Monday, 16 February 2015

Carbon Monoxide

I thought I knew about Carbon Monoxide poisoning, and then I went to an (R)CEM day - and learnt a whole lot more about it.

Numbers
4,000 attend ED with CO
200 hospitalised

50 die each year

Pathophysiology

CO binds to haemoglobin with an affinity of approximately 220 times that of oxygen. CO shifts the oxygen dissociation curve to the left. It also binds to myoglobin and mitochondrial cytochrome oxidase, impairing ATP production. Somehow it also binds to platelets so that nitric oxide gets released.

CO binds to fetal haemoglobin and shifts the already left-shifted fetal oxyhaemoglobin dissociation curve further to the left. The half-life of CO in the fetus is longer than in the Mum.
CO is produced continuously in the body as a by-product of haem breakdown. This leads to a normal baseline COHb concentration of about 0.5%. In pregnancy and haemolytic anaemias this can rise towards 5%. 

Animal research – dogs given COHb 65-70%. They died.
2/3 of blood removed, and replaced with already poisoned blood - they were fine. They were then injected with CO, and were also fine. 


- Cigarette smoking leads to COHb concentrations of up to about 12% in heavy smokers 
Non-smokers living away from urban areas have carboxyhaemoglobin concentrations of between 0.4% and 1.0%. 
In an urban or industrial setting, concentrations of up to 5% may be considered normal.

- Methylene chloride (dichloromethane), which is found in some paint strippers and sprays. Once in the liver, is converted to carbon monoxide. Methylene chloride is stored in body tissues and released gradually.

- Electric fires, fires, wood burning stoves, barbeques, shesha pipes gas - anything. 

Clinical Presentation
With prolonged exposure her symptoms could lead to hypotension, seizures, cerebral oedema, metabolic acidosis and respiratory failure.

4.3% of 1758 patients presenting to 4 EDs had raised COHb levels
Unsuspected positive cases 3.5%: 
COPD 7.5% 
Headache 6.3%
Flu-like 4.3% 
Chest pain 3.3% 
Seizures 2.1% 

More Severe Symptoms:
The appearance of intoxication or a personality change
Impaired mini mental-state examination
Vertigo and ataxia
Breathlessness and tachycardia
Chest pain (due to angina or myocardial infarction)
Loss of consciousness 
Seizure or multiple seizures
Abnormal neurological signs including blindness, deafness, and extrapyramidal effects. 
High risk features include chest pain, history of unconsciousness, any continuing neurological symptoms or signs (especially cerebellar features) and pregnancy due to the effects on the foetus

Chronic Presentation
Chronic fatigue
Emotional distress
Memory deficits
Difficulty concentrating
Sleep disturbances
Vertigo 
Neuropathy
Paraesthesias
Recurrent infection
Polycythaemia
Abdominal pain
Diarrhoea

Clinical Assessment

Neuro exam: including tests of coordination and balance, MMSE, short term memory for chronic.

Cherry red skin is only seen in severe poisoning, with levels >20.

Investigations
CO reading taken from a breath analyzer - lactose intolerant patients have raised H2 in their expired breath which can interfere with the readings. 
Venous blood should be taken into anti-coagulant and sent to the laboratory. 

Treatment
Administration of oxygen speeds the elimination of CO from the body. Without therapy, the elimination half life of CO is 4-6 hours. Administration of high flow oxygen by a tight fitting mask at normal atmospheric pressure reduces half life to approximately 76 mins. 
Get your NIV out – you just need the seal and the flow rates of O2 high enough to maintain 100% oxygen. Even manually holding a BVM over them with a good seal does the job.

There is debate about the added value provided by hyperbaric oxygen. A COHb concentration of >20% should be an indication to consider hyperbaric oxygen especially if has any other symptoms:
Loss of consciousness at any stage
Neurological signs other than headache
Myocardial ischaemia/arrhythmia diagnosed by ECG
The patient is pregnant

If metabolic acidosis persists despite correction of hypoxia and adequate fluid resuscitation consider correction with intravenous sodium bicarbonate.


I wrote a powerpoint presentation on this - ages ago - feel free to use and adjust... 


References
Toxbase

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