4 year old Mariska is brought in by ambulance with her mother after being rescued from a house fire. Neither appear to have sustained any significant injuries as they were woken by the screech of the smoke detector but your resident is worried about possible carbon monoxide poisoning and wants to do an arterial blood gas to rule it out.
- Carbon monoxide (CO) is an odourless, colourless, tasteless gas that preferentially binds to haemoglobin rendering it unavailable for oxygen transport.
- CO poisoning in children manifests in a similar fashion as it does in adults but occurs sooner.
- Capillary blood gases are a lot kinder than arterial blood gases.
- There is a lack of hard evidence on the best way to treat mild to moderate CO poisoning and no data on long term effects.
- Prevention is better than cure – fit a CO detector in your home.
Every year the newspapers publish heartbreaking headlines about children who have died as a result of carbon monoxide poisoning. Fortunately these tragic cases are rare. Approximately 450 adults die in the US every year as a result of accidental poisoning but there is no data on the actual number of cases in the paediatric population. There is also little data on the number of cases of exposure and any figure is liable to be a gross underestimate. It’s a problem that’s rarely on our list of differential diagnoses.
Symptoms can be very subtle and nondescript
- Weakness or clumsiness
- Nausea or vomiting
- Blurry vision
- Shortness of breath
- Flu-like illness
or more serious.
- Cardiac arrest
This retrospective case series from Ankara suggested that, as in adults, severity of symptoms relates to degree of exposure, with the worst symptoms to be found with levels greater than 25%. These affected kids were much more likely to present with neurological symptoms such as headache, syncope or seizures compared to children with lower levels of carboxyhaemoglobin. Because infants and children have a higher basal metabolic rate than adults they are likely to become symptomatic earlier than their parents. Fortunately this also means they are likely to recover more quickly.
There is also some evidence that poisoning can lead to potential long-term neuropsychiatric sequelae such as subtle personality changes and memory impairment but again paediatric data is lacking.
Carbon monoxide is a byproduct of the incomplete burning of carbon containing fuel. In most cases of domestic CO poisoning this is as a result of poor or unventilated space heaters that use kerosene or natural gas.
Carbon monoxide exerts its toxic effects via three mechanisms. It preferentially binds to haemoglobin reducing its oxygen binding capacity, it shifts the oxygen dissociation curve to the left thus inhibiting the release of bound oxygen in the periphery and it acts as a direct cellular toxin by impairing aerobic metabolism.
Remember that fetal haemoglobin has a greater affinity for carbon monoxide than adult haemoglobin and so neonates are particularly susceptible.
Oxygen saturation monitors can be falsely reassuring in the setting of CO poisoning. Carboxyhameoglobin (CoHb) levels may be detected directly from either an arterial or venous blood gas sample with a high degree of correlation between results. A capillary gas is equally as effective and a lot less painful. Although high levels may give you a diagnosis, partially treated cases may already have low levels.
Small, portable transcutaneous devices also exist but are not readily available in most EDs.
Whilst immediate first aid involves removing the child from the source, high flow oxygen is the mainstay of treatment. The half-life of COHb is 320 minutes whilst breathing room air and about 30-90 minutes with an FiO2 of 1.0. With hyperbaric oxygen therapy reduces this time even further to only about 15 minutes (at 2.5 ATM and 100% O2) though there is controversy over its utility. We can only extrapolate from adult data as age less than 18 has been an exclusion criteria in all of the major trials. Whilst there have been positive and negative results the 2011 Cochrane review summarised the results nicely – there’s not enough evidence either way to determine whether HBO reduces the incidence of delayed neuropsychiatric damage. In the case of moderate to severe poisoning it would be worth consulting your local hyperbaric service.
The two basic principles are avoiding it happening in the first place and detecting leaks early. The former can be done by using properly serviced heaters in well ventilated rooms. The latter can be done by fitting a ceiling mounted carbon monoxide detector. This device, similar to a smoke detector, emits an ear-piercing shriek if higher than expected levels of carbon monoxide are detected.
You convince your resident that although it is possible that Mariska had an elevated CO level when she was rescued, the investigation that they want to do is likely to be more harmful than helpful. You persuade them to do a capillary gas instead and are satisfied with an undetectable CO level.
HT to Charlotte Davies (@OneLongPlait) for the idea behind this post.
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