Mortality due to anaesthesia

The UK is probably comparable to other developed countries where the mortality has been found to be approximately 1:10,000. UK anaesthetists will administer approximately 18,000–30,000 anaesthetics in their careers, so the chances of being involved in a tragedy are not negligible.

Causes of anaesthetic mortality

Mistakes or complications associated with drug administration cause the majority of deaths (approximately 40%). Deaths due to airway management have declined and in the last reported study from the USA, problems with airway management caused the same number of fatalities as central venous line insertion (20%).

Causes of airway mortality

The causes continue to be loss of airway during attempts at intubation, perforation of the pharynx, oesophagus or trachea, and oesophageal intubation. Rarer causes are aspiration of stomach contents, laryngospasm and bronchospasm.

Loss of airway at intubation

The mechanism is unclear but the scenario is well recognized –a patient, who was initially possible to ventilate with a mask, becomes impossible to ventilate after repeated attempts to intubate. Possible causes are swelling of tissues due to trauma, laryngospasm, operator fatigue and panic. The message is clear – do not make repeated fruitless attempts to intubate. Aim to make one ‘optimal best attempt’ with all factors properly arranged.

Perforation of pharynx/oesophagus or trachea

The cause of death is usually sepsis following the development of mediastinitis or cervical abscess. The mortality rate from mediastinitis is about 20%. Acute airway obstruction or respiratory failure can result from surgical oedema or pneumothorax. Only half of the perforations are recognized (pneumothorax or subcutaneous air) at the time. Perforation can occur during easy intubation, but is strongly associated with difficult intubation.

The placement of a cuffed tube in the trachea offers the highest level of airway maintenance and protection, and is often the most appropriate route for provision of mechanical ventilation. This chapter concentrates on characteristics of the tube rather than the means of inserting it into the trachea.

History

Intubation of the trachea via the larynx was first described by Macewen of Glasgow in 1878 but only began to gain popularity in the 1920s with the description of blind nasal intubation by Rowbotham and Magill. This technique was quick, accurate and gained early control of the airway. Magill made his own tubes, which he cut from lengths of rubber tubing, bevelling the ends and smoothing the tips with sandpaper before storing them in a biscuit tin to attain the desired curve.

Various materials are used in the construction of tracheal tubes and each has its own properties (Table 7.1). Traditional mineralized red rubber tubes are still in use but began to be replaced with plastic tubes from the 1950s. Plastic tubes are made of polyvinyl chloride or polyurethane, offer a number of advantages and are widely used. Silicone tubes are available but less widely used as they are more expensive to manufacture. Polyvinyl chloride and polyurethane tubes may be siliconized as part of the manufacturing process. In the past, some materials used in the manufacture of tracheal tubes have shown evidence of tissue toxicity. Implant testing (IT) or cell culture are mandatory to exclude toxicity when a new material is used, without it the device cannot be marketed.

Design

Standard tubes have a preformed curve that approximately conforms to the anatomy of the pharynx and which aids insertion and resists kinking of the tube in situ.

Light-guided intubation was introduced in 1959 by Yamamura. Since then several intubation devices which use transillumination have been developed. In 1995 Hung introduced the Trachlight, which has proved to be the most popular device. This instrument has been thoroughly evaluated in almost 50 published articles and has been proven to be safe, effective and easy to use in most cases.

Principle of transillumination

The light-guided intubation technique is based on transillumination of the soft tissues of the neck from inside out. A lighted stylet is placed in the endotracheal tube (ET) in such a way that the light source marks the tip of the ET. ET and stylet are introduced into the upper airway and transillumination indicates the position of the tip of the ET in the patient. When transillumination is seen externally at the level of the cricothyroid membrane, the tip of the ET is considered to be distal to the vocal cords, in the larynx. When the ET is further advanced into the trachea, the tip can be followed by means of a moving transilluminated spot.

Instrument

The Trachlight consists of three parts: a handle, a semi-rigid pliable metal stylet and a hollow flexible lightwand (Figure 11.1). The stylet fits inside the hollow lightwand and the ET is mounted over the lightwand. When the stylet is clicked into the active position, the combination of both components plus the ET is semi-rigid but pliable. During the process of intubation the stylet can be retracted out of the lightwand which turns the lightwand flexible again. This is important because the rigidity facilitates manoeuvring the lightwand through the vocal cords into the larynx, while the flexibility enables safe advancement of the ET further into the trachea.

Difficulty with the airway and cervical spine disease are associated because adequate flexion/extension movement at the cranio-cervical junction is an essential requirement of airway management.

The three main problems are as follows:

1. Difficult direct laryngoscopy.

2. Post-operative airway obstruction.

3. Spinal cord damage.

Difficult direct laryngoscopy

This is common when the occipito-atlanto-axial complex is involved by disease. Osteoarthritis/cervical spondylosis can involve the upper two cervical vertebrae and their joints, but top end disease is most often due to rheumatoid arthritis, tumours and iatrogenic interventions (fixation devices) (Figures 19.1 and 19.2).

Poor mouth opening is common in cervical disease. There is an association between cervical arthritides, especially rheumatoid and temporo-mandibular joint (TMJ) disease. Impaired cranio-cervical extension has been shown to prevent full mouth opening. The mechanism is uncertain.

Grade 3 laryngoscopy occurs in about 7% of patients with cervical spondylosis, but the prevalence can be over 40% in patients having surgery for cervical rheumatoid arthritis.

Post-operative airway obstruction

The reported incidence of this has been as high as 6%, with re-intubation being required in 2%. The classic cause is a haematoma in the wound after anterior cervical surgery. However, the haematoma may be small, or even absent. The problem seems to be due to interference with venous and lymphatic drainage, and it is commoner after long operations. Airway obstruction due to tissue swelling is a particular problem after front and back surgery, where a patient has anterior surgery followed by posterior surgery in the prone position. Airway obstruction after cervical surgery is sometimes neglected because of misunderstandings about symptoms and signs.

Difficult airway management problems are encountered more commonly during anaesthesia for ear, nose and throat (ENT) and maxillofacial procedures than probably any other branch of surgery. Surgery varies from high volume cases such as tonsillectomy and dento-alveolar surgery to complex head and neck cancer and craniofacial surgery. For a successful outcome these shared airway procedures require thorough pre-operative evaluation and close co-operation between anaesthetist and surgeon with an understanding of each other's problems and knowledge of specialist equipment.

General ENT and maxillofacial procedures

Airway management for ENT and maxillofacial procedures needs to take account of specific problems and a number of techniques may be useful (Table 24.1):

• Once surgery has commenced the anaesthetist is remote from the airway making adjustments to the airway more difficult and disruptive.

• Head and neck positioning for surgery and movements during surgery require a secure airway.

• For nasal and intraoral surgery the airway requires protection by soiling from blood and debris.

• Oropharyngeal and nasopharyngeal packs should be specifically recorded and accounted for at the end of the procedure.

• Direct inspection and suction clearance of blood and debris from the oro–nasopharynx should be undertaken at the end of the procedure.

Face mask

Until the introduction of the laryngeal mask airway (LMA) many simple ear procedures were undertaken with a face mask. The increased use of the LMA, compared to a face mask, for simple ear procedures is due to its ability to provide a clearer airway with a more effective airtight seal allowing better ventilation and monitoring of tidal gases. The anaesthetist's hands are also freed and factors which make facemask anaesthesia difficult, for example the edentulous bearded patient, have little impact on the quality of the airway with an LMA.

Introduction

There is a fundamental problem attached to extubation. If the tube is removed when airway reflexes are recovered then the presence of the tube will make the patient uncomfortable and sympathetically stimulated. If the tube is removed before the reflexes have returned, then the patient is at risk of aspiration and loss of airway. The former situation may be safer but aesthetically displeasing, while the latter may be nicer to watch but less safe.

Few (if any) experienced anaesthetists approach extubation with total confidence that their patient will suffer no adverse event and their own dignity will be unruffled. It is also true that nowadays we get much less practice in extubation because the laryngeal mask has drastically reduced the need for intubation.

There have never been, and there are unlikely to be, any randomized controlled trials of extubation methods. Extubation remains the ‘Cinderella’ of management of the difficult airway. In the UK the Difficult Airway Society Guidelines do not include advice on the management of extubation, although similar guidelines from Canada and the USA include some advice. Articles on the management of extubation are few and far between, and by nature are full of opinion and very little in the way of evidencebased medicine. Basic questions, such as does coughing at extubation have any influence on postoperative complications like haematoma formation after neuro-, ENT (ear, nose and throat), plastic or eye surgery are unanswered.

However, anaesthesia providers are increasingly recognizing the importance of the emergence, extubation and recovery period as an important contributor to anaesthetic morbidity, and focusing efforts at improving delivery of a quality service in this area.

Anaesthetic trauma

Dental damage

Dental damage most often occurs during direct laryngoscopy, the upper incisors are most frequently damaged and diseased or restored teeth are most at risk. Avulsed teeth should be stored in milk or saline (never water) for possible re-implantation by dental surgeons. Dental damage is the most common cause of a civil action against anaesthetists, and a careful assessment of pre-anaesthetic dental condition may prevent subsequent claims.

Naso/pharyngeal/oesophageal damage

Damage to the nasal turbinates, even complete avulsion, or perforation of the nasopharyngeal mucosa can occur during nasal intubation. Intubation over a guide, such as a suction catheter or endoscope is recommended. Se psis following pharyngeal or oesophageal perforation during intubation is one of the principal causes of anaesthetic mortality. Urgent treatment is required (antibiotics and drainage) and patients in whom perforation is suspected should be closely observed for symptoms (sore throat, cervical or chest pain, dysphagia) and signs (fever).

Glottic damage

A persistent sore throat, weak cough and dysphonia should prompt a laryngoscopy. Possible causes include subluxation of an arytenoid cartilage (Figure 22.2), vocal cord palsy or haematoma (Figure 22.3). The diagnosis of arytenoid subluxation has followed intubations that were regarded as easy by the anaesthetist, but Paulsen et al failed to induce arytenoid subluxation in cadaver specimens despite strenuous efforts. They proposed that trauma at intubation causes ‘cricoarytenoid joint dysfunction’.

Tracheal damage

Acute

Perforation or rupture can occur during intubation and is not always obvious immediately. The cardinal sign is subcutaneous air. A tracheal tube may need to be placed below a rupture. The use of fibreoptic placement of two microlaryngoscopy tubes has been reported.

Introduction

Most anaesthetists are worried about the airway in obstetrics. This worry goes back over many years to the high incidence of difficult and failed intubation/ventilation that regularly used to feature in the Reports on Confidential Enquiries into Maternal Deaths until 15–20 years ago. Partly through better training/facilities and partly through greater use of regional anaesthesia in obstetrics, the incidence of such problems in recent reports is low. However, the possibility of unexpected difficulty remains (see below) and there is concern that recent changes in anaesthetic training in the UK have led to more limited exposure of trainees to clinical anaesthesia in general, and to general anaesthesia for Caesarean section in particular, resulting in an increase in airway problems.

The airway and pregnancy

There are several reasons why problems with the airway may be more common and serious in pregnant women than in other patients. First, technical aspects of airway management are more difficult; second, management is often particularly stressful because of the urgency of the case, increasing the importance of human factors and third, the consequences of difficulty are more serious (Table 26.1).

Prediction of difficulty with the airway is notoriously inaccurate and is covered in detail in Chapter 15. Although there are many studies of prediction of difficulty, most have excluded pregnant women. The factors that might assist prediction of difficulty are thought, in general, to be the same as for non-pregnant patients.

Non-anaesthetic airway problems

Airway problems may occur as a result of pregnancy and labour alone, albeit rarely. Pre-eclampsia is common and results in oedema which may be marked, although airway obstruction in the absence of instrumentation would be rare.

Decontamination is the combination of processes, including cleaning, disinfection and/or sterilization, used to render a reusable medical device safe for further use. It is important because of the risk of transmission of hospital acquired infections and transmissible spongiform encephalopathies. The recommendations of the Department of Health are clear and detail that single use disposable airway equipment should be used where possible. Where this is not possible, equipment should be sterilized by heat (autoclave) and where this is not possible cold, chemical sterilization or disinfection by an automated process should be used. The term disinfection means the elimination of all vegetative pathogenic organisms such as bacteria and viruses, and sterilization indicates the elimination of all pathogens including spores. For a terminally sterilized medical device to be labelled sterile, the theoretical probability of there being a viable microorganism present on the device should be equal to or less than 1 × 10–6.

An additional infective agent, the abnormal prion protein PrPSc, has great public health implications as it is the probable mechanism of transmission of variant Creutzfeldt–Jakob disease (vCJD). This prion protein cannot be eliminated by chemical means or temperatures reached in the standard autoclave. Temperatures approaching those required for incineration are needed. Reusable equipment should never be used in patients known or suspected of having vCJD unless the equipment is to be destroyed after use. In the case of flexible fibrescopes, dedicated endoscopes may be obtained from the National CJD Surveillance Unit in Edinburgh. All trusts should have Infection Control Committees and Teams which are responsible for preparing infection control policies to conform to all national and international standards and recommendations.

Single use, disposable

Single use equipment eliminates the risk of cross infection between patients through the airway device.

Retrograde intubation is a misnomer since intubation is undertaken in the normal direction over a guide-passed retrogradely. The first description by Butler and Cirillo was of a catheter passed through a healing tracheostomy, up between the vocal cords and out of the mouth, over which a tracheal tube was railroaded. The first author to describe puncture of the intact cricothyroid membrane (CTM) was Waters (1963), then working in Africa. His method was to use a Tuohy needle to puncture the CTM, feed up ‘a yard’ of plastic tubing similar to an epidural catheter until it exited the mouth and intubate over the catheter.

A literature search in the mid-1990s revealed about 500 retrograde intubations, about 300 of which were in cadavers. Mean intubation time was about 40 s with a 98% success rate. It is, therefore, a simple, effective technique supported by a reasonable body of published work. There are two variants, namely blind and fibreoptic assisted. For the blind technique a purpose-built kit (Figure 13.1) is available from Cook Critical Care, and this will be used for description of the technique. Developments since the original description of blind retrograde by Waters have focused particularly on the guide. The epidural catheter has been replaced by a guidewire with a J-tip, with a diameter sufficient to pass through a 20-G catheter but enough stiffness to provide support for intubation. Intubation does not occur over the guidewire alone, because the difference in size between the wire and tracheal tube means that the tube tip is highly likely to impinge on the larynx. The wire is made fatter and stiffer by a bougie (or wire-stiffener) slid anterogradely, over which the tracheal tube is railroaded.

Worldwide, obstructive sleep apnoea (OSA) is the most common medical disorder affecting sleep, afflicting about 5% of the middle aged in the UK and about 80% of those affected are male. The full range of sleep disorders has been classified by the American Sleep Disorders Association (Table 17.1). The high prevalence of OSA means that anaesthetists are frequently involved with such patients presenting either for surgery as part of treatment for the condition itself or for unrelated surgery. This chapter deals with the pathophysiology of the condition and its presenting features, investigations and treatment. It concludes with management during the peri-operative period. Sleep apnoea in children is also considered briefly.

Definition and presentation of OSA

OSA is defined as a repetitive obstruction of the upper airway during sleep causing hypoxaemia with arterial oxygen desaturation, which leads to a reduced quality of sleep. When this results in excessive daytime sleepiness (EDS), the condition is often referred to as sleep apnoea syndrome. There is almost always a history of snoring that has worsened over the preceding few years, often causing patients to sleep separately from their partners. The obstruction to the airway may arise from a number of discrete anatomical causes that are listed in Table 17.2, but in the absence of abnormal anatomy, it is relaxation of the pharyngeal constrictor muscles during sleep that allows collapse of the airway. There is evidence that these muscles are hypertonic during wakefulness, and that fatigue may become unmasked during sleep leading to excessive relaxation. Physical associations with OSA include being male, a collar size of 17 in. or greater, a large tongue and often some degree of retrognathia.

Dr Peter Murphy (Figure 12.1) was the first to use a flexible endoscope ('scope) to intubate the trachea, while a Registrar at the National Hospital, Queen Square in 1967. He moved to Chicago shortly afterwards, where he still practises.

Flexible fibreoptic intubation is a tremendously useful technique, which every anaesthetist should attempt to master. The advantage of vision, the pre-eminent sense, needs no explanation.

Fibreoptic laryngoscopy is easier in the awake patient. The less experienced you are, the more inclined to an ‘awake’ procedure you should be.

Flexible fibreoptic intubation is not a panaceabecause of the following:

1. It is relatively slow, not suitable for really rapid airway management.

2. Blood and secretions can easily prevent vision.

3. An air space is required, which may be absent when there is soft tissue swelling.

4. It can be impossible to access a glottis displaced from the midline.

Blind techniques have succeeded when flexible fibreoptics have failed.

How to do it

Make sure the equipment is correct

1. a Use a television (TV) camera and screen if possible (a bigger view and helpful for assistants).

2. b Adjust the focus and orientation of the camera.

3. c Use a mains-powered light source if possible, although battery-powered ones are adequate.

4. d The easiest tracheal tube to pass off a flexible laryngoscope is the silicone intubating laryngeal mask airway (silicone ILMA) tube, followed by the ‘Flexilum’-reinforced tube (Mallinkrodt). Ordinary tubes can be used but do not pass as easily.

5. e Wind the pilot tube round the tube and fix (loosely) with tape.

6. f Use the smallest size of tube you can; 6mm for women and 7mm for men is usual.

Physics of airflow

Gas flowing through tubes can be characterized by possessing either laminar or turbulent flow, or more usually, a mixture of both. Before we explore these, we need to familiarize ourselves with a fundamental thermodynamic concept detailed below.

Laminar flow

In laminar flow, although gas molecules in different parts of the ‘stream’ have different velocities, the vectors of these velocities are parallel, as shown in Figure 3.1.

For a viscous Newtonian fluid such as air, molecules can be thought of as being arranged in slippery ‘sheets’ or ‘layers’ and these have been schematically labelled 1, 2, 3, etc. in Figure 3.1. The sheet of molecules closest to the wall of the tube (sheet 1) is stationary and bound to the wall of the tube. Each sheet of molecules can exert a force on its neighbour (‘shear’ force) so that a slow moving sheet will tend to retard a quicker moving neighbour, and quicker moving sheets will tend to drag slower neighbours along. For example, the shear forces exerted on sheet 2 by the stationary sheet 1 would tend to retard it, but the shear force exerted by its other neighbour, the faster moving sheet 3, would tend to increase its velocity. The sheets of molecules in the middle of the stream (e.g. sheet 3) are the least influenced by the static layers at the edges, and so these have the highest velocity. The amount of grip which one sheet has on another (the shear force) is a property of the viscosity of the fluid. For low viscosity (i.e. slippery) fluids, the stationary boundary layer has little ‘grip’ on its neighbour and so the velocity of subsequent layers rises very quickly as one moves towards the centre of the stream.

Defining, describing and identifying ‘difficult’ airways are difficult tasks. The obviously problematic patients are, in a sense, no problem, since everybody understands the situation and suitable tactics,staff and equipment can be deployed. The majority of difficult patients that anaesthetists encounter look ‘more-or-less’ normal. We are still unable to identify these patients with acceptable accuracy. Anaesthetists must be prepared to adapt their technique if difficulty occurs.

There can be no doubt that the advent of the laryngeal mask airway (LMA) has decreased the frequency of difficulty with the airway. Many patients who would have been intubated have an LMA instead, so that episodes of difficult laryngoscopy are avoided, and when difficult laryngoscopy is encountered an LMA is often used instead of a tracheal tube.

Having found a difficult airway, it is by no means easy to describe the problem in a way which will be of practical help to subsequent anaesthetists. It has been shown that most anaesthetists ignore a history of difficult intubation, which may be because the problem was not accurately defined or they believe that they can do better. Many scoring systems have been proposed, but the only one that has stood the test of time is Cormack and Lehane's description of direct laryngoscopy.

Causes of difficulty

1. a Who you are

2. b Where you are

3. c What equipment and drugs you have

4. d Who you have to help you

5. e What the patient is like

Introduction

Misplacement of a tracheal tube is a common complication. The tube may pass into the oesophagus or a main bronchus; this may occur at initial placement or the tube may migrate to one of these positions after movement of the patient. Oesophageal intubation is a preventable complication, which is lethal if unrecognized and which every anaesthetist must remain constantly on guard against.

Reasons for confirming tracheal intubation

The prompt recognition of misplacement is important because of the following:

1. • If not corrected, oesophageal placement will lead to:

2. – insufflation of the stomach when attempting to ventilate the lungs (in children this can lead to bradycardia and in all ages it increases the risk of regurgitation of stomach contents);

3. – hypoxia, brain damage and/or death.

4. • If not corrected, bronchial placement may lead to:

5. – hypoxaemia from shunting of blood;

6. – lung or lobar collapse;

7. – increased airway pressure and possible barotrauma/volutrauma.

8. • Consequently it is vital to:

9. – differentiate between a tube which has passed through the vocal cords from one which is in the oesophagus;

10. – determine that a tube through the cords is in the trachea and not in a main bronchus.

A variety of tests has been devised to do each of these. The tests reviewed here do not comprise a comprehensive list, but are commonly used, reliable or both. The reliability is evaluated according to estimates of the frequencies of false positives (related to specificity) and negatives (related to sensitivity) (Table 9.1). When reading publications on this topic it is important to be clear as to whether the authors are describing their results in terms of the ability of the test to identify tracheal or oesophageal placement.

Fine lingerie itself is rather tedious: it is the context that makes it exciting. The same is true for anatomy: topology alone is for idiots-savants. The following lines instead offer a selective account of the functional anatomy of the adult head, neck and airway as it applies to anaesthetic clinical practice.

The mouth

The mouth is dominated by the tongue, a muscular instrument of pleasure – gastronomic and linguistic. For anaesthetists, little else counts but its size. It may be swollen acutely (as in angioneurotic oedema), but is also susceptible to disproportionate enlargement in trisomy 21, myxoedema, acromegaly and glycogen storage diseases, among others.

Angioneurotic oedema can cause such swelling as to fill the entire pharynx, preventing both nasal and mouth breathing and making a percutaneous subglottic airway necessary for survival. Less dramatically, a large tongue (relative to the submandibular space) can hinder direct laryngoscopy. That is, manoeuvered with reasonable force, the laryngoscope blade should squeeze the posterior tongue so as to allow a direct view of the glottis. If the tongue is too large, or the jaw hypotrophied, it may not be possible to see the glottis over the compressed tongue.

Within the mouth, the tongue is like a thrust stage in a theatre. It is surrounded by two tiers of teeth (stalls and royal circle), and a series of trapdoors, wings and flies (Figure 1.1).

Each tooth consists of calcified dentine, cementum and enamel surrounding a cavity filled (if the tooth is alive) with vessels and nerves. Each tooth is held in its socket in the jaw by a periodontal ligament. If a tooth is inadvertently knocked out, the sooner it is returned to its socket the better.

Introduction

A trained anaesthetist rarely has significant difficulty in maintaining a patent airway and ventilating the patient's lungs. When difficulty occurs the degree of difficulty encountered covers a spectrum, at the far end of which is the situation where the patient is consuming oxygen faster than the anaesthetist can deliver it; for the purpose of this chapter this is the definition of the lost airway. Other terms are ‘can't ventilate, can't intubate' (CVCI) or ‘can't intubate, can't oxygenate’ (CICO). This extreme situation is rare in elective general surgery with an incidence of 1 : 10,000 to 1 : 50,000. Diagnosis and management must be complete within minutes of the onset of the problem to have a chance of a successful outcome.

Pathophysiology

An adult patient requires 200–250 ml min–1 of oxygen to sustain life. After induction of anaesthesia if this amount of oxygen cannot be delivered to the patient's lungs they will use up their reserves over 1–2 min (longer if well preoxygenated, sooner in the obese or those with increased requirements) and then they will start to desaturate. Without intervention to increase oxygen delivery to the lungs the patient will die. How long an individual patient will survive will depend on many factors. Anecdotal accounts of death due to airway obstruction (judicial execution by hanging) suggest that about 10 min of complete airway obstruction will result in cardiac arrest.

Most general anaesthetic agents decrease upper airway tone tending to lead to compromise of the natural upper airway and minor degrees of difficulty with facemask ventilation around the time of induction of anaesthesia.