oxygen prescribing

Question 1

prescribe oxygen in critically unwell patients

Answer 1

if the patient is critically unwell and not at risk of T2RF (or unsure about risk), initially prescribe high-flow oxygen (15L/min) through a non-rebreather mask

Question 2

non-critically ill patients who are at risk of T2RF

Answer 2

• 88-92% target saturations
• start 1-2L/min via nasal cannulae or 28% FiO2 via white venturi
• perform an ABG

if:
a. respiratory acidosis (pH < 7.35 and CO2 > 6.0 kPa):
- continue with 88-92% target
- seek senior help and consider NIV

b. hypercapnia (pH>7.35, CO2 > 6.0 kPa)
- continue with 88-92% target
- repeat ABG in 60 min; if resp acidosis, seek senior help and consider NIV

c. normocapnic/hypocapnic (CO2 < 6.0 kPa):
- change to 94-98% target
- repeat ABG in 60min;
if CO2 > 6.0kPa, get senior help and
continue 94-98% if no known hypercapnia risk
aim 88-92% if known hypercapnic risk

Question 3

oxygen prescribing in non-critically ill patients who are not at T2RF risk

Answer 3

• 94-98% target sats
• if <85%: 15L/min via non-rebreather mask
• if 85-93%: 1-2L/min via nasal cannulae or 28% FiO2 via white venturi
• perform an ABG

CO2 >6.0kPa or signs of CO2 retention (e.g., drowsiness, headache, flapping tremor):
- change to 88-92% target if likely undiagnosed COPD/chronic type 2 resp failure
- seek senior help and consider NIV

Question 4

what is the P/F ratio? why is it used for?

Answer 4

• use the P/F ratio to check if the patient’s pO2 responds adequately to the supplemental oxygen

P/F ratio = PaO2 on ABG (“P”) divided by FiO2 (“F”)

• the FiO2 must be expressed as a decimal (e.g., 40% FiO2 = 0.4)
• the normal P/F ratio is 55kPa or 400 mmHg (depending on whether PaO2 is measured in kPa or mmHg)
• calculate the P/F ratio instead of the old-fashioned way of calculating an adequate PaO2 for a patient on supplemental oxygen (subtracting 10 from the FiO2)

Question 5

when and how to increase oxygen

Answer 5

• if a patient’s oxygen sats do not reach their target within 3-5 mins of administering, the flow rate/FiO2 (if using Venturi) should be increased
• 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 - FiO2 28% at 4 L/min)

if the patient’s oxygen requirements increase so much that they do not respond to 15L/min via a non-rebreather mask:
- re-assess the patient (A-E)
- inform senior clinician
- consider patient’s ceiling of case/escalation status. if they are for full escalation: high-flow nasal oxygen/CPAP/NIV/intubation and ventilation

Question 6

options for escalation

Answer 6

high-flow nasal oxygen (HFNO)
- compared to a non-rebreather mask, can delivery oxygen at a greater FiO2 (up to 100%) and flow rate (up to 60L/min)
- usually only available in high-dependency/intensive care enviornments

continuous positive airway pressure (CPAP)
- used in T1RF (PaO2 < 8.0 kPa)
- e.g., cardiogenic pulmonary oedema

non-invasive ventilation (NIV)
- used in T2RF (PaO2 < 8.0kPa AND PaCO2 >6.0 kPa)
- e.g., COPD exacerbation

Question 7

weaning and discontinuing oxygen

Answer 7

• wean down the flow rate/FiO2 if the patients sats are at least at the higher end of their target sats for 4-6 hours consecutively
• wean by small increments e.g., from a yellow Venturi/35% FiO2 to a white Venturi/28% FiO2
• once the patient is stable on 1-2L/min via nasal cannulae, you can cease oxygen completely
• monitor the patient’s oxygen sats for 5 mins without supplemental oxygen; if they remain within their target sats, measure their oxygen sats in 1 hour (and then use clinical judgement regarding when you will measure them again)

Question 8

harms of over-oxygenation

Answer 8

some studies have shown that over-oxygenating a patient (aiming for sats 96-100%) is associated with increased risk of death in acute illness
- increased reactive oxygen species leading to cellular damage or death
- systemic vasoconstriction (inc. cerebral vasoconstriction), leading to organ hypoperfusion
- false reassurance: respiratory deteriorations may be detected later if a patient is left on high-flow oxygen (as it would require a more significant deterioration to desaturate on high-flow oxygen compared to lower-flow oxygen)