The operation had been a long, but it had been a success. The patient had been taken into the recovery room and was being looked after by theatre staff as they were slowly woken up. In theatre the anaesthetist was talking with colleagues about the operation.

Suddenly, a member of staff put their head through the door of the recovery room and looking at the anaesthetist said, “doctor there’s a problem in the recovery room.”

Upon entering the recovery room, the anaesthetist found that the patient, had started to recover but was biting down on the reinforced laryngeal mask airway (LMA). The anaesthetist tried to encourage the patient to stop biting, but that didn’t work. The patient bit right through the LMA and this part was removed from his mouth. Remarkably the patient could still breathe through the bitten off end. A few minutes later the patient had recovered enough to spit the remnants of the LMA out. The photographs below clearly show the aftermath. 

Thankfully, that was a good outcome both for the patient and the anaesthetist and their team. But there are two other scenarios that could have occurred:

• The patient could have broken their teeth and suffered dental damage. I wrote about this last year and pointed out the consequences both from a repair perspective and a financial one for the patient and the hospital.
• Another more serious scenario is that the patient obstructs the lumen of the LMA or the LMA blocks the upper airway. There is a real risk of desaturation and negative pressure pulmonary oedema. This is a dangerous and potentially fatal condition. Negative pressure pulmonary oedema (NPPE) or post obstruction pulmonary oedema (POPE) is a clinical entity of great relevance in anaesthesiology and intensive care. The presentation of NPPE can be immediate or delayed, which therefore necessitates immediate recognition and treatment by anyone directly involved in the perioperative care of a patient.⁽¹⁾

So, what do we know about negative pressure pulmonary oedema or Post Obstruction Pulmonary Oedema?

There are few studies in the public domain that look at the incidence of NPPE. The incidence of NPPE has been reported to be 0.05%–0.1% of all anaesthetic practices. However, it is suggested that it occurs more commonly than is generally documented. According to one estimate, NPPE develops in 11% of all patients requiring active intervention for acute upper airway obstruction ⁽²⁾ . In a small review of case reports where laryngeal mask is cited, 60% reported that the patient bit through the LMA and of that group ⅔ reported that the patient developed a pulmonary oedema ⁽³⁾ .

The review concluded, ”The vast majority of the papers found are case reports, though a single survey suggests that biting of an unguarded laryngeal mask airway (LMA) is not an uncommon event. Complications of biting include airway obstruction and the development of negative pressure pulmonary oedema, neither of which would be welcome events in the resuscitation area.”

In a U.K. national survey of the use of bite guards and critical incidents involving the laryngeal mask airway ⁽³⁾ a postal questionnaire was sent to 451 anaesthetists with a 42% response rate. 63% of consultants, 45% of SpRs and 43% of recovery staff never used a bite guard in conjunction with a laryngeal mask airway of any sort. However, biting of a laryngeal mask airway by a patient, resulting in airway obstruction, had been experienced by 18 users of the flexible laryngeal mask airway (7.3%) and 71 users of the standard laryngeal mask airway (18.8%).

The recovery staff reported an average of two incidents per month of laryngeal mask airway obstruction. The authors concluded that the use of a bite guard with a laryngeal mask airway is an uncommon practice but the occurrence of airway obstruction with the laryngeal mask airway is high.

An upper airway obstruction is the cause of negative pressure pulmonary oedema. A blocked or broken LMA caused by biting is one cause. Others include hanging, strangulation, upper airway tumours, foreign bodies, croup, choking, migration of Folly’s catheter balloon used to tamponade the nose in epistaxis, near drowning, goitre mononucleosis, big tonsils, hypertrophic adenoids, or a redundant uvula.

Once the upper airway is obstructed a very large, negative, intrathoracic pressure is generated by the patient’s increased effort to breathe. This causes pulmonary oedema or fluid build-up in the lungs resulting in acute respiratory failure. The onset of pulmonary oedema is usually rapid (within a few minutes after signs of upper airway obstruction). The patient will become agitated, may look frightened, will breathe rapidly, may become tachycardic, crackling sounds or rales may be heard with a stethoscope and pulmonary secretions become frothy and pink as progressive oxygen desaturation occurs.

Quick thinking and action are required to remove the blockage causing this emergency. If the blockage were caused by a broken LMA the patient would need to be rapidly re-anaesthetised and paralysed to allow the LMA to be removed. This would also allow reoxygenation to occur if the patient were desaturated. This intervention not only exposes the patient to more drugs but if desaturation carries on for long enough the situation can become an anaesthetic emergency. The Difficult Airway Society Guidelines for the management of tracheal extubation⁽⁴⁾ recommend the following for the management of negative pressure oedema.

1. Treat the cause: relieve the airway obstruction.
2. Administer 100% O2 with full facial CPAP mask. In addition to relieving upper airway obstruction, CPAP may reduce oedema formation by increasing mean intrathoracic pressure and minimise alveolar collapse by increasing functional residual capacity, improving gas exchange, and reducing the work of breathing.
3. Nurse the patient sitting upright.
4. If there is fulminant pulmonary oedema with critical hypoxaemia, tracheal intubation and mechanical ventilation with PEEP are necessary. Less severe hypoxia responds to supplemental oxygen and ⁄ or non-invasive ventilation, or CPAP.
5. Intravenous opioids may help reduce subjective dyspnoea.
6. Chest radiography may exclude other complications of difficult airway management and causes of hypoxia (gastric aspiration, pre-existing infection, pneumothorax, barotrauma, pulmonary collapse).
7. Frank haemoptysis may necessitate direct laryngoscopy and ⁄ or flexible bronchoscopy.
8. Diuretics are often administered, but their efficacy is unproven.

The Difficult Airway Society also comment,” Post-obstructive pulmonary oedema may be prevented through use of a bite block during emergence.”

And so, let us finally consider the economics of managing a patient who develops negative pressure oedema from biting through their LMA. The first thing to say is that the patient would probably need to spend more time recovering in hospital either in the recovery room, on a ward, HDU or even ICU. Further investigations such as a chest x-ray or blood gas analysis might be needed. Interventions as described in the Difficult airway Society Guidelines may also be required.

Uncovering the daily cost of a hospital bed is not easy and the data is quite old. A stay in a hospital bed without factoring in investigations and/or interventions would cost approximately $1800/day in the USA, $AUD1000/day in Australia and £400/day in the UK. Private healthcare charges would be higher. In most health care systems around the world the daily cost of an ICU bed is in 4 figures. In the USA it is approximately $6000/day, Australia approximately $AUD4000/day and the UK approximately £2000/day. A bite block such as BiteMe™ costs $1.48 per patient and would reduce the incidence of negative pressure pulmonary oedema resulting in fewer patients needing to spend extra time in ICU.

I leave you to make your own mind up when it comes to cost effectiveness.

So, what can we determine from this article?

• The incidence of NPPE is poorly understood and probably under reported.
• NPPE can result in acute respiratory failure which is a dangerous and potentially fatal condition.
• Biting through a laryngeal mask airway (LMA) is not an uncommon event.
• Despite being recommended by the Difficult Airway Society the use of a bite block with a laryngeal mask airway is not a common practice.
• Using a bite block in conjunction with an LMA would reduce the incidence of potentially fatal negative pressure pulmonary oedema caused by a patient biting through their LMA.
• Using a bite block such as BiteMe™ to prevent NPPE caused by the patient biting through the LMA and the upper airway becoming blocked is a more cost-effective option than having the patient spend extra time in ICU.

By using a specifically designed bite block such as BiteMe™. Which is made of a very strong, but soft, plastic that resists the shear forces of a human bite very well reduces the risk of desaturation and/or Negative pressure pulmonary oedema if the patient’s airway device becomes obstructed.

The combination of the soft plastic surrounding a closed air-filled space means that when a patient bites down, there are two forces opposing the bite. This means BiteMe™ has a spongy recoil and therefore reduces the risk of the patient severing the LMA if they start biting during emergence.

References

1. Bhaskar B, Fraser JF. Negative pressure pulmonary edema revisited: Pathophysiology and review of management. Saudi J Anaesth. 2011 Jul-Sep; 5(3): 308–313.
2. Tami TA, Chu F, Wildes TO, Kaplan M. Pulmonary edema and acute upper airway obstruction. Laryngoscope. 1986;96:506–9.
3. Heptinstall E, Heptinstall L. Should Bite Guards Be Used with Laryngeal Mask Airways In Adults? Best Evidence Topics Database (BestBETS). March 2015.
4. Popat M (Chairman),Mitchell V, Dravid R, Patel A, Swampillai C, Higgs A. Difficult Airway Society Guidelines for the management of tracheal extubation. Anaesthesia 2012, 67, 318–340

Author: Niall Shannon, European Business Manager, Innovgas

This article is based on research and opinion available in the public domain.

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