21. Carbon Monoxide Poisoning

Question 1

You are asked to see a male patient who has been found in his garage
with the engine of his car running. His face is cherry red.

What is the likely diagnosis?

Answer 1

The patient is likely to be suffering from carbon monoxide poisoning.

It must not be forgotten that there could be co-existent
problems such as self-poisoning with tablet / alcohol

Question 2

What is carbon monoxide poisoning?

Answer 2

The usual definition is a level of greater than 20% carboxyhaemoglobin in the
blood.

Co-oximetry is needed to make the diagnosis.

Symptoms are:

20−30%
Throbbing headache
Irritability
Fatiguability
Poor judgement
Nausea and vomiting

30−40%
Confusion
Syncope on exertion

> 50%
Coma
Convulsions
Death

Question 3

How does carbon monoxide poisoning affect O2 delivery at tissue level?

Answer 3

CO and O2 both bind to the a-chain of the haemoglobin molecule, but CO has
about 250 times the affinity for the ferrous iron complex compared with O2.
This is a competitive and reversible effect.

Tissue oxygen delivery is affected in two ways:

1. There is a reduction in the availability of oxygen-binding sites and
   therefore a reduction in oxygen carrying capacity.
2. CO interacts with the remaining haemoglobin molecule to increase the
   affinity for the oxygen it carries.
   This results in a left shift of the
   oxyhaemoglobin dissociation curve such that haemoglobin is less keen to
   give up oxygen at tissue level.

Thus, oxygen delivery is further reduced.

Question 4

How would you manage carbon monoxide poisoning?

Answer 4

Resuscitation should always follow the ABC approach.

The primary objective is to reverse tissue hypoxia.
Removal of CO is of secondary importance.

Oxygen is the treatment that accomplishes both of these goals.

The patient should be given 100% oxygen.

In severe cases hyperbaric oxygen therapy has been used
(but is still controversial).

Hyperbaric oxygen therapy (HBO) can provide nearly
all the body’s oxygen needs purely from the dissolved oxygen.

Question 5

Poor prognostic indicators are:

Answer 5

Increased duration of exposure

Increasing age

Low GCS at time of admission.

Question 6

Current criteria for HBO therapy are:

Answer 6

Carboxyhaemoglobin >20%

Loss of consciousness at any stage

Cognitive impairment

Neurological signs and symptoms (except headache)

Myocardial ischaemia/arrhythmia

Pregnancy

Difficulty assessing (e.g. concurrent drug over dose).

Question 7

Transfer?

Answer 7

Co-existent problems such as other forms of self-poisoning or burns should
also be addressed.

The decision to transfer a patient to a hyperbaric chamber
(usually in a different hospital) must take into account the
carboxyhaemoglobin level, the time needed to transfer the patient and the
patient’s fitness for transfer.

Question 8

What is the half-life of COHb in air, 100% O2 and hyperbaric O2?

Answer 8

Air (21% O2) 240−300 minutes

100% O2 (1 atm) 40−80 minutes

100% O2 (3 atm) 23−25 minutes

As can be seen from these figures,
simply administering 100% oxygen will
significantly expedite the removal of carbon monoxide.