NAP4 – two years on

NAP4, the 4th National Audit Project of the Royal College of Anaesthetists (RCoA) and the Difficult Airway Society (DAS) on ‘Major Complications of Airway Management in the United Kingdom’, was published in March 2011.

Two years on, the key findings of the report continue to resonate. These include:

• A high failure rate of emergency cannula cricothyroidotomy
• Failure to correctly interpret a capnograph trace leading to several oesophageal intubations going unrecognised in anaesthesia.
• Numerous cases where awake fibreoptic intubation (AFOI) was indicated but was not used.
• Problems arising when difficult intubation was managed by multiple repeat attempts at intubation.

Poor airway assessment, a failure to plan for failure and poor judgement were also identified as key clinical themes in a number of the cases reported. Such is the breadth of NAP4, that the above represent little more than a highly selective short-list.

Interestingly, when the report was launched, one of the authors highlighted in a presentation on ‘Aspiration of gastric contents and of blood‘, which can be seen as a podcast on the RCoA web-site, that ‘there’s nothing new in NAP4’, referring to the fact that one of the major findings of the report (perhaps the major finding?), that aspiration of gastric contents was the single commonest cause of death in anaesthesia events, was also the finding of a report published in Anaesthesia way back in 1956 entitled, ‘Deaths associated with anaesthesia: A report on 1,000 cases‘. So, despite all the advances in airway management and anaesthesia over the last 50 years, aspiration remains a major concern.

For many, I am sure NAP4 did highlight a lot that was new, or at the very least, NAP4 probably provided evidence to support what logic and personal experience had suggested might be true. The ultimate judgement on the success of the report may only become evident in the years ahead, when it can be assessed what practical changes have been made in light of the many recommendations of the report.

A number of posters and abstracts at national and international conferences have already assessed or reported on changes implemented in their departments in the light of NAP4. At the annual DAS meeting in 2011, these included the following:

Audit comparing supraglottic airway (SAD) device use at a DGH to NAP4 guidelines. Thomas S – This poster reported on SAD use in obese patients, use in procedures with risk of reflux or previous difficult intubation and supraglottic training and awareness of NAP4.

Post NAP4 – Implications for intensive care nursing. Lamb RG et al – A report on a project looking at basic awareness of ICU nursing staff regarding Rapid Sequence Induction (RSI) and their familiarity with difficult airway equipment. The results were used to assess the need for further education of nursing staff who may be expected to assist with RSI.

Capnography use in ICU. Measuring up to NAP4. Cole S et al – A poster reporting on the results of a survey to measure capnography use in ICUs across Scotland and to describe factors influencing use.

Capnography use outside of theatres in the Northern Deanery before and after publication of NAP4. Metcalfe SE et al – An audit on use of capnography in the UK for patients undergoing anaesthesia and being intubated irrespective of location.

Some of the recommendations of NAP4, such as each department of anaesthesia having a ‘Departmental Airway Lead’, have long been advocated by the UK Difficult Airway Society. Many hospitals already have an airway lead, but following discussions between DAS and the RCoA, the college council has endorsed a strong recommendation that all anaesthesia departments should conform with this NAP4 recommendation. The responsibilities of the position should include, the overseeing of local airway training, ensuring local policies exist and are disseminated for predictable airway emergencies, liasing specifically with ICU and emergency departments to ensure consistency, and ensuring that difficult airway equipment is appropriate to the local guidelines and standardised within the organisation.

Further potential responsibilities have also been outlined. The RCoA intends to maintain a database of departmental airway leads. DAS also plans two follow-up surveys to study the impact of NAP4. A National survey of institutional responses to NAP4 and a national ‘sprint audit’ to collect national data on practice and activity over a short period. We await the results with interest. See the DAS Newsletter – Projects Edition December 2012 pp6-7 for further details

NAP5 – Accidental Awareness during General Anaesthesia in the United Kingdom, has just been published, but two years on from publication, its predecessor remains essential reading.

Classification of supraglottic airways

Early classification and scoring systems for supraglottic airways
In recent years, a number of attempts have been made to categorise supraglottic airways (SADs). In 2004, A Proposed Classification and Scoring System for Supraglottic Sealing Airways: A Brief Review by Miller was published in Anesthesia & Analgesia. This categorised SADs by the sealing mechanism. Three primary groups were identified:

• Cuffed perilaryngeal sealers – such as the Laryngeal Mask Airway®
• Cuffed pharyngeal sealers – such as the Cuffed Oropharyngeal Airway (COPA®)
• Uncuffed anatomically preshaped sealers – such as i-gel®

This classification was further subdivided, so cuffless perilaryngeal sealers could be either ‘directional’ or ‘non-directional’, and cuffed pharyngeal sealers could be designated as ‘with’ or ‘without’ oesophageal sealing. The sealing mechanisms were described in detail, in conjunction with force vectors, frictional force and whether a device was reusable or single-use; or incorporated a mechanism to provide additional protection against aspiration. Whatever the merits of this system, it now appears unnecessarily complex, although to be fair, Miller’s objective was limited to providing a consistent method for evaluating and understanding the mechanisms of action of any given SAD.

In the same paper, Miller provided a provisional scoring of airways, which suggested a number of desirable features appropriate for a SAD for routine use in anaesthesia. This included easy insertion by a non-specialist, stable airway once positioned, sufficient sealing quality to apply positive pressure ventilation, a good first-time insertion success rate, minimal associated risk of aspiration, and minimal risk of cross-infection and serious side effects. These attributes remain valid today, although to this list we could probably now add latex free and atraumatic, requiring minimal training for safe and effective use, incorporation of a bite block, MRI compatibility, and perhaps suitability for use as a conduit for intubation.

International standard for supralaryngeal airways
In 2009, five years after Miller’s paper, the International standard, ISO 11712:2009(E) Anaesthetic and respiratory equipment – Supralaryngeal airways and connectors was published. This standard included five classifications of supralaryngeal designs as follows:

• Cuffed oropharyngeal airway, where the ventilatory opening is located at the base of the tongue and a sealing surface is located in the oropharynx.

• Laryngeal masks, where the ventilatory opening is surrounded by the cuff, which forms a seal with the periglottic tissues. The ventilatory opening and the cuff seal usually represent the most distal portion of the device.

• Pharyngeal or pharyngeal-esophageal tube, where a cuff surrounds the ventilatory tube in a circumferential fashion and is located proximal to the ventilatory opening. This design compartmentalizes the pharynx, with the cuff serving as a sealing divider between the proximal and distal pharyngeal compartments, and the ventilatory opening(s) are located in the distal pharyngeal compartment.

• Pharyngeal airway liner, which is represented by the streamlined liner pharyngeal airway (SLIPA^TM). This is a shell-like device that, upon insertion, expands the soft tissues of the neck. The tension of the elastic neck soft tissues that surround the device provides the sealing mechanism. The ventilatory opening is located within the shell in the periglottic area.

• Device with a soft, gel-like, non-inflatable cuff and widened, concaved buccal cavity stabiliser. The sealing mechanism is created by the soft non-inflatable cuff accurately mirroring the anatomy of the laryngeal inlet to create an impression fit, without the need for cuff inflation.

 This classification did little to reduce complexity.

 1st and 2nd generation devices
In the same year as ISO 11712 was published,  White, Cook and Stoddart, in a review article published in  ‘Pediatric Anesthesia’, entitled, A critique of elective pediatric supraglottic airway devices categorised SADs into 1st and 2nd generation devices. A 1st generation device was described as a ‘simple airway tube’ and 2nd generation as a device that ‘incorporates specific design features to improve safety by protecting against regurgitation and aspiration’. I am not sure if this was the first published description of this method of classification, but its simplicity had immediate appeal, and it quickly became established as the most popular method for classifying SADs.

 It has since been used in numerous published clinical studies, review articles and conference lectures and a number of recommendations regarding use of 2nd generation devices were made in the 4th National Audit Project of the Royal College of Anaesthetists (RCoA) and The Difficult Airway Society (DAS) report, Major complications of airway management in the United Kingdom.

Of course, 2nd generation devices are not all the same, so the clinical evidence for each device regarding safety and efficacy still needs to be reviewed and assessed individually. Designation as a 2nd generation device does not in itself confirm superiority of performance, but the classification does provide useful information about basic product design characteristics, such as whether the device incorporates a mechanism for the management of regurgitant fluid.

In conclusion, although initial classifications of SADs provided some useful information, they were also complex, and as a result never really obtained widespread use or acceptance. The more recent classification of SADs into 1st or 2nd generation devices has proved popular, is widely used and provides valuable information regarding basic product design. Safety and efficacy of individual devices still needs to be reviewed and assessed individually.

A basic diagram highlighting the differences between 1st and 2nd generation devices is shown below (Fig 1). An Infographic, with additional background information, is also available (Fig 2). Please contact me if you would like a pdf copy of the infographic.

Fig 1

Fig 2 – Infographic