CO poisoning

Question 1

Oxygen

Common indications

Answer 1

1. To increase tissue oxygen delivery in states of hypoxaemia
2. To accelerate reabsorption of pleural gas in pneumothorax
3. To reduce the half-life of carboxyhaemoglobin in CO poisoning

Question 2

Oxygen

MOA

Answer 2

• An abnormally low Partial pressure of oxygen (PO2) in arterial blood (PaO2), termed hypoxaemia, may be a consequence of a wide range of disease processes
• Its effect is to reduce the delivery of oxygen to tissues (hypoxia), forcing them to use anaerobic metabolism for energy generation
• Supplemental oxygen therapy increases the PO2 in alveolar gas, driving more rapid diffusion of oxygen into the blood
• The resultant increase in PaO2, increase delivery of O2 to tissues, which in effect ‘buys time; while the underlying disease is corrected
• In Pneumothorax, supplemental oxygen therapy has an additional benefit of reducing the fraction of nitrogen in alveolar gas
• This accelerates the diffusion of nitrogen out of the body
• Since pleural air is composed mostly of nitrogen, this increases its rate of reabsorption
• In CO poisoning, oxygen competes with CO to bind with haemoglobin and thereby shortens the half-life of carboxyhaemoglobin, returning haemoglobin to a form that can again transport oxygen to tissues

Question 3

Oxygen

Important adverse effects

Answer 3

• The most common adverse effects are related to the delivery device (e.g. discomfort of the mask) or its lack of water vapour leading to dry mouth/throat
• The latter can be improved by using a humidification system
• Except in pneumothorax and CO poisoning, there is little to be gained from having abnormally high PaO2, there is some evidence that it may be harmful
• However, this concern should not lead you to withhold oxygen in critical illness or states of severe hypoxaemia in which oxygen is lifesaving

Question 4

Oxygen

Prescription

Answer 4

• Oxygen therapy should always be guided by a written prescription except in emergencies when it may initially be administered without a prescription
• The oxygen prescription is usually found on a dedicated section of the drug chart or a separate chart
• Its key feature is the target oxygen saturation range, as measured by pulse oximetry (SpO2)
• The target SpO2 should be 94-98% in most patients and 88-92% in those with chronic type 2 respiratory failure
• For the initial delivery device, in general, prescribe a reservoir mask in critical illness and patients with SpO2 <85%; a venturi (28%) for patients in chronic type 2 respiratory failure, and nasal cannulae for everyone else

Question 5

Oxygen

Monitoring

Answer 5

• SpO2 monitoring is essential in all patients receiving oxygen for acute illness
• The device and or flow rate should be adjusted as necessary to keep the SpO2 within the target range
• In addition, arterial blood gas measurement is essential in patients with a critical illness, those with chronic type 2 respiratory failure or at risk of hypercapnia and those with hypoxaemia that is unexpected, progressive or disproportionate to their illness