing monitoring using appropriate early warning scores (i.e. NEWS). Patients should undergo blood gas analysis within 1 hour of requiring increased oxygen.

Question 21

Titration of oxygen

Answer 21

Stage one - Blue 24% Venturi (2-3 L/min) / Nasal cannulae 1 L/min
Stage two - White 28% Venturi (4-6 L/min) / Nasal cannulae 2 L/min
Stage three - Yellow 35% Venturi (8-12 L/min) / Nasal cannulae 4 L/min
Stage four - Red 40% Venturi (10-15 L/min) / Nasal cannulae 5-6 L/min / Simple face mask 5-6 L/min
Stage five - Green 60% Venturi (12-15 L/min) / Simple face mask 7-10 L/min
Stage six - Non-rebreathe with reservoir mask 60-85% (15 L/min)

Question 22

When monitoring patients on oxygen ensure you look for signs of respiratory deterioration, particularly if you are required to titrate oxygen upwards. These may include:

Answer 22

Tachypnoea (esp. >30 bpm)
Desaturation
Increasing oxygen requirement
High NEWS score

Question 23

Respiratory support

Answer 23

Non-invasive and invasive ventilation are a form of respiratory support to aid ventilation.

Question 24

Patients with severe hypoxaemia may require respiratory support to aid ventilation. In brief, respiratory support is a method of providing pressure-based ventilation to support a patients breathing and improve recruitment of alveoli. There are two major types of respiratory support:

Answer 24

Non-invasive: does not require tracheal intubation

Invasive: requires tracheal intubation and mechanical ventilation

Question 25

Non-invasive ventilation

Answer 25

Non-invasive ventilation (NIV) is a way of supporting ventilation without having to undergo invasive tracheal intubation. NIV is indicated if high inspired oxygen concentrations (>60%) are not sufficient to maintain adequate oxygenation in patients with respiratory failure.

Question 26

There are several methods of delivering NIV:

Answer 26

High-flow nasal cannula oxygen (HFNC): often referred to as ‘Optiflow’. Provides a high concentration of oxygen (up to 80%) and high flow rate (up to 70 L/min). Provides a small amount of positive end expiratory pressure (PEEP - 2-5 cmH20). This essentially helps to recruit alveoli and prevent collapse.
Continuous positive airway pressure (CPAP): similar to HFNC, CPAP provides a high concentration of oxygen (up to 80%) and high flow rate (up to 70 L/min). It provides a higher amount of PEEP (up to 10 cmH20), which prevents airway collapse, improves alveoli recruitment and improves ventilation/perfusion matching.
Bi-level Positive Airway Pressure (BiPAP): this provides air pressure during both inspiration and expiration. The expiratory positive airway pressure (ePAP) is similar to PEEP and helps to prevent alveoli collapse and recruit more alveoli for gas exchange. The inspiratory positive airway pressure (iPAP) assists inspiration and is important for ventilation (exchange of gas). The pressure difference between ePAP and iPAP helps increase tidal volume and minute ventilation. BiPAP is usually reserved for patients with, or at risk of, type 2 respiratory failure.

Question 27

CPAP vs BiPAP

Answer 27

Continuous positive airway pressure (CPAP): similar to HFNC, CPAP provides a high concentration of oxygen (up to 80%) and high flow rate (up to 70 L/min). It provides a higher amount of PEEP (up to 10 cmH20), which prevents airway collapse, improves alveoli recruitment and improves ventilation/perfusion matching.
Bi-level Positive Airway Pressure (BiPAP): this provides air pressure during both inspiration and expiration. The expiratory positive airway pressure (ePAP) is similar to PEEP and helps to prevent alveoli collapse and recruit more alveoli for gas exchange. The inspiratory positive airway pressure (iPAP) assists inspiration and is important for ventilation (exchange of gas). The pressure difference between ePAP and iPAP helps increase tidal volume and minute ventilation. BiPAP is usually reserved for patients with, or at risk of, type 2 respiratory failure.

Question 28

Invasive ventilation

Answer 28

Invasive ventilation is also known as mechanical ventilation. It involves general anaesthesia, invasive tracheal intubation with placement of a tracheal breathing tube, and connection to a mechanical ventilator that does the breathing for the patient.

Question 29

ECMO - what is this and when is it used?

Answer 29

In very severe cases, patients may require Extracorporeal membrane oxygenation (ECMO), which is essentially an artificial lung. This is a very specialised treatment that involves diverting a patients blood to an artificial oxygenator before being pumped back to the patients body. It is used when mechanical ventilation is not sufficient to maintain adequate oxygenation and there is deemed a reasonable chance of recovery.

Question 30

The diagram below shows a summary of oxygen prescribing for acutely hypoxaemic patients in hospital.

Answer 30

Question 31

A 43-year-old is seen in accident and emergency resuscitation area with severe shortness of breath. They started to feel unwell today with a tight chest and wheeze. They called the ambulance and on arrival, the saturations were 78% on room air. They were rushed to the hospital and put on 15L oxygen via a non-rebreathe mask. They have no significant past medical history, smoke 20 cigarettes a day with a 40-pack-year history, and drink minimal alcohol. An urgent chest x-ray shows no major collapse or consolidation. An arterial blood is taken, which is shown:

pH 7.36 (7.35-7.45)

PO2 24.4 (11.3-12.6 kPa)

PCO2 8.2 (4.7-6.0 kPa)

Bicarbonate 34 (21-29 mmol/L)

Lactate: 2.9 (0.5-1.6 mmol/L)

What is the most appropriate management?

A Urgent referral to intensive care for intubation and ventilation
B Liaise with the ED consultant about starting bi-level Positive Airway Pressure
C Stop the oxygen and repeat the blood gas in one hour
D Switch to nasal cannula aiming saturations 88-92%
E Titrate down the oxygen aiming for saturations 88-92% and repeat the blood gas

Answer 31

This patient has evidence of hypercapnia and they are at risk of type 2 respiratory failure (T2RF).
This is a common scenario in clinical practice. The patient has no formal diagnosis of lung disease but given the heavy smoking history, hypercapnia and compensatory rise in bicarbonate they likely have undiagnosed COPD. In this case, it would be appropriate to change the target saturations to 88-92%, titrate down the oxygen and repeat the blood gas to see if there is an improvement in hypercapnia and acid/base balance.

In a patient without a known diagnosis that increases the risk of T2RF such as COPD, it may be difficult to determine whether they should have a lower target saturation at 88-92%.

Features that can suggest a patient is at risk of T2RF:

• Heavy smoker
• Severe emphysematous changes on imaging
• Compensated respiratory acidosis (i.e. raised PaCO2 & raised bicarbonate)
• History of obstructive sleep apnoea
• Large neck or very obese
• Rising PaCO2 with oxygen therapy

If in doubt, always ask a senior for advice on target saturations.

Question 32

A 29-year-old is admitted with an acute exacerbation of asthma. Their saturations are currently 97% on 2L of oxygen via nasal cannulae.

Approximately, what percentage of oxygen is being delivered to the patient?

A	20%
B	24%
C	28%
D	36%
E	60%

Answer 32

As a general rule, each litre of oxygen via nasal cannulae provides an additional 4% of oxygen.
We breathe atmospheric oxygen at 20-21%. Using 1L of oxygen via nasal cannulae means the patient is getting approximately 24% oxygen. Using 2L of oxygen via nasal cannulae means the patient is getting approximately 28% oxygen. However, the amount of oxygen is actually more variable because it is affected by respiratory rate and the amount delivered via the nose as compared to the mouth. Up to 6 litres can be given via nasal cannula but In general, more than 4 litres is uncomfortable for patients and causes dryness of the nasal passages.

Question 33

What are the appropriate target saturations in a patient not at risk of type 2 respiratory failure?

A	90-100%
B	94-98%
C	88-92%
D	96-100%
E	>94%

Answer 33

Target saturations in those not at risk of type 2 respiratory failure are 94-98%.
There are two major target saturations for patients being treated with oxygen:
- 94-98%: patients not at risk of type 2 respiratory failure
- 88-92%: patients with, or at risk of, type 2 respiratory failure

Note that some patients may have very specific target saturations set by their respiratory team.

Question 34

What percentage of oxygen can venturi masks deliver?

A	20-24%
B	24-60%
C	30-80%
D	24-32%
E	20-100%

Answer 34

Venturi masks come in different colours that can each deliver a set concentration of oxygen between 24-60%.
Venturi masks are designed to entrain a set amount of oxygen and air giving a fixed concentration delivered to the patient. It is important to use the flow rate (L/min) suggested on the side of the mask.

Question 35

A 72-year-old is admitted with acute shortness of breath. He has been feeling unwell for the last few days with a productive cough. He can normally manage a walk down to his local shop about 200 metres away but has been unable to leave the house in the last 24 hours due to breathlessness. He has a background of severe chronic obstructive pulmonary disease. On examination, there is a widespread polyphonic wheeze with poor air entry. Saturations on room air are 79%. He is started on high-flow oxygen and his sats improve to 98%.

What is the most appropriate management with regard to his oxygen?

A Continue high flow oxygen until bloods are back
B Switch to simple face mask and aim saturations 94-98%
C Stop the oxygen and observe again in 10 minutes
D Switch to Venturi mask and titrate down the oxygen aiming saturating 88-92%
E Switch to Optiflow and aim saturations 88-92%

Answer 35

This patient has severe COPD and is at risk of oxygen-induced hypercapnia and should have target saturations at 88-92%.
In patients with, or at risk of, type 2 respiratory failure (T2RF) higher levels of oxygen can induce or worsen hypercapnia due to a combination of ventilation/perfusion mismatch and increased physiological deadspace. Therefore, we can get oxygen-induced hypercapnia. These patients require a lower target oxygen saturation at 88-92%. Venturi masks provide a fixed oxygen concentration and are easier to titrate.

Question 36

A 52-year-old man is seen on the surgical ward following an appendicectomy. The foundation year one is called because the saturations are 88% on room air. He has a background of hypertension and hypercholesterolaemia only. He is currently taking oral morphine immediate relief on an as required basis for pain. On review, he appears well and there are crepitations at the right lung base. Pupils are equal and reactive to light. Observations are HR 80/min, BP 134/89 mmHg, RR 20/min, T 37.7º.

What is the most appropriate management regarding his oxygen?

A Start high flow oxygen 15 L non-rebreathe mask
B No supplemental oxygen needed
C Advise to set up a 60% Venturi mask
D Advise to initiate 1-2 L of oxygen via nasal cannulae
E Call the on call physiotherapist to provide chest physio

Answer 36

This patient has experienced an episode of desaturation post-operatively that can be managed with simple supplemental oxygen (i.e. 1-2L of oxygen via nasal cannulae).
Oxygen is a drug and its administration requires a formal prescription. In an acute situation, it is usually appropriate to give high flow oxygen whilst awaiting more observations and clinical assessment. In this situation, the patient is stable with mild hypoxia. There is no history of, or risk, for type 2 respiratory failure and there is no evidence of opioid toxicity. Therefore, initiating simple supplemental oxygen is appropriate. Oxygen can then be titrated upwards as needed to meet target saturations of 94-98%.

Incorrect answers
A: This patient is not critically unwell, so initiation of high flow oxygen is not needed at this stage

B: This patients target saturations will be 94-98% so they need supplemental oxygen

C: There is no need to jump straight to a 60% venturi, starting with nasal cannulae will suffice

E: Chest physio may be suitable for patients who struggle to clear secretions leading to mucous plugging and hypoxia. There is no need for chest physio in this case