We read with interest the study by Hamaekers et al [1] on equipment currently used for oxygenation during complete airway obstruction. The authors commented that physical properties of such devices have major clinical consequences and that alternatives may be safer. We feel that their laboratory work does not support this assertion and that the venture is contrary to current guidelines [2].

The study measured pressure within a 2 mm trans-tracheal catheter during attachment to insufflation devices. Catheter flow during intermittent insufflation was then assessed. The first two devices used three-way taps to change from in-line to a side-on configuration that substantially reduced catheter-tip pressure. The authors branded the pressure with the in-line configuration (the currently prevalent technique) as potentially dangerous, but did not study this further.

However, the relevant pressure is the pressure within the trachea itself. This will be substantially below the value of 71.1 cm H₂O found within the catheter. Whilst egress down the catheter may be impeded, the risk of barotrauma was not quantified. Additionally, given that total upper airway occlusion is an extremely rare finding, elevated tracheal pressure from catheter insufflation is still likely to generate some pressure-release via the upper airway.

The side-on configuration used by the authors requires an additional section to join the catheter and tap Luer lock ports: the rarely seen male/male connector. This necessity is at odds with the need for simple, readily available equipment in the emergency situation.

The paper then presents timings for insufflation and for egress of 1000 ml of oxygen. No explanation is given for the choice of this large tidal volume or its clinical validity. Inclusion of either device increased egress time compared to an unconnected catheter. Egress time was longer with the side-on tap configuration than with the Oxygen Flow Modulator. No statistical comparison is offered for the respective minute volumes. This is surprising given the apparent efficiency of the Oxygen Flow Modulator for egress.

The authors conclude that modifying the in-line configuration of the three-way tap will make emergency jet ventilation safer. Their statement is not borne out by this study. Switching to a side-on configured tap is complicated by the need for a male/male connector. Switching to a novel device would be supported by it achieving a significantly superior minute volume across a range of clinically appropriate tidal volumes; this has yet to be demonstrated for the Oxygen Flow Modulator.

This study confirmed that bidirectional flow through a 2 mm trans-tracheal catheter in the laboratory is possible. Despite acknowledging the inevitability of hypercapnia in the clinical setting, the authors considered this a technique for slowly re-establishing oxygenation. Faced with an obstructed upper airway, guidelines indicate the need to convert to a definitive airway as soon as possible [2], and we suggest that the focus of management should be on minimising this duration.

L Cohen

H Morris

D Vaughan 

Northwick Park Hospital,

Middlesex, UK.

No external funding and no competing interests declared

References

1. Hamaekers AE, Borg PA, Enk D. A bench study of ventilation via two self-assembled jet devices and the Oxygen Flow Modulator in simulated upper airway obstruction. Anaesthesia 2009; 64: 1353–8.

2. Henderson JJ, Popat MT, Latto IP, Pearce AC. Difficult Airway Society guidelines for management of the unanticipated difficult intubation. Anaesthesia 2004; 59: 675–94.

We thank Dr. Cohen and colleagues for their letter. We agree that in the case of an obstructed upper airway, after placing of a small-bore transtracheal catheter, the focus should be on rapidly converting to a definitive large-bore airway. However, if the emergency ventilation device connected to the 2 mm inner diameter transtracheal catheter is able to act as a bidirectional airway, the patient may be oxygenated with limited risk of barotrauma and haemodynamic instability, whilst trying to establish a definitive airway.

The results of our bench study showed that the widely advised, self-assembled emergency ventilation device (based on a three-way stopcock connection to a transtracheal catheter) is unable to act as a bidirectional airway and is inherently dangerous. Connecting this self-assembled device to an oxygen flow of 15 l.min^-1 resulted in a continuous flow to the patient with a pressure of 71 cm H₂0 at the tip of the catheter. This was due to inadequate flow and pressure release through the open 2 mm inner diameter side port of the stopcock. This maximum pressure will only be reached in the trachea and lungs if the upper airway remains completely blocked. As our colleagues stated, total upper airway occlusion is a rare finding. However, in an emergency situation one does not know at what pressure the airway will open. This opening pressure depends on the anatomy and the presence of oedema or laryngospasm. Even an airway pressure of 30 cm H₂0 may lead to a decrease in cardiac output due to reduction in venous return [1]. Any increase in backpressure, resulting from insufficient flow release, will impede expiration and further reduce the achievable minute volume. We believe that an attempt to prevent pressure build-up during the expiratory phase is necessary.

By changing the configuration of the self-assembled device (connecting the transtracheal catheter to the side-port; device B in our study), any pressure build-up during the expiratory phase may be completely avoided and even may result in a slightly negative pressure at the tip of the catheter. We hypothesised that this negative pressure could facilitate expiration. However, linear regression modeling of the results of the 1 litre in- and expiration test (a volume chosen to minimize measurement inaccuracy) showed that expiration times of device B were significantly longer (p < 0.001) compared to passive backflow through the Oxygen Flow Modulator.

Our experimental laboratory work conforms to national guidelines and should not be misinterpreted as an attempt to contravene them. The Difficult Airway Society guidance regarding a ventilation system for a transtracheal catheter states: ‘an adjustable high pressure device (driven by gas pipeline pressure) with a Luer lock connection is recommended’ [2]. However, the need for readily available equipment should not mean we configure equipment which may be unsafe. In our opinion, self-assembled three-way stopcock-based ventilation devices should not be recommended; as better options are available.

Ankie Hamaekers

Pieter Borg

Dietmar Enk

Dr. Enk is the inventor of the Oxygen Flow Modulator and receives royalty payments from Cook. No other competing interests declared.

References

1. Jellinek H, Krafft P, Hiesmayr M, Steltzer H. Measurement of right ventricular performance during apnea in patients with acute lung injury. Journal of Trauma 1997; 42: 1062-7.

2. Henderson JJ, Popat MT, Latto IP, Pearce AC: Difficult Airway Society guidelines for management of the unanticipated difficult     intubation. Anaesthesia 2004; 59: 675-94.