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We would like to comment on Scrase and Woollard's review of ventilation via needle and surgical cricothyroidotomy [1]. They recommend ‘that needle cricothyroidotomy with low pressure ventilation systems are no longer used… (in) out-of-hospital scenarios’. We agree. Nevertheless, the practicalities, physics and physiology of ventilation through a cricothyroidotomy they presented seem muddled.

We believe there should be a common understanding for terms used in this field and the authors' terminology could be misunderstood. They refer to a ‘jet ventilator’ for ventilation through a ‘needle (cannula) airway’ and state ‘the term “jet ventilation” is sometimes inaccurately applied to low pressure systems’. They apply these terms to a high pressure gas source delivering conventional ventilation (normal tidal volumes at a relatively low frequency). A high pressure source is needed to overcome the resistance of a narrow cannula. However, the pressure changes in the trachea during this type of ventilation are similar to conventional ventilation [2]. This type of ventilation should not be confused with ‘jet ventilation’ via a cannula with very low tidal volumes (a few millilitres), at high frequency (usually > 100 breaths per minute), which comparatively reduces peak intratracheal pressure (high frequency jet ventilation). We believe the use of the term ‘jet ventilation’ to describe conventional ventilation delivered by a high pressure source is potentially confusing, as is the term ‘jet ventilator’. In the context of ventilation after emergency cricothyroidotomy, the terms ‘volume ventilation’ and ‘injector’ may be preferable.

Similarly, we believe that the authors' grouping of oxygen sources into ‘low’ or ‘high’ pressure is misleading. They define a ‘high pressure’ source as ‘up to 50 psi’ and administration of ‘15 l.min−1’ as ‘a low pressure system’. Oxygen at 15 l.min−1 is most commonly sourced from cylinders or ‘wall oxygen’ in hospitals. Both are sourced by oxygen at 400 kPa (4 atmospheres, 4 bar, 58 psi) and are therefore high pressure sources. An oxygen ‘injector’ such as a Sanders injector also has a source pressure of 4 atmospheres. However, the driving pressure of modern ‘injectors’, such as the Manujet (VBM GmbH, Sulz, Germany), can be set from low to high (0.5–4 atmospheres) and 1 atmosphere (14.5 psi) from a Manujet is entirely adequate to achieve lung inflation via a small cricothyroid catheter in a healthy adult. We believe Scrase and Woollard have confused high and low pressure systems with high and low flow. They infer that oxygen at 15 l.min−1 cannot provide adequate ventilation. Although an ‘injector’ may deliver in excess of 1000 ml.s−1[3], ‘wall oxygen’ or cylinders set to 15 l.min−1 can deliver 250 ml.s−1. Therefore both can inflate the chest by 500 ml within 0.5–2 s.

We believe the important difference between the ability of a system to inflate the chest and provide ventilation should be clear and emphasised. When ventilating a patient with complete upper airway obstruction using a cricothyroid cannula, the critical aspect is not lung inflation but lung deflation. This is the limit both to generating adequate minute ventilation and ensuring patient safety. Exhalation of 500 ml via a 14G cannula takes at least 30 s [4] and is of no practical use. Delivering further breaths without allowing full expiration will inevitably lead to barotrauma, which can be rapidly life-threatening [5]. However, in cases of incomplete obstruction the amount of exhalation through the upper airway may be greater than expected and inflation via a cannula might be safe if administered with care. During ventilation via the patient's normal airway, the upper airway is drawn inwards in inspiration and tends to collapse. However, in expiration, positive intraluminal pressure tends to expand and open the upper airway [6]. As a result, inspiratory airway obstruction is more common than expiratory. In ‘can’t intubate, can't ventilate' (CICV) crises the patient's airway is often sufficiently patent during expiration to allow exhalation. In one series, 86% of patients with CICV retained a patent expiratory route [7]. This implies that in the majority of cases of upper airway obstruction, safe transtracheal ventilation with an injector will produce rapid inspiration followed by slow low pressure exhalation. Delivery of oxygen at 15 l.min−1 from wall or cylinder would achieve slower inspiration, also followed by slow low pressure exhalation. Increasing degrees of expiratory obstruction would lead to ever slower exhalation. However, provided that the operator observed the chest to fall carefully, ventilation could safely continue, albeit at a slower rate and with a reduced minute ventilation.

The authors conclude that ventilation using a 15 l.min−1 oxygen flow will inevitably fail and suggest it ‘should no longer be taught’. We disagree with their conclusions and do not believe the literature supports their view. We agree that use of a surgical airway overcomes all the difficulties associated with small cannulae and that this technique should be more widely taught and deployed.

Of less relevance perhaps to emergency physicians, but crucial to anaesthetists, is oxygen delivery from an anaesthetic machine, which can deliver 15 l.min−1 via a flowmeter and 30–60 l.min−1 when the oxygen flush is depressed. Scrase and Wollard quote Craven and Vanner [8] as demonstrating failure of a ‘low pressure’ system and success of a ‘high pressure’ system. The ‘high pressure’ system was an injector driven by 2 atmospheres pressure (200 kPa). However, all other systems were attached to the common gas outlet of the anaesthetic machine. Anaesthetic machine flushes achieve pressures of only 30–60 kPa (0.3–0.6 atmospheres, 4.35–8.7 psi) and an attached anaesthetic breathing system with a conventional reservoir bag only 6 kPa (0.87 psi). The back bar ‘blows off’ at 40–60 kPa depending on the modernity of the machine. These are all arguably ‘low pressure’ sources. Similarly, when Ryder et al. [4] tested four systems for inflating the lungs via a 14 G cannula, all except the injector were driven from a Boyles machine oxygen flush. Neither study used oxygen at 4 atmospheres (wall or cylinder) as a source. Neither is useful therefore in drawing conclusions about the author-defined ‘low pressure’ systems driven by oxygen at 4 atmospheres.

To summarise, there are three methods of achieving oxygenation and ventilation via a cricothyroidotomy.

  • • 
    Small cannula (2–3 mm) techniques (usually inserted as cannula over needle) require a high pressure gas source to overcome device resistance and rely on a patent upper airway for exhalation. Entrainment may augment inspiratory flow.
  • • 
    Large cannula (> 4 mm) techniques (usually inserted using a Seldinger technique) enable ventilation with lower pressures but require either a cuff, or the upper airway to be obstructed, to prevent loss of driving gas through the upper airway. Entrainment is minimal or non-existent.
  • • 
    Surgical airway techniques allow placement of a large tracheal tube and, if cuffed, allow conventional (low pressure) ventilation.

The fact that Scrase and Woollard's paper achieved publication without clarification of these issues is indicative of the confusing nature of this topic and a lack of general understanding. Safe management of CICV is dependent on prompt decision-making, skilled deployment of correctly selected equipment and correct application of ventilation. This requires clear, accurate education: something that current airway training for UK anaesthetists may not always achieve.

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