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Correspondence Oxygen cylinder fire – an update Colleagues may be interested to know that an interim report has been prepared, following the investigation by the Health and Safety Executive (HSE) and BOC Ltd, into the fire that we experienced on our intensive care unit (ICU) in Bath in 2011 [1]. A CD-sized medical oxygen cylinder caught fire whilst being prepared for a patient who was being transferred to another hospital. The oxygen valve was badly burnt in the fire, destroying any of the evidence that may have helped to identify the cause of the ignition. This means that the exact cause of the fire may never be known. Although the HSE have closed their case, BOC and GCE (the valve manufacturer) are continuing with their investigation to try and identify any possible factors that may have caused the cylinder to ignite. The interim report was prepared at the request of our local Coroner, as part of her investigation into the death of the patient who suffered burns in the fire [2]. The investigation found that the fire appeared to start inside the
cylinder valve, rather than being caused by an external factor. This is supported by witness descriptions of the event, with sparks seen coming from the valve outlet before it caught fire, and from the extent of the damage to the internal components of the cylinder valve. It is probable that many factors were involved in the exact chain of events, and it must be emphasised that all causes of the fire suggested below are speculative. To provide an insight into the investigation, it should be noted that a fire requires three elements to all be present: a fuel (something that will burn); a source of ignition (such as a naked flame, spark or elevated temperature to start the fire); and oxygen (to support the ignition).
The fuel Within a medical cylinder, there are a number of ‘fuels’ that could burn. These include the non-metallic components, such as the O-rings, nylon seats and even the metal components (which will burn if heated to a high enough temperature). The non-metallic components are essential for the valve to function both correctly and reliably, and, to mini-
mise the chance of ignition, these components are carefully selected for safe use within the medical cylinder valves. These components are extensively tested in a high-pressure oxygen environment as part of the CE marking of the valve. Even under potential adverse operating conditions, they are normally perfectly safe. Entry of other possible external contaminant fuels was also considered during the investigation, but thought to be highly unlikely. Specifically, alcohol hand gels appear not to have been a likely factor, seeing as the fire started within the cylinder valve. A small quantity of PTFE tape (used to make a gas-tight seal when fitting the valve to the cylinder) and a small piece of carbon fibre material were also found in the valve filter, but neither of these were thought to be the cause of the ignition.
The source of the ignition The O-rings, valve seats and the lubricants used in the valve all have an auto-ignition temperature (AIT), which is the temperature at which the material will spontaneously ignite. The valve components will all have an AIT above 300 °C in a
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100% oxygen environment. However, if under normal operating conditions the temperature in the valve does rise above the AIT (which may have occurred in the Bath oxygen cylinder fire), they can ignite. This, in turn, will release energy that could ignite adjacent materials that have a higher AIT. In this way, a ‘kindling chain’ can be started in the valve, escalating the fire to the point where the integrity of the valve is breached. Adiabatic heating of the gas, which would naturally raise the temperature of the gas in the valve, was considered as a possible cause of the high temperature. Adiabatic heating occurs when gas, flowing through the valve, comes up to a dead end or bend in the gas passage. The instantaneous compression of the gas as the molecules hit the dead end causes the temperature to rise instantaneously. This may lead to an ignition, by providing sufficient heat to raise the temperature above the AIT limits for the adjacent components. This mechanism is thought to be extremely unlikely as the flow rate selected was only 2 l.min 1. Having eliminated adiabatic compression as the source of the ignition, the only other potential causes could have been: a) friction of moving parts within the valve b) the impingement of a particle on a surface with a relatively low AIT. As the gas flows through the narrow passages in the valve, it tends to pick up any small particles that may be present. When one of these particles hits any sur512
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face within the valve it will generate heat. The resultant increase in temperature would have been very ‘local’, but would have been sufficient to cause a ‘fuel source’ to ignite. This potentially could have then led to adjacent materials burning in the ‘kindle chain’. c) contamination, such as a hydrocarbon. The presence of a contaminant can also reduce the AIT of the components used in the valve.
The oxygen Increasing the concentration of oxygen present will also lead to the lowering of the AIT of the valve components, and increase the speed of the fire. Oxygen was clearly present in the highest concentration (100%), which would have caused the fire to start more easily and to spread much more quickly than if it had been in air. With regard to the fire in Bath, it is likely that the source of ignition was most probably a combination of a number of factors. Hence, a particle impinging on a contaminated component in the valve, that had an elevated temperature due to friction, may have led to the initial ignition. This could then have ignited an adjacent O-ring or seat material within the valve, possibly in combination with the lubricant on the O-ring. Depending on where the initial ignition occurs, the ‘fuel’ can either be consumed quickly (with minimal consequences), or lead to other components (such as the valve’s metal body) catching fire. In the Bath incident, the brass
in the regulator seat burnt right through, creating a large hole. Although brass does not burn very easily, there would have been enough damage to allow the highpressure gas to vent into the lowpressure part of the valve. Once the high pressure oxygen escaped into the regulator’s low pressure chambers, the relief valve (which protects the regulator from high pressures) would have opened and allowed large quantities of oxygen to vent very quickly. With the sparks from the initial burning of the brass, this would have set the plastic guard alight, leading to ignition of the bedding. As there was a significant amount of oxygen present in the bedding, this would have burnt ferociously, so much so that the fire spread to the curtains around the bed space and the ceiling tiles above, and, when the cylinder was pushed to the floor, to the floor tiles around the bed. An issue with fires of this nature is that large quantities of toxic fumes are generated, potentially affecting other patients and healthcare staff within the area. The Coroner’s inquest into the death of the patient who was burnt as a result of the fire took place in July 2013. The Coroner recorded a verdict of “death due to natural causes, contributed to by the effects of the burns she suffered in the fire” [2]. Within the last four years, three other fires involving CD oxygen cylinders have been reported in the UK, which all appear to have had many similarities with the Bath incident [3]. They all followed a very similar chain of events, but no-one was injured in any of these
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other incidents. Their investigations have all shown that the damage to each valve was very similar to the Bath incident, but that the actual cause has still not been identified. F. E. Kelly R. Hardy Royal United Hospital Bath, UK Email: fi
[email protected] P. Henrys BOC Ltd Guildford, UK No external funding and no competing interests declared. Previously posted on the Anaesthesia correspondence website: www.anaesthesia correspondence.com.
References 1. Kelly FE, Hardy R, Hall EA, et al. Fire on an intensive care unit caused by an oxygen cylinder. Anaesthesia 2013; 68: 102–4. 2. Tremelling L. Coroner decides terrifying RUH fire contributed to death of OAP. http://www.bathchronicle.co.uk/Coronerdecides-terrifying-RUH-contributed-death/ story-19509150-detail/story.html (accessed 07/03/14). 3. Safe Anaesthesia Liason Group: promoting fire safety on intensive care and in theatre. https://www.rcoa.ac.uk/system/ files/SALG-FIRE-SAFETY_0.pdf (accessed 07/03/2014). doi:10.1111/anae.12698
Anaesthetists and accurate database recording We read with interest the article by White et al. [1] on outcome by mode of anaesthesia for hip fracture surgery and would like to discuss our experience from the recent Anaesthesia Sprint Audit Project (ASAP).
During ASAP, additional consultant anaesthetist-entered data about type of anaesthesia were uploaded to the National Hip Fracture Database (NHFD), in addition to the standard NHFD dataset, entered by an orthopaedic research co-ordinator, within which the type of anaesthesia was also recorded, therefore allowing us to compare data accuracy by both method. During the ASAP period, we uploaded full data for 100% of eligible cases, but found a discrepancy rate between the ASAP and NHFD data for type of anaesthesia involving 4/101 (4%) patients. Two patients were recorded as having had general anaesthesia when they received spinal anaesthesia, one (who died within 30 days postoperatively) was recorded as having had spinal anaesthesia despite having received general anaesthesia, and one was recorded as having had general + spinal anaesthesia when they received general anaesthesia with nerve block. We agree with the authors, therefore, that it is not possible to rule out inaccurate data collection as one of the causes for failure to demonstrate important differences between various anaesthetic techniques. Our experience reiterates the concerns of others [2–4] who have stressed the importance of active involvement of anaesthetists in order to reduce inaccuracies in the data collection and entry process. However, data collection and entry for just the three months of the ASAP collection period involved a considerable amount of consultant anaesthetic time, such as is probably unfeasible in the longer term.
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The aim of ASAP is to strengthen the evidence base for hip fracture anaesthesia, but the conclusions and future recommendations will rely heavily upon the accuracy of the data entered. D. B. Jumani Warrington & Halton Hospitals NHS Foundation Trust Warrington, UK Email:
[email protected] S. H. McClelland Aintree University Hospital NHS Trust Liverpool, UK No external funding and no competing interests declared. Previously posted on the Anaesthesia correspondence website: www.anaesthesia correspondence.com.
References 1. White SM, Moppett K, Griffiths R. Outcome by mode of anaesthesia for hip fracture surgery. An observational audit of 65 535 patients in a national dataset. Anaesthesia 2014; 69: 224–30. 2. Howes BW, Clarke PA, Cook TM. The National Joint Registry may fail to collect accurate, validated anaesthetic data. Anaesthesia 2009; 64: 694–5. 3. Cook T. Anaesthetists engagement in National Joint Registry data collection. Anaesthesia 2014; 69: 180. 4. Sessler DI. Big Data – and its contributions to peri-operative medicine. Anaesthesia 2014; 69: 100–5. doi:10.1111/anae.12673
Big Data – of the people, for the people, by the people We share Dr Sessler’s enthusiasm for Big Data [1], and welcome such
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