Selecting the wrong filter: a difficult patient safety issue

Her Majesty’s Coroner for East London issued a Prevention of Future Deaths (PFD) report following the deaths of two patients in April 2021 at the London Nightingale Hospital in incidents related to breathing system filters [1]. This article aims to highlight the main issues of which clinicians should be aware.

The inquest has not been completed at the time of writing and detailed information is not available. In both cases there was a serious incident where the wrong filter was found to have been used within the breathing system of an ICU ventilator; these two cases came within a cluster of similar incidents known to the Coroner. An independent expert advised that the “non-standardised colour coding used by manufacturers of these filters, the number of different types of filters with different names, the variable optimal position of the filters, and whether a wet or a dry breathing system is being used, results in an extremely confusing situation” and “The confusion over breathing system filters and HMEs is widespread among ICU staff”.

Heat and moisture exchange (HME) filter blockage has been reported by SALG [2]. A simple internet images search confirms the wide array of size, shape and colour of these devices, creating identification difficulties even for experts. One author took photographs of all filters identified at the Anaesthesia and Critical Care 2021 trade exhibition in London to demonstrate the variability (Figure 1). Major suppliers had different or little indication of filter type at the point of care. One company (Dräger, Lübeck, Germany) has colour coding, red and blue, and specific labelling (Figure 1 upper right), which is different to another (Intersurgical, Wokingham, UK) who use green and yellow but no labelling (Figure 1 upper middle). Other companies had unclear differentiation (Pall, Port Washington, NY, USA; Armstrong, Comber, Northern Ireland). Additionally, whether a HME or heated humidification is used, a viral/ bacterial filter is commonly placed at the expiratory outlet of the ventilator. Although there is a vast array of filter nomenclature and function, for simplicity in this article we will refer to: wet circuits using heated humidification; HME; and non-HME viral filters. Complicating matters further, there is a wide range of efficiency of HMEs and thus they may be designated for short- or longer-term use.

During this pandemic, incorrect filter selection and positioning in the breathing system has been more likely to occur because less experienced staff are working in high-stress unfamiliar environments, garbed in PPE, and potentially using unfamiliar ventilators not designed for ICU use. Many COVID-19 patients require prone positioning, making a tube blockage extremely challenging to correct in a timely manner.

There are two mechanisms related to filters that can contribute to a blockage.

1. The filter may block if a HME filter is placed in a wet circuit. A non-HME filter is likely to be safer, although it should be monitored. The NHSE issued a Patient Safety Alert in 2015 following 76 incidents where a HME had accidentally been left attached to a wet circuit [3]. They noted that, in some cases, nursing staff had noticed that patients were deteriorating but they could not identify the cause of the problem. They also noted that HMEs become obstructed for other reasons (e.g. occluded with sputum or through regular administration of nebulised drugs), and also there are incidents of dry and cold air being supplied to the patient as no artificial humidification device was attached to the ventilation system. They broadly recommended local review and education. Doyle et al. published a simulation study examining the problem of accidental dual humidification with a HME and a wet circuit [4]. These breathing systems developed rapid critical airway occlusion after a delay of > 10 and < 24 hours of use. In a survey of ICU clinical directors, 95% reported that both HMEs and heated humidifiers were used at the same time for different patients on their unit, and 25% recalled episodes where a dual humidification error had occurred [4]. In a further simulation study two years following the Patient Safety Alert, local educational efforts, and a near miss in the ICU, knowledge retention about the simultaneous use of two artificial humidification systems within the same breathing circuit was poor even among experienced staff from a pool who had received specific training [5]. When specifically asked to check an incorrect breathing system containing a HME and heated humidification, only 30% of trained staff recognised and undertook corrective measures.

2. Tracheal tube blockage with secretions. Lack of humidification predisposes to thickened secretions and tube blockage if either no filter or a viral/ bacterial filter is placed at the T-piece and there is no active circuit humidification. A high profile tragedy occurred when a heated humidifier was discontinued and no HME was used [6].

Figure 1 (above)

Figure 2a. (left) HME Filter must be at patient T-piece and active humidification must not be used.

Figure 2b. (right) Viral/bacterial filter must be EITHER at expiratory limb of ventilator OR at patient end for infection control ONLY if adequate active humidification provided.

Off-label use of circle systems

During the pandemic the ’off-label’ use of anaesthetic machines to ventilate critical care patients was common and necessary. This brought breathing system problems not usually seen in routine anaesthetic use, where continuous use of a single circuit and filter is typically measured in single-digit hours or less. Circle systems were often used, at times with low fresh gas flow to conserve oxygen. There was a desire to have viral filters at the T-piece to prevent potential aerosolisation and droplet spread during tubing disconnection, which required a choice between a HME or non-HME viral filter, increasing the intricacy of an already complex situation. A circle system at high gas flows supplies dry cold gas and requires a HME viral filter. A non-HME viral filter would predispose the patient to tracheal tube obstruction with secretions. A circle system at low flows becomes waterlogged over time and can normally tolerate a viral filter, but a HME will usually block within 24 h. A circle system at a ‘moderate’ flow rate would lie somewhere in between, making the choice of filter a matter of guesswork.

The existing lack of standardisation within and between manufacturers is likely to continue for some time because of the global nature of product manufacture and the slow rate at which changes can be made to international standards. The Association of Anaesthetists continues to make representations to manufacturers and standard-setting organisations via its long-established contacts, but in the meantime, clinicians need to understand the risks and put local procedures in place to mitigate these risks.

Conflict of interest: PY is developing and has a patent for a heat colour change HME and non-HME to assist with recognition of filter misplacement. Medovate Humidicare

Tim Meek
Consultant Anaesthetist
James Cook University Hospital, Middlesbrough

Twitter: @drtimmeek

Peter Young
Consultant Anaesthetist
Queen Elizabeth Hospital, Kings Lynn

Twitter: @peterjyoung101

References 

1. Courts and Tribunals Judiciary. Prevention of Future Deaths, Kishorkumar Patel and Kofi Aning, 2021. https://www.judiciary.uk/publications/kishorkumar-patel-and-kofi-aning/ (accessed 22/11/2021). 
2. Safe Anaesthesia Liaison Group. SALG Publications, July 2020 – March2021. https://www.salg.ac.uk/salg-publications/patient-safety-update/ (accessed 22/11/2021). 
3. NHS England. Patient safety alert: the risk of using different airway humidification devices simultaneously, 2015. https://www.england.nhs.uk/publication/patient-safety-alert-risk-using-different-airway-humidification-devices-simultaneously/ (accessed 22/11/2021). 
4. Doyle A, Mariyaselvam M, Wijewardena G, English N, Gent E, Young P. The simultaneous use of a heat and moisture exchanger and a heated humidifier causes critical airway occlusion in less than 24 hours. Journal of Critical Care 2015; 30: 863.e1-3. 
5. Patel V, Mariyaselvam MZA, Peutherer C, Young PJ. Accidental dual humidification in intensive care units: repeated alerts and system changes are not enough. Journal of Critical Care 2018; 47: 159-63. 
6. Courts and Tribunals Judiciary. Regulation 28: Report to prevent future deaths, 2017. https://www.judiciary.uk/wp-content/uploads/2017/02/Entenrman-2017-0011.pdf (accessed 22/11/2021).