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11/13/24, 7\:43 PM Guide | Oxygen prescribing

Oxygen prescribing

Table of contents

Introduction

Oxygen is one of the most commonly prescribed drugs in hospitals. Oxygen prescribing is a core task for any doctor, and it is
an important part of the UK Medical Licensing Assessment curriculum.
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This article will cover when and how to prescribe oxygen in adults, and highlight some potential dangers of over-oxygenation.

Indication

Oxygen is indicated for hypoxaemia, not breathlessness.
Aim for oxygen saturations of 94-98%, or 88-92% in those at risk of type 2 respiratory failure. The British Thoracic Society (BTS)
recommends clinical assessment if the oxygen saturation falls ≥3% below the patient's target.
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Treating the underlying cause
Supplemental oxygen does not treat the underlying cause of hypoxaemia.
If oxygen needs to be prescribed, you should also take a systematic approach to identify why the patient has
desaturated. Always involve a senior clinician if you are concerned or unsure how to manage the patient.

Types of oxygen delivery devices

There are various oxygen delivery devices used in clinical practice.
Oxygen is delivered at
For every increase in 1L/min, the fraction of inspired oxygen (FiO ) increases by 4% (e.g. 1L/min = 24% FiO , 2L/min = 28% FiO
2 2 2
etc).
Table 1. The uses of each main oxygen delivery device.
3,4,5
Delivery device Description
Provides low-
Used in mild hypoxaemia when the
patient is not critically unwell
Nasal cannulae
While the maximum
6L/min, do not exceed 4L/min as this
would dry out the nasal passages,
leading to irritation
Do not use if the patient mainly
breathes through their mouth
https\://app.geekymedics.com/osce-guides/prescribing/oxygen-prescribing/ 1/611/13/24, 7\:43 PM Venturi mask
Simple face mask (Hudson mask)
Non-rebreather mask (reservoir
mask)
Guide | Oxygen prescribing
Delivers a precise FiO 2
is set to a speci
if the
Venturi masks are colour-coded
based on the FiO delivered\:
2
Blue = 24% (set at 2L/min)
White = 28% (set at 4L/min)
Orange = 31% (set at 6L/min)
Yellow = 35% (set at 8L/min)
Red = 40% (set at 10L/min)
Green = 60% (set at 15L/min)
Venturi masks deliver a precise FiO ,
2
so are used in COPD patients as you
need precise titrations of oxygen to
maintain target saturations of 88-92%
Increasing the
recommended level for each mask
colour may not increase the FiO 2.
If
you need to up-titrate the oxygen
delivered, you must change the mask
colour
Delivers oxygen between 5-10L/min
However, does not allow precise
control of FiO 2
delivered, so Venturi
masks are preferred
Delivers up to 15L/min, which
approximately equates to 70-90% FiO
2
(does not deliver 100% FiO 2
as some
room air will escape into the mask due
to the mask not being perfectly
adherent to the face)
The reservoir bag is
before the mask is applied to the
patient, which means the patient
inhales oxygen from the reservoir bag
and the direct oxygen source
A one-way valve prevents exhaled air
from entering the reservoir bag (the
exhaled air exits via vents on the sides
of the mask)
Used if the patient is critically unwell
or has high oxygen requirements (e.g.
saturations \<85% in a healthy person)
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Figure 1. Nasal cannulae [3]

Prescribing oxygen

Use an ABCDE approach. If the patient is not breathing, call for help and commence resuscitation. This would involve inserting
airway adjuncts and applying high-
If the patient is breathing, you should assess how unwell they are.

Critically unwell patients

If the patient is critically unwell and not at risk of type 2 respiratory failure (or you are unsure about their risk), initially prescribe
high-
If the patient is at risk of type 2 respiratory failure (e.g. COPD patients who are known to be CO 2
start at a lower FiO 2
using a Venturi mask and up-titrate if required. Aim for oxygen saturations of 88-92%.
retainers), it may be safer to
This is due to the risks of type 2 respiratory failure in these patients if you over-oxygenate them. Use clinical judgement and
seek senior support early in these cases

Non-critically unwell patients

If the patient is not critically unwell, prescribe oxygen based on Figure 5.
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Figure 5. Prescribing oxygen in acutely hypoxaemic patients (adapted from the BTS guideline for oxygen use in healthcare and emergency settings) [2]
Aim for 100% oxygen saturations in patients with pneumothorax as this reportedly increases the resolution speed.
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Oxygen prescriptions

Since oxygen is a drug, it must be prescribed on a drug chart (paper or electronic). Most drug charts have a section for oxygen
prescribing.
Usually, the prescriber would need to specify the target oxygen saturations, oxygen delivery device and desired
rate/FiO
2.
As with any drug, if oxygen can be stopped, then the prescription on the drug chart should be crossed o
electronic prescribing platforms).

Monitoring oxygen saturations

In the UK, oxygen saturations are part of the patient's basic observations (vital signs) which make up their National Early
Warning Score (NEWS).
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The frequency of observations depends on how high the NEWS is (the higher the score, the more frequently it is measured).
Usually, nursing sta
Document clear instructions if you wish for it to be measured at a certain frequency.
In addition, give clear instructions to nursing sta
\<92%).
For a patient who has just been initiated on oxygen/acutely unwell patients, it is advisable to monitor oxygen at least hourly
until they stabilise (e.g. are within their target saturations for 4-6 hours consecutively).
P/F ratio
Use the P/F ratio to check if the patient’s pO 2
responds adequately to the supplemental oxygen.
P/F ratio = PaO 2 2
on ABG (“P”) divided by FiO (“F”)
The FiO 2 2
must be expressed as a decimal (e.g. 40% FiO = 0.4).
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The normal P/F ratio is 55kPa or 400mmHg (depending on whether PaO 2
Calculate the P/F ratio instead of the old-fashioned way of calculating an adequate PaO 2
oxygen (subtracting 10 from the FiO ).
2
is measured in kPa or mmHg).
for a patient on supplemental

Increasing oxygen

If a patient's oxygen saturations do not reach their target within 3-5 minutes of administering oxygen, the 2
using a Venturi mask) should be increased.
2
(if
If the patient becomes critically unwell, increase to 15L/min via a non-rebreather mask
If the patient is not critically unwell, consider increasing oxygen by increments (e.g. from 3L via nasal cannulae to a white
Venturi mask - FiO 2
28% at 4L/min)
If a patient’s oxygen requirements increase so much that they do not respond to 15L/min via a non-rebreather mask\:
Re-assess the patient (ABCDE assessment)
Inform a senior clinician if you have not done so already
Consider the patient's ceiling-of-care/escalation status. If they are for full escalation, they may need high-
oxygen (HFNO)/continuous positive airway pressure (CPAP)/non-invasive ventilation (NIV)/intubation and ventilation. This is
a complex decision based on various factors, including the patient's ABG results (summarised below and covered in detail
here)
Options for escalation
High-2
(up to
100%) and
Continuous positive airway pressure (CPAP)\: used in type 1 respiratory failure (PaO 2
\<8.0kPa), for example cardiogenic
pulmonary oedema
Non-invasive ventilation (NIV)\: used in type 2 respiratory failure (PaO 2 2
\<8.0kPa AND PaCO >6.0kPa), for example a
COPD exacerbation

Weaning and discontinuing oxygen

Wean down the 2
hours consecutively.
if the patient's oxygen saturations are at least at the higher end of their target saturations for 4-6
Wean by small increments (e.g. from a yellow Venturi/35% FiO to a white Venturi/28% FiO ). This is usually performed by
2 2
nursing sta
Once the patient is stable on 1-2L/min via nasal cannulae, you can cease oxygen completely.
Monitor the patient's oxygen saturations for 5 minutes without supplemental oxygen. If they remain within their target
saturations, measure their oxygen saturations in 1 hour (and then use clinical judgment regarding when you will measure them
again).
2

Harms of over-oxygenation

Some studies have shown that over-oxygenating a patient (aiming for saturations 96-100%) is associated with an increased risk
of death in acute illnesses.
8
There are multiple possible explanations for the harms of over-oxygenation\:
Increased reactive oxygen species, leading to cellular damage or death.
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Systemic vasoconstriction (including cerebral vasoconstriction), leading to organ hypoperfusion.
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False reassurance\: respiratory deteriorations may be detected later if a patient is left on high-
require a more signi
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Regularly review a patient’s oxygen requirement to ascertain if it can be weaned down.

Reviewer

Dr Neeraj Shah
Specialist registrar in Respiratory Medicine (ST6)
Guy’s and St Thomas’ NHS Foundation Trust

References

Source\: geekymedics.com
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