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Fires cause burns and these injuries are obvious but injuries to the lungs and airways from smoke inhalation are often less apparent and may not present until 24-36 hours after exposure. In 2011-2012, being overcome by gas, smoke or toxic fumes was partly or wholly the cause of death in 53% of fatalities. A further 19% were due to a combination of burns and being overcome by gas or smoke.
A study from North West England found that the overall admission rate to hospital for smoke inhalation or burns was 0.29/1,000 population per year. Another found that 10-30% of all burns admissions had smoke inhalation injury. The groups most at risk are the under-5s and the over-75s, with a male-to-female preponderance of about 2:1. Mortality is highest in the elderly and risk increases with social deprivation.
Risks are increased by being in a confined space, by the duration of exposure, by substances being burned that may emit various poisons and by pre-existing respiratory disease.
Mechanism of injury
There are three basic ways that damage occurs. Heat causes thermal damage, gases cause asphyxiation and there may be irritation of the lungs or airways.
Generally, heat damage is limited to the oropharynx, as heat is soon dissipated but steam and explosions may carry it rather deeper.
Asphyxiation results in tissue hypoxia. The fire may take a significant amount of oxygen from the air leaving only 10-13% oxygen, giving a pO2 of barely half the normal value. Even small amounts of carbon monoxide (CO) will aggravate the situation, as the gas has 200 times the affinity of oxygen for haemoglobin so that an atmosphere of 21% oxygen and 0.1% CO will cause the blood to leave the lungs 50% saturated with oxyhaemoglobin and 50% saturated with carboxyhaemoglobin.
CO also binds to myocardial myoglobin and reduces the contractility of the heart. Combustion of plastics, polyurethane, wool, silk, nylon, nitriles, rubber and paper products can all lead to cyanide production. Only 20-40% of the population carry the gene needed to smell hydrogen cyanide. It has an almond-like odour. It is 20 times as toxic as CO and can cause immediate respiratory arrest.If patients with smoke inhalation have cardiovascular complications then cyanide poisoning should be considered.
Irritation of the lungs and airways will incite an inflammatory response with bronchospasm and an outpouring of fluid. This seems very much like the inflammatory response in acute asthma. Aerolised steroids may be of benefit but further research is required.Chlorine and oxides of sulphur and nitrogen cause acids, whilst ammonia is alkaline. Highly soluble substances tend to aggravate the upper airways but less soluble poisons like phosgene and nitrogen oxides reach the lungs. Airways can be blocked by soot particles along with bronchospasm and the outpouring of inflammatory exudate. Pulmonary oedema may take a while to develop and so observation after smoke injury is important.
- Look at the patient. Check whether breathing is normal or laboured. Note whether there is cyanosis. Note whether the chest wall moves normally and symmetrically.
- Assess the airway but, if there is any risk of cervical spine trauma, be careful with the neck.
- Note respiratory rate. Listen to the chest.
- Note level of consciousness, pulse rate, blood pressure and peripheral circulation.
- If the patient is not fully conscious and alert the Glasgow Coma Scale should be employed.
- Note any injuries and burns, undressing the patient as required and possibly removing smoldering clothes. Check the back too.
- Respiratory assessment is required in anyone with possible smoke injury. It may form a baseline, as conditions can deteriorate after rescue.
- Check for signs of deteriorating respiratory function and treat aggressively before the situation becomes desperate.
- Hoarseness and change in the voice may herald serious problems and tachypnoea is a bad sign.
- Black sputum suggests excessive exposure to soot .
- Note rhonchi, rales, wheeze and use of accessory muscles of respiration.
- Facial burns show nearness to the fire. Other burns demonstrate an inability to escape.
- A baseline CXR may be useful for comparison if pulmonary oedema ensues. Early CXR is often normal and a normal film should not give too much reassurance. Later features can include atelectasis, pulmonary oedema and acute respiratory distress syndrome.
- Blood gases should be performed, including carboxyhaemoglobin and acid/base balance.
- A pulse oximeter may give false readings by interpreting carboxyhaemoglobin as oxyhaemoglobin. Co-oximetry, a 4-wavelength technique of light refractance to measure carboxyhaemoglobin and oxyhemoglobin accurately, gives a more accurate assessment.
- U&E and creatinine as a baseline are also important if there are substantial burns or crush injuries.
- ECG may show evidence of cardiac ischaemia, especially after cyanide exposure .
- Bronchoscopy may be very useful in identifying erythema, oedema, ulceration, the presence of carbonaceous material and atelectasis.
- Cyanide may be emitted in household fires. Diagnosis of cyanide poisoning may be difficult in the early stages. An elevated plasma lactate may be found in asphyxiation, under-resuscitation, CO poisoning or associated traumatic injury.[5, 11]However, a serum lactate level above 10 mmol/L is sensitive and specific for cyanide poisoning in fire victims without burns.
- Immediate management at the scene involves extracting the patient as rapidly as is safe, bearing in mind other possible injuries and getting out into the fresh air. Then (and only once clear of the fire!) oxygen should be given at high flow rate and humidified. Establish venous access, assess briefly and then transport with the minimum of delay. The most experienced people at dealing with smoke injury are in a burns unit and there may well be burns too.
- A patient who has suffered smoke inhalation should be assumed to have CO poisoning and be treated accordingly. High flow 100% O2 significantly reduces the half life of CO in the blood. CO poisoning is responsible for most of the deaths which occur before reaching hospital. If it causes cardiac arrest, the chance of resuscitation is extremely poor. A systematic review highlighted that further research is needed about the mode of delivery of hyperbaric oxygen and mooted the possibility of portable devices, especially in mild cases.
- One study found that of 41 patients with smoke inhalation injury, 8 required intubation. Intubation was positively correlated with physical examination findings of soot in the oral cavity, facial burns and body burns.If there is a significant risk of intubation being required it should be performed early or else oedema may make it technically more difficult or impossible. Damage to the mucosa of the trachea makes it more vulnerable and so the endotracheal tube cuff should not be over-inflated. Even allow a little leakage.
- Once in hospital, intubation may need to be supported by mechanical ventilation. When instituting ventilation a balance has to be struck between providing sufficient oxygenation and causing as little collateral harm as possible.Lower tidal volumes and plateau pressures with high positive end-expiratory pressure (PEEP) and permissive hypercapnia have become widely accepted management regimes for patients with acute lung injury resulting from smoke inhalation.High-frequency ventilation has also been found helpful.
- Inhalation injury is not always associated with an increased requirement for fluids unless other burns injuries are present. There is a danger that over-replacement of fluid can increase the risk of pulmonary oedema. Fluid resuscitation should be guided by urine output and hemodynamic parameters of the individual patient.
- Inhaled anticoagulation regimens (eg, nebulised or aerosolised heparin, heparinoids, antithrombins, or fibrinolytics) are increasingly being used to manage smoke inhalation-associated acute lung injury.
- Prophylactic antibiotics may help to reduce mortality.
- The treatment of cyanide poisoning has traditionally been with amyl and sodium nitrite. These create methaemoglobinaemia which can be dangerous when there is CO poisoning too. Increasingly, they are being substituted with hydroxocobalamin, which is safe enough to be used in pre-hospital care.
- Patients with acute asthma or chronic obstructive pulmonary disease may develop bronchospasm that needs treating in the usual way.
Patients who have suffered smoke inhalation but are not definite candidates for admission should be monitored in A&E for 4-6 hours before discharge. The following point to the need for admission:
- Exposure in a closed space for more than 10 minutes.
- Thick, black sputum.
- PaO2 below 8 kPa (60 mm Hg) or metabolic acidosis.
- Carboxyhaemoglobin above 15%.
- Arteriovenous oxygen difference (on 100% oxygen) greater than 13.33 kPa (100 mm Hg).
- Burns to the face.
- One American study looking at children aged 10-18 years, admitted with inhalation injury over a 10-year period (a sample of 850), reported a mortality rate of 16.4%.A study of elderly patients with burns showed that smoke injury increased mortality by 400%.This may be in part because of more severe exposure but, the presence of burns with smoke injury, means that the patient has to be treated more seriously.
- Subglottic stenosis, bronchiectasis, pulmonary oedema and atelectasis can occur.
- It seems quite possible that smoke injury may trigger susceptibility to asthma. Polyvinyl chloride (PVC) is a particular risk.
- Do not rely on CO levels in A&E to assess severity of exposure. The level at the scene may have been more relevant but beware of complications taking many hours to arrive.
- If a patient is discharged they must be given written instructions on the warning signs which would indicate the need to return immediately. Those with normal vital signs and examination and with short exposure may safely be discharged. The experience from disaster triage involving smoke inhalation is that patients without dermal burns and with normal bronchoscopy, chest radiography and normal blood gases (including carboxyhaemoglobin levels) have a low risk of complications.
Due to its relative rarity, information on prognosis is not abundant. However, in one series of 96 patients, 13 patients developed immediate respiratory failure resulting from ventilatory insufficiency, 4 patients died and vocal cord and tracheal stenosis were noted in 5 patients and 1 patient respectively. In patients with reduction in pulmonary function, improvement was seen after three months, with no further changes being observed within the subsequent three months. An American review quoted an increased mortality by a maximum of 20% over that predicted by age and extent of cutaneous burn alone. Pneumonia in these patients further increased mortality by a maximum of 40%. One estimate suggested that 75% of deaths following burn injury could be accounted for by inhalation injury but more recent cohort studies have suggested a decreasing mortality attributable to this cause.A study of 13 patients suffering from isolated inhalation injury who required mechanical ventilation were all independent with activities of daily living at eight months. All were back to work, except for two who continued to need artificial airways.
The prevention of smoke injury is largely the prevention of fire but, if it does occur, then early warning is necessary. Smoke detectors save lives. An American study showed an 80% drop in fire related morbidity and mortality in a high-risk area, However, alarms only work if an effective battery is in situ and many people are lax about checking this. Even those less likely to respond so swiftly to an alarm, like the very young, the elderly, the infirm and those intoxicated by drugs or alcohol, may benefit.Programmes to give away smoke alarms have not been randomised and American experience suggests that the batteries are not kept in order. One study suggests that the use of an alarm which plays a recording of the parent's voice is more likely to wake a child than a conventional residential tone smoke alarm. Initiatives to increase the uptake of alarms, such as incorporating them into child surveillance programmes, require further evaluation. Alarms which have a low 'nuisance' level (eg, which do not sound unnecessarily) seem to provide the most effective prevention.
Deaths in children aged under 5 are sometimes associated with fire play and these are not usually prevented by smoke alarms, due to the behaviour of the children. Interventions to prevent fire play in this age group may be more successful.
The choice of household furnishings is important in terms of risk of emission of toxic gases on burning as well as combustibility. There are relevant laws about materials that may be used in the manufacture of furniture.
Further reading and references
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