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Critical incidents: the respiratory system
CLINICAL ANAESTHESIA
• The compliance of the system is noted. Many causes of airway obstruction reduce airway compliance. The varied pressure and ‘feel for the airway’ afforded by hand ventilation aids diagnosis and may help to overcome airway obstruction, aiding treatment. Listen to the chestt to determine the presence or absence of breath sounds on both sides of the chest and the presence of additional sounds, such as added wheeze in bronchospasm. The essential feature of this approach is that treatment of hypoxaemia is initiated at the same time as attempts are made to establish the cause. Subsequent definitive treatment depends on establishing the diagnosis.
Critical incidents: the respiratory system Michael Taylor
Critical incidents are events that cause harm or have the potential to cause harm if not recognized and acted on. Those affecting the respiratory system are among the most common critical events in anaesthesia. In the course of any general anaesthetic the anaesthetist must: • control the contents of the inspired gases delivered by the anaesthetic machine, maintaining a safe inspired oxygen concentration • maintain the airway, allowing passage of gases between the anaesthetic machine and the lungs (an adequate minute ventilation must be maintained by facilitating spontaneous respiration or by controlled ventilation) • protect the airway from contamination. Failure of any part of this process may lead to the rapid development of hypoxaemia. The body has little or no stored oxygen to draw on in the event of a critical incident. If delivery of oxygen to the alveoli is interrupted, deoxygenated haemoglobin in mixed venous blood passing through the pulmonary capillaries fails to combine with oxygen. This deoxygenated blood returns from the lungs to the systemic circulation, reducing the amount of oxygen delivered to the tissues. As the tissues continue to extract oxygen (at 250–300 ml/min) from this already desaturated blood, the oxygen content of the blood falls further before returning to the lungs. If there is still no oxygen in the alveoli, desaturation continues rapidly. The anaesthetist must rapidly establish the cause of the failure of oxygen delivery and institute remedial treatment if hypoxic tissue damage is to be prevented.
Prevention of critical incidents The prevention of critical incidents relies on the key points in Figure 1. The value of preoxygenation The purpose of preoxygenation is to create a store of oxygen in the lungs before induction of anaesthesia. It is achieved by delivering 100% oxygen to the patient via a face mask, which is closely applied to prevent indrawing of room air. The aim is to displace nitrogen from the functional residual capacity (FRC) with oxygen. The best means of achieving this nitrogen washout is open to debate (3 minutes preoxygenation versus 5 minutes preoxygenation versus 4 vital capacity breaths), but whichever means
Prevention of critical incidents • A thorough preoperative assessment of the patient by an anaesthetist of appropriate experience and expertise • The availability of appropriate anaesthetic equipment and experienced support staff • Thorough checking of equipment. The Association of Anaesthetists has issued revised guidance on the anaesthetic machine check; this is available in laminated form and should be attached to every anaesthetic machine (http://www.aagbi. org/pdf/Checklist_A4.pdf). f The checks ensure: • functioning of all monitoring, including an oxygen analyser, capnograph and pulse oximeter with appropriate alarm limits • functioning of the pressurized oxygen supply and reserve oxygen cylinder • functioning of the anti-hypoxia device and the emergency oxygen bypass control • the integrity of the breathing circuit and ventilator • the presence of functioning ancillary equipment, including airway devices, suction and a tipping trolley • the presence of an alternative means of positive-pressure ventilation
Immediate management of hypoxaemia under anaesthesia The anaesthetist is usually alerted to developing hypoxaemia by the falling pitch of the pulse oximeter tone. The immediate management of this situation should include the following steps. Look at the patient to confirm the situation and hand ventilate the patient’s lungs using the breathing circuit and reservoir bag. Turn the fresh gas flow to 100% oxygen and confirm the delivered concentration using an oxygen analyser. This institutes immediate treatment of hypoxaemia while providing the following diagnostic information. • The integrity of the oxygen supply to the patient via the anaesthetic machine and breathing system is confirmed. Should this system fail, an oxygen cylinder and self-inflating bag should be used. If all else fails, mouth-to-mouth or mouth-to-tube ventilation may be required.
• Knowledge of the safety features of the anaesthetic machine are key to the prevention of respiratory critical incidents
Michael Taylorr is Consultant Anaesthetist at Bristol Royal Infirmary. He qualified from Cambridge University and the Royal London Hospital, and trained at the Bristol School of Anaesthesia. He specializes in thoracic and vascular anaesthesia.
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• Full monitoring of the patient, including the watchful presence of the anaesthetist throughout the case 1
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is chosen, it can be monitored by watching the rise in expired oxygen from 18% to steady state using a gas analyser. The extra oxygen stored in the FRC continues to oxygenate the mixed venous blood returning to the lungs for much longer than if the alveoli contain both oxygen and nitrogen. If the patient becomes apnoeic or the airway obstructs this may prolong the time taken for hypoxaemia to develop from seconds to minutes. This time can prove vital, and preoxygenation must be considered should any respiratory difficulty be anticipated before induction of anaesthesia.
to the situation of upper airway obstruction including stridor described below.
The obstructed airway Existing upper airway obstruction The essential features of upper airway obstruction are: • partial airway obstruction may rapidly become complete on induction of anaesthesia • intubation by direct laryngoscopy may be impossible. The key presenting feature of upper airway obstruction is stridor. Patients with stridor at rest cause most concern to anaesthetists because this suggests a reduction in airway diameter of at least 50%. Of particular concern are patients with tumours affecting the upper airway who may present to medical care late in the course of development of their condition, when airway obstruction may be far advanced. Investigations should include: • flexible nasendoscopy (without spraying anaesthetic on the larynx because this may precipitate complete airway obstruction) may provide key information on the site and nature of airway obstruction, particularly perilaryngeal tumours • CT scanning of the neck and upper airway may identify the site and nature of the obstruction. It is essential in the anaesthetic management of these patients to have a back-up plan, should the first approach fail. Close liaison with the surgical team is essential. To attempt awake fibre-optic intubation is most often inappropriate in patients with upper airway obstruction causing stridor. Applying topical anaesthesia to the partially obstructed airway may precipitate laryngospasm and complete airway obstruction. Use of the fibre-optic bronchoscope may cause friable tumours around the larynx to bleed, obscuring the view down the scope and worsening airway obstruction.
Critical incident reporting and analysis The term critical incident derives from the aviation industry. When applied to anaesthesia, the aim of critical incident reporting is to record events or circumstances that have caused, or have the potential to cause, patient harm. In this context, a transient episode of hypoxaemia occurring under anaesthesia that is quickly recognized and successfully treated may be reported as a critical incident for the following reasons: • if not recognized and treated, the event may have progressed to cause patient harm; in these circumstances, the consequences of profound tissue hypoxia (e.g. hypoxic brain damage) • an analysis of the circumstances surrounding the event may lead to changes in practice to prevent similar events occurring. Critical incident reporting schemes are a central tool of risk management, the process of reducing the frequency and cost of adverse events in anaesthesia.
Distinguishing difficult intubation and obstructed airway Thorough assessment of the airway is an essential part of anaesthetic preoperative assessment. For anatomical reasons, it is crucial to differentiate between the patient who is known to be, or may be, difficult to intubate and the patient with an obstructed airway. The anaesthetic management of these two groups of patients is different, and problems may be encountered if the anaesthetic management appropriate for one group is applied to the other.
Patients with stridor at rest definitely requiring preliminary tracheostomy under local anaesthetic may have: • severe stridor • large tumour • fixed hemilarynx • gross anatomical distortion • larynx not visible on nasendoscopy. They should not be given sedation. The use of helium/oxygen mixtures may improve flow past the airway obstruction until tracheostomy is performed.
Difficult intubation The physical characteristics that predispose patients to being difficult to intubate include: • limited mouth opening, temporomandibular joint abnormality, trismus, scarring, fibrosis, local swelling • protruding upper teeth, high arched palate, receding mandible • reduced neck mobility • congenital syndromes or abnormalities known to predispose to difficult intubation (e.g. Marfan’s syndrome, Treacher Collins syndrome). Some patients who are anatomical variants of normal are difficult to intubate. Tests aimed at identifying anatomical and radiological characteristics that predict difficult intubation have been developed, but they lack sensitivity and specificity. The most reliable indicator of ease or difficulty of intubation may be the observations recorded on a previous anaesthetic chart. Anaesthetic management – a full description of the management of predicted difficult intubation is beyond the scope of this article. It may be appropriate to manage the airway in these cases by awake fibre-optic intubation (see page 287). This is in contrast
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Patients with stridor at rest in whom intubation is considered possible Anaesthetic managementt – the initial plan for these patients is to perform an inhalation induction and subsequent intubation. This should be performed in the operating theatre with an ENT surgeon on hand, scrubbed to perform a tracheostomy if intubation is impossible. It may be prudent to identify anatomical landmarks with a marker pen before attempting induction. The essential safety feature of an inhalational rather than an intravenous induction is that the patient’s spontaneous ventilation is maintained throughout the procedure until the airway is secured, either by intubation or by a tracheostomy. It is much easier for the patient’s own respiratory muscles to draw gases past an upper airway obstruction from below by inspiration than it is
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for the anaesthetist to attempt to force gases past the obstruction from above by bag-and-mask ventilation. The flow of gases in the airway naturally takes the path of least resistance. If there is any gas movement at all caused by the patient’s own inspiratory effort, it must go through the larynx and then past the cause of airway obstruction. There is no alternative path for the gases to take. The patient’s own inspiratory effort is vital. Thus, a carefully applied face mask delivering sevoflurane and oxygen via continuous positive airway pressure (CPAP) may be successful in maintaining the airway while deepening anaesthesia. By contrast, if the patient’s own inspiratory effort is inhibited (as may happen with even the most cautious intravenous induction) and bag-and-mask ventilation is attempted there are alternative paths for the gases to take. Gas flow is more likely to leak at the side of the face mask or to pass into the oesophagus (increasing the risk of regurgitation) than it is to take the path of most resistance past the airway obstruction into the lungs. Complete airway obstruction and rapid desaturation ensue. Practical conduct of inhalational induction – sevoflurane delivered in 100% oxygen is the anaesthetic agent of choice for inhalational induction for airway obstruction. The use of nitrous oxide should be avoided, because although it may speed induction of anaesthesia it limits the concentration of oxygen that can be delivered. The aim is to attain sufficient depth of anaesthesia to allow direct laryngoscopy. This may require a considerable amount of time and patience. The temptation to take over respiration should the patient become apnoeic during inhalational induction should be resisted because it is unlikely to be successful for the reasons outlined above. A cautiously inserted nasal airway may overcome the respiratory obstruction that follows immediate loss of consciousness during inhalational induction, whereas attempts to insert an oral airway in light planes of anaesthesia may precipitate coughing, laryngospasm and complete airway obstruction. The insertion of nasal airways may be facilitated by the application of 5% cocaine spray to both nostrils before induction of anaesthesia. Laryngoscopy should be attempted only once sufficient depth of anaesthesia has been obtained. Physical signs reflecting this include a fall in blood pressure and the pupils becoming small and central. If sufficient depth is not achieved with sevoflurane then it may be necessary to switch to halothane, but dysrhythmias are more common in the presence of hypercapnia. Again, time and patience are required. Should complete airway obstruction occur without achieving sufficient depth of anaesthesia, tracheostomy must be performed immediately. If an adequate view is obtained on direct laryngoscopy, then intubation should be performed. Depth of anaesthesia should be such that muscle relaxants are not required for intubation. If an adequate view is not obtained, it should be possible to proceed to surgical tracheostomy while maintaining deep inhalational anaesthesia. The safety of this approach relies on maintaining the patient’s own respiratory effort. Therefore, muscle relaxants should not be used until the airway is secured, by intubation or tracheostomy.
for symptoms of obstructive airways disease. The aphorism ‘all that wheezes isn’t asthma’ applies to these patients. Diagnostic tests include: • flow volume loops (which may identify previously undiagnosed lower airway obstruction) are useful in differentiating between physical airway obstruction and obstructive airways disease • CT and MRI may be essential to establish the diagnosis and determine the extent of the obstruction within the trachea and bronchi. Patients with known lower airway obstruction may present on anaesthetic lists for diagnostic rigid bronchoscopy or endoluminal stent placement. Practical conduct of anaesthesia: tracheal and bronchial obstructions cannot be relieved by tracheostomy and the upper airway and conditions for intubation are likely to be entirely normal. In this situation an inhalational induction of anaesthesia confers no advantage, because the back-up plan of performing a tracheostomy with the patient still breathing is not an option. The best plan is thorough preoxygenation of the patient, followed by intravenous induction and relaxation with suxamethonium. Two scenarios may follow: • the lower airway remains patent; diagnostic or therapeutic rigid bronchoscopy may then be performed by the surgeon • the lower airway collapses and lower airway obstruction develops; the only thing that may re-establish the airway is to pass a rigid bronchoscope through the obstruction, which may be very difficult (fortunately this is rare). For patients in extremis with obstructing tracheobronchial malignancy it is possible to perform rigid bronchoscopy under local anaesthesia only.1 The crucial element of this approach is that all of the patient’s remaining expiratory drive is preserved while allowing a therapeutic intervention that may buy time to allow palliative radiotherapy. The only other means of providing oxygenation in patients with critical lower airway obstruction is to establish cardiopulmonary bypass before induction of anaesthesia. This requires careful planning and may require transfer to a cardiothoracic centre. Unexpected airway obstruction following induction of anaesthesia Transient airway obstruction immediately following intravenous induction of anaesthesia, before intubation or insertion of a laryngeal mask airway, is common. It is usually overcome by the following measures. • Insertion of a Guedel airway. Optimize the patient’s position with simple manoeuvres (e.g. chin lift, jaw thrust). • Application of increased positive pressure via the face mask (turn up the gas flows, screw down the adjustable pressurelimiting valve, and increase the inspired oxygen to 100%). • Optimize the seal between the face mask and the skin. Use two hands to apply the face mask and get a second operator to squeeze the reservoir bag. • Use CPAP to overcome transient laryngospasm (Figure 2). • Insertion of a laryngeal mask. • Use a muscle relaxant to break airway rigidity and permit hand ventilation. The use of muscle relaxants in this scenario may appear contradictory, because the return to spontaneous respiration may be critically delayed if measures to regain the airway fail. Nevertheless this may be the right thing to do if the cause of
Existing lower airway obstruction Anaesthetic management: obstruction of airway below the larynx presents an entirely different challenge to the anaesthetist. The patients may be asymptomatic, or may have been investigated
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difficulty is failure of the airway to relax properly. If there is any doubt in this situation always use suxamethonium rather than a long-acting muscle relaxant. The earlier return to spontaneous respiration afforded may be life saving. This airway obstruction develops into a critical event when these measures are ineffective and rapid measures are required to restore the airway. As this situation develops it is crucial to remember the following. • Call for help early. An experienced second pair of hands is essential. • Serious difficulties may arise as the vicious circle of repeated failed attempts to intubate and developing laryngeal oedema progresses. It is vital to know when to bail out and try to wake the patient up or attempt another emergency airway technique. If these measures fail, the patient’s oxygen saturations will continue to fall, and rapid intervention is required. The next step is usually to attempt intubation. Suxamethonium is the muscle relaxation of choice, providing rapid onset and offset if difficulties arise. Fortunately, intubation is often reasonably straightforward in these circumstances, provided the anaesthetist retains a cool head. Occasionally intubation proves difficult, and desaturation continues. Further rapid intervention is required.
Optimum intubating position
35˚
3
Percutaneous cricothyrotomy or needle cricothyrotomy is the last resort when all other attempts to establish an airway and oxygenate the patient have failed. It typifies the type of extremely rare critical incident that may happen to an anaesthetist only once in a career, but which requires prompt, calm management. The benefit of rehearsing the management of such incidents mentally and (ideally) in simulated conditions cannot be over-emphasized. Percutaneous cricothyrotomy y – the ideal equipment is a specific device designed to allow rapid placement of a tracheal tube through the cricothyroid membrane. Several commercial kits are available. These vary in technique (Seldinger versus non-Seldinger), size of tube (3.0–6.0 internal diameter), and time required for tube placement. It is vital that the anaesthetist is familiar with the specific kit used in their institution. The time to do this is not when the saturations are unrecordable. The advantage of this approach, as opposed to needle cricothyrotomy, is that it allows relatively normal ventilation of the trachea using the catheter mount and breathing system already at hand. Upward leaks via the vocal cords may be reduced to an acceptable level by closing the mouth and pinching the nose. Needle cricothyrotomy (Figure 5) is a more difficult technique, and should be reserved for situations where formal cricothyrotomy equipment is unavailable. Specific needles have been designed for this approach, but in desperate straits, the use of a large-bore
Techniques to overcome difficult intubation • Optimize the position of the head and neck (Figure 3). • Change laryngoscope, because a blade of different length or shape may improve the view. The McCoy laryngoscope improves the view of the laryngeal inlet in critical conditions. • Manipulate the position of the larynx. Backwards, Upwards and Rightwards Pressure (BURP) on the larynx may bring the laryngeal inlet into view (Figure 4). • Use a bougie. There should be a maximum of three attempts to intubate, by whatever means. If intubation fails despite these measures, the options are emergency laryngeal mask insertion or cricothyrotomy (percutaneous or needle). Emergency laryngeal mask insertion: there are numerous case reports of the layngeal mask re-establishing the airway in the ‘can’t ventilate, can’t intubate’ situation. It may be life saving.
Aligning the laryngeal inlet with the visual axis Apply backwards, upwards and rightwards pressure (BURP)
Laryngospasm • Laryngospasm occurs when the vocal cords are stimulated in light planes of anaesthesia • If the patient is awake enough, they will cough. If they are deep enough, or the vocal cords are paralysed, there will be no response Prevention • Avoid stimulation of the vocal cords during light planes of anaesthesia, specifically during induction or emergence Treatment • Deepen the anaesthetic via small increments of propofol • Apply 100% oxygen via continuous positive airway pressure • Break the spasm with a small dose of suxamethonium • Avoid intubation if possible on emergence, because the problem may recur on repeat extubation 2
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15˚
4
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Preventing regurgitation and aspiration
Needle cricothyrotomy a Finger palpates cricothyroid membrane
Richard Vanner
T T2
T1
H
C Laryngeal inlet
Aspiration is the process by which material is carried from the pharynx to the lower respiratory tract. Aspiration occurs in 1/4000 anaesthetics for elective surgery and 1/900 for emergency surgery. If a tracheal tube is used, aspiration is just as likely after extubation as during induction. The clinical outcome depends on the volume and nature of the aspirate, its distribution and the host defence mechanisms. The consequences can vary from relatively benign to fulminant acute respiratory failure and death. Aspiration of solids or semi-solids may cause airway obstruction and hypoxia. Aspiration of acidic gastric contents can cause a pneumonitis with bronchospasm, pulmonary oedema and hypoxia (Mendelson’s syndrome). Of those patients who aspirate, 64% have no respiratory sequelae, 20% require ventilation in ICU for more than 6 hours and 5% die. Mortality may be as high as 50% if acute respiratory distress syndrome (ARDS) develops. If no symptoms or signs are present 2 hours after an episode of aspiration, respiratory sequelae are unlikely.
E
b Confirm correct placement by free aspiration of air
T T2
T1
H
C Laryngeal inlet
E
E, epiglottis H, hyoid bone T, thyroid cartilage C, cricoid cartilage T1, T2, tracheal rings
5
Predicting risk of aspiration Patients presenting for elective surgery who have not eaten for 6 hours or drunk for 2 hours (and do not fall into the high-risk groups below) are at low risk of aspiration and no precautions need to be taken to prevent the pulmonary aspiration of gastric contents. The risk of morbidity from the precautions outweighs the risks of aspiration in these patients. Patients with a higher risk of aspiration during anaesthesia are those: • who are not fasted when they present for surgery • with oesophageal strictures • with delayed gastric emptying (bowel obstruction, upper gastrointestinal cancer, critical illness, acidosis, pain, taking opioids, diabetes mellitus with an autonomic neuropathy or high blood glucose) • with an incompetent lower oesophageal sphincter (oesophagitis, previous oesophageal surgery, women more than 20 weeks pregnant). These patients require precautions to prevent regurgitation and aspiration pneumonitis.
intravenous cannula or Tuohy needle has been described. The limitations of the technique are as follows. • The needles are of such narrow gauge that they require high pressure to inject oxygen into the airway. This requires a jet ventilator, which is seldom immediately available in the standard anaesthetic room or operating theatre. Alternatives to jet ventilation have been described, using combinations of green oxygen tubing and syringes or tracheal tubes and syringes attached to the breathing system. Assembling this equipment is fraught with difficulty in the emergency situation. • Non-reinforced needles (e.g. intravenous cannulae) are liable to collapse and cause obstruction once passed through the cricothyroid membrane. • Needle cricothyrotomy allows injection of oxygen only under high pressure. If the upper airway is completely obstructed, there is no way for gases to leave the lungs. Thus, there is no route to allow elimination of carbon dioxide and there is a high risk of causing barotrauma.
KEY REFERENCE 1 Conacher I D, Curran E. Local anaesthesia and sedation for rigid bronchoscopy for emergency relief of central airways obstruction. Anaesthesia 2004; 59: 290–2.
Richard Vannerr is Consultant Anaesthetist at Gloucester Hospitals NHS Trust. He trained in anaesthesia at the Middlesex, St Thomas’ and Great Ormond Street Hospitals, London, and in Bristol. His research interests are in the function of the upper oesophageal sphincter, cricoid pressure, and the difficult airway. He is a generalist anaesthetist but also does obstetrics and paediatrics.
FURTHER READING Mason R, Fielder C P. The obstructed airway in head and neck surgery. Anaesthesia 1999; 54: 625–8.
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• The compliance of the system is noted. Many causes of airway obstruction reduce airway compliance. The varied pressure and ‘feel for the airway’ afforded by hand ventilation aids diagnosis and may help to overcome airway obstruction, aiding treatment. Listen to the chestt to determine the presence or absence of breath sounds on both sides of the chest and the presence of additional sounds, such as added wheeze in bronchospasm. The essential feature of this approach is that treatment of hypoxaemia is initiated at the same time as attempts are made to establish the cause. Subsequent definitive treatment depends on establishing the diagnosis.
Critical incidents: the respiratory system Michael Taylor
Critical incidents are events that cause harm or have the potential to cause harm if not recognized and acted on. Those affecting the respiratory system are among the most common critical events in anaesthesia. In the course of any general anaesthetic the anaesthetist must: • control the contents of the inspired gases delivered by the anaesthetic machine, maintaining a safe inspired oxygen concentration • maintain the airway, allowing passage of gases between the anaesthetic machine and the lungs (an adequate minute ventilation must be maintained by facilitating spontaneous respiration or by controlled ventilation) • protect the airway from contamination. Failure of any part of this process may lead to the rapid development of hypoxaemia. The body has little or no stored oxygen to draw on in the event of a critical incident. If delivery of oxygen to the alveoli is interrupted, deoxygenated haemoglobin in mixed venous blood passing through the pulmonary capillaries fails to combine with oxygen. This deoxygenated blood returns from the lungs to the systemic circulation, reducing the amount of oxygen delivered to the tissues. As the tissues continue to extract oxygen (at 250–300 ml/min) from this already desaturated blood, the oxygen content of the blood falls further before returning to the lungs. If there is still no oxygen in the alveoli, desaturation continues rapidly. The anaesthetist must rapidly establish the cause of the failure of oxygen delivery and institute remedial treatment if hypoxic tissue damage is to be prevented.
Prevention of critical incidents The prevention of critical incidents relies on the key points in Figure 1. The value of preoxygenation The purpose of preoxygenation is to create a store of oxygen in the lungs before induction of anaesthesia. It is achieved by delivering 100% oxygen to the patient via a face mask, which is closely applied to prevent indrawing of room air. The aim is to displace nitrogen from the functional residual capacity (FRC) with oxygen. The best means of achieving this nitrogen washout is open to debate (3 minutes preoxygenation versus 5 minutes preoxygenation versus 4 vital capacity breaths), but whichever means
Prevention of critical incidents • A thorough preoperative assessment of the patient by an anaesthetist of appropriate experience and expertise • The availability of appropriate anaesthetic equipment and experienced support staff • Thorough checking of equipment. The Association of Anaesthetists has issued revised guidance on the anaesthetic machine check; this is available in laminated form and should be attached to every anaesthetic machine (http://www.aagbi. org/pdf/Checklist_A4.pdf). f The checks ensure: • functioning of all monitoring, including an oxygen analyser, capnograph and pulse oximeter with appropriate alarm limits • functioning of the pressurized oxygen supply and reserve oxygen cylinder • functioning of the anti-hypoxia device and the emergency oxygen bypass control • the integrity of the breathing circuit and ventilator • the presence of functioning ancillary equipment, including airway devices, suction and a tipping trolley • the presence of an alternative means of positive-pressure ventilation
Immediate management of hypoxaemia under anaesthesia The anaesthetist is usually alerted to developing hypoxaemia by the falling pitch of the pulse oximeter tone. The immediate management of this situation should include the following steps. Look at the patient to confirm the situation and hand ventilate the patient’s lungs using the breathing circuit and reservoir bag. Turn the fresh gas flow to 100% oxygen and confirm the delivered concentration using an oxygen analyser. This institutes immediate treatment of hypoxaemia while providing the following diagnostic information. • The integrity of the oxygen supply to the patient via the anaesthetic machine and breathing system is confirmed. Should this system fail, an oxygen cylinder and self-inflating bag should be used. If all else fails, mouth-to-mouth or mouth-to-tube ventilation may be required.
• Knowledge of the safety features of the anaesthetic machine are key to the prevention of respiratory critical incidents
Michael Taylorr is Consultant Anaesthetist at Bristol Royal Infirmary. He qualified from Cambridge University and the Royal London Hospital, and trained at the Bristol School of Anaesthesia. He specializes in thoracic and vascular anaesthesia.
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• Full monitoring of the patient, including the watchful presence of the anaesthetist throughout the case 1
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is chosen, it can be monitored by watching the rise in expired oxygen from 18% to steady state using a gas analyser. The extra oxygen stored in the FRC continues to oxygenate the mixed venous blood returning to the lungs for much longer than if the alveoli contain both oxygen and nitrogen. If the patient becomes apnoeic or the airway obstructs this may prolong the time taken for hypoxaemia to develop from seconds to minutes. This time can prove vital, and preoxygenation must be considered should any respiratory difficulty be anticipated before induction of anaesthesia.
to the situation of upper airway obstruction including stridor described below.
The obstructed airway Existing upper airway obstruction The essential features of upper airway obstruction are: • partial airway obstruction may rapidly become complete on induction of anaesthesia • intubation by direct laryngoscopy may be impossible. The key presenting feature of upper airway obstruction is stridor. Patients with stridor at rest cause most concern to anaesthetists because this suggests a reduction in airway diameter of at least 50%. Of particular concern are patients with tumours affecting the upper airway who may present to medical care late in the course of development of their condition, when airway obstruction may be far advanced. Investigations should include: • flexible nasendoscopy (without spraying anaesthetic on the larynx because this may precipitate complete airway obstruction) may provide key information on the site and nature of airway obstruction, particularly perilaryngeal tumours • CT scanning of the neck and upper airway may identify the site and nature of the obstruction. It is essential in the anaesthetic management of these patients to have a back-up plan, should the first approach fail. Close liaison with the surgical team is essential. To attempt awake fibre-optic intubation is most often inappropriate in patients with upper airway obstruction causing stridor. Applying topical anaesthesia to the partially obstructed airway may precipitate laryngospasm and complete airway obstruction. Use of the fibre-optic bronchoscope may cause friable tumours around the larynx to bleed, obscuring the view down the scope and worsening airway obstruction.
Critical incident reporting and analysis The term critical incident derives from the aviation industry. When applied to anaesthesia, the aim of critical incident reporting is to record events or circumstances that have caused, or have the potential to cause, patient harm. In this context, a transient episode of hypoxaemia occurring under anaesthesia that is quickly recognized and successfully treated may be reported as a critical incident for the following reasons: • if not recognized and treated, the event may have progressed to cause patient harm; in these circumstances, the consequences of profound tissue hypoxia (e.g. hypoxic brain damage) • an analysis of the circumstances surrounding the event may lead to changes in practice to prevent similar events occurring. Critical incident reporting schemes are a central tool of risk management, the process of reducing the frequency and cost of adverse events in anaesthesia.
Distinguishing difficult intubation and obstructed airway Thorough assessment of the airway is an essential part of anaesthetic preoperative assessment. For anatomical reasons, it is crucial to differentiate between the patient who is known to be, or may be, difficult to intubate and the patient with an obstructed airway. The anaesthetic management of these two groups of patients is different, and problems may be encountered if the anaesthetic management appropriate for one group is applied to the other.
Patients with stridor at rest definitely requiring preliminary tracheostomy under local anaesthetic may have: • severe stridor • large tumour • fixed hemilarynx • gross anatomical distortion • larynx not visible on nasendoscopy. They should not be given sedation. The use of helium/oxygen mixtures may improve flow past the airway obstruction until tracheostomy is performed.
Difficult intubation The physical characteristics that predispose patients to being difficult to intubate include: • limited mouth opening, temporomandibular joint abnormality, trismus, scarring, fibrosis, local swelling • protruding upper teeth, high arched palate, receding mandible • reduced neck mobility • congenital syndromes or abnormalities known to predispose to difficult intubation (e.g. Marfan’s syndrome, Treacher Collins syndrome). Some patients who are anatomical variants of normal are difficult to intubate. Tests aimed at identifying anatomical and radiological characteristics that predict difficult intubation have been developed, but they lack sensitivity and specificity. The most reliable indicator of ease or difficulty of intubation may be the observations recorded on a previous anaesthetic chart. Anaesthetic management – a full description of the management of predicted difficult intubation is beyond the scope of this article. It may be appropriate to manage the airway in these cases by awake fibre-optic intubation (see page 287). This is in contrast
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Patients with stridor at rest in whom intubation is considered possible Anaesthetic managementt – the initial plan for these patients is to perform an inhalation induction and subsequent intubation. This should be performed in the operating theatre with an ENT surgeon on hand, scrubbed to perform a tracheostomy if intubation is impossible. It may be prudent to identify anatomical landmarks with a marker pen before attempting induction. The essential safety feature of an inhalational rather than an intravenous induction is that the patient’s spontaneous ventilation is maintained throughout the procedure until the airway is secured, either by intubation or by a tracheostomy. It is much easier for the patient’s own respiratory muscles to draw gases past an upper airway obstruction from below by inspiration than it is
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for the anaesthetist to attempt to force gases past the obstruction from above by bag-and-mask ventilation. The flow of gases in the airway naturally takes the path of least resistance. If there is any gas movement at all caused by the patient’s own inspiratory effort, it must go through the larynx and then past the cause of airway obstruction. There is no alternative path for the gases to take. The patient’s own inspiratory effort is vital. Thus, a carefully applied face mask delivering sevoflurane and oxygen via continuous positive airway pressure (CPAP) may be successful in maintaining the airway while deepening anaesthesia. By contrast, if the patient’s own inspiratory effort is inhibited (as may happen with even the most cautious intravenous induction) and bag-and-mask ventilation is attempted there are alternative paths for the gases to take. Gas flow is more likely to leak at the side of the face mask or to pass into the oesophagus (increasing the risk of regurgitation) than it is to take the path of most resistance past the airway obstruction into the lungs. Complete airway obstruction and rapid desaturation ensue. Practical conduct of inhalational induction – sevoflurane delivered in 100% oxygen is the anaesthetic agent of choice for inhalational induction for airway obstruction. The use of nitrous oxide should be avoided, because although it may speed induction of anaesthesia it limits the concentration of oxygen that can be delivered. The aim is to attain sufficient depth of anaesthesia to allow direct laryngoscopy. This may require a considerable amount of time and patience. The temptation to take over respiration should the patient become apnoeic during inhalational induction should be resisted because it is unlikely to be successful for the reasons outlined above. A cautiously inserted nasal airway may overcome the respiratory obstruction that follows immediate loss of consciousness during inhalational induction, whereas attempts to insert an oral airway in light planes of anaesthesia may precipitate coughing, laryngospasm and complete airway obstruction. The insertion of nasal airways may be facilitated by the application of 5% cocaine spray to both nostrils before induction of anaesthesia. Laryngoscopy should be attempted only once sufficient depth of anaesthesia has been obtained. Physical signs reflecting this include a fall in blood pressure and the pupils becoming small and central. If sufficient depth is not achieved with sevoflurane then it may be necessary to switch to halothane, but dysrhythmias are more common in the presence of hypercapnia. Again, time and patience are required. Should complete airway obstruction occur without achieving sufficient depth of anaesthesia, tracheostomy must be performed immediately. If an adequate view is obtained on direct laryngoscopy, then intubation should be performed. Depth of anaesthesia should be such that muscle relaxants are not required for intubation. If an adequate view is not obtained, it should be possible to proceed to surgical tracheostomy while maintaining deep inhalational anaesthesia. The safety of this approach relies on maintaining the patient’s own respiratory effort. Therefore, muscle relaxants should not be used until the airway is secured, by intubation or tracheostomy.
for symptoms of obstructive airways disease. The aphorism ‘all that wheezes isn’t asthma’ applies to these patients. Diagnostic tests include: • flow volume loops (which may identify previously undiagnosed lower airway obstruction) are useful in differentiating between physical airway obstruction and obstructive airways disease • CT and MRI may be essential to establish the diagnosis and determine the extent of the obstruction within the trachea and bronchi. Patients with known lower airway obstruction may present on anaesthetic lists for diagnostic rigid bronchoscopy or endoluminal stent placement. Practical conduct of anaesthesia: tracheal and bronchial obstructions cannot be relieved by tracheostomy and the upper airway and conditions for intubation are likely to be entirely normal. In this situation an inhalational induction of anaesthesia confers no advantage, because the back-up plan of performing a tracheostomy with the patient still breathing is not an option. The best plan is thorough preoxygenation of the patient, followed by intravenous induction and relaxation with suxamethonium. Two scenarios may follow: • the lower airway remains patent; diagnostic or therapeutic rigid bronchoscopy may then be performed by the surgeon • the lower airway collapses and lower airway obstruction develops; the only thing that may re-establish the airway is to pass a rigid bronchoscope through the obstruction, which may be very difficult (fortunately this is rare). For patients in extremis with obstructing tracheobronchial malignancy it is possible to perform rigid bronchoscopy under local anaesthesia only.1 The crucial element of this approach is that all of the patient’s remaining expiratory drive is preserved while allowing a therapeutic intervention that may buy time to allow palliative radiotherapy. The only other means of providing oxygenation in patients with critical lower airway obstruction is to establish cardiopulmonary bypass before induction of anaesthesia. This requires careful planning and may require transfer to a cardiothoracic centre. Unexpected airway obstruction following induction of anaesthesia Transient airway obstruction immediately following intravenous induction of anaesthesia, before intubation or insertion of a laryngeal mask airway, is common. It is usually overcome by the following measures. • Insertion of a Guedel airway. Optimize the patient’s position with simple manoeuvres (e.g. chin lift, jaw thrust). • Application of increased positive pressure via the face mask (turn up the gas flows, screw down the adjustable pressurelimiting valve, and increase the inspired oxygen to 100%). • Optimize the seal between the face mask and the skin. Use two hands to apply the face mask and get a second operator to squeeze the reservoir bag. • Use CPAP to overcome transient laryngospasm (Figure 2). • Insertion of a laryngeal mask. • Use a muscle relaxant to break airway rigidity and permit hand ventilation. The use of muscle relaxants in this scenario may appear contradictory, because the return to spontaneous respiration may be critically delayed if measures to regain the airway fail. Nevertheless this may be the right thing to do if the cause of
Existing lower airway obstruction Anaesthetic management: obstruction of airway below the larynx presents an entirely different challenge to the anaesthetist. The patients may be asymptomatic, or may have been investigated
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difficulty is failure of the airway to relax properly. If there is any doubt in this situation always use suxamethonium rather than a long-acting muscle relaxant. The earlier return to spontaneous respiration afforded may be life saving. This airway obstruction develops into a critical event when these measures are ineffective and rapid measures are required to restore the airway. As this situation develops it is crucial to remember the following. • Call for help early. An experienced second pair of hands is essential. • Serious difficulties may arise as the vicious circle of repeated failed attempts to intubate and developing laryngeal oedema progresses. It is vital to know when to bail out and try to wake the patient up or attempt another emergency airway technique. If these measures fail, the patient’s oxygen saturations will continue to fall, and rapid intervention is required. The next step is usually to attempt intubation. Suxamethonium is the muscle relaxation of choice, providing rapid onset and offset if difficulties arise. Fortunately, intubation is often reasonably straightforward in these circumstances, provided the anaesthetist retains a cool head. Occasionally intubation proves difficult, and desaturation continues. Further rapid intervention is required.
Optimum intubating position
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Percutaneous cricothyrotomy or needle cricothyrotomy is the last resort when all other attempts to establish an airway and oxygenate the patient have failed. It typifies the type of extremely rare critical incident that may happen to an anaesthetist only once in a career, but which requires prompt, calm management. The benefit of rehearsing the management of such incidents mentally and (ideally) in simulated conditions cannot be over-emphasized. Percutaneous cricothyrotomy y – the ideal equipment is a specific device designed to allow rapid placement of a tracheal tube through the cricothyroid membrane. Several commercial kits are available. These vary in technique (Seldinger versus non-Seldinger), size of tube (3.0–6.0 internal diameter), and time required for tube placement. It is vital that the anaesthetist is familiar with the specific kit used in their institution. The time to do this is not when the saturations are unrecordable. The advantage of this approach, as opposed to needle cricothyrotomy, is that it allows relatively normal ventilation of the trachea using the catheter mount and breathing system already at hand. Upward leaks via the vocal cords may be reduced to an acceptable level by closing the mouth and pinching the nose. Needle cricothyrotomy (Figure 5) is a more difficult technique, and should be reserved for situations where formal cricothyrotomy equipment is unavailable. Specific needles have been designed for this approach, but in desperate straits, the use of a large-bore
Techniques to overcome difficult intubation • Optimize the position of the head and neck (Figure 3). • Change laryngoscope, because a blade of different length or shape may improve the view. The McCoy laryngoscope improves the view of the laryngeal inlet in critical conditions. • Manipulate the position of the larynx. Backwards, Upwards and Rightwards Pressure (BURP) on the larynx may bring the laryngeal inlet into view (Figure 4). • Use a bougie. There should be a maximum of three attempts to intubate, by whatever means. If intubation fails despite these measures, the options are emergency laryngeal mask insertion or cricothyrotomy (percutaneous or needle). Emergency laryngeal mask insertion: there are numerous case reports of the layngeal mask re-establishing the airway in the ‘can’t ventilate, can’t intubate’ situation. It may be life saving.
Aligning the laryngeal inlet with the visual axis Apply backwards, upwards and rightwards pressure (BURP)
Laryngospasm • Laryngospasm occurs when the vocal cords are stimulated in light planes of anaesthesia • If the patient is awake enough, they will cough. If they are deep enough, or the vocal cords are paralysed, there will be no response Prevention • Avoid stimulation of the vocal cords during light planes of anaesthesia, specifically during induction or emergence Treatment • Deepen the anaesthetic via small increments of propofol • Apply 100% oxygen via continuous positive airway pressure • Break the spasm with a small dose of suxamethonium • Avoid intubation if possible on emergence, because the problem may recur on repeat extubation 2
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Preventing regurgitation and aspiration
Needle cricothyrotomy a Finger palpates cricothyroid membrane
Richard Vanner
T T2
T1
H
C Laryngeal inlet
Aspiration is the process by which material is carried from the pharynx to the lower respiratory tract. Aspiration occurs in 1/4000 anaesthetics for elective surgery and 1/900 for emergency surgery. If a tracheal tube is used, aspiration is just as likely after extubation as during induction. The clinical outcome depends on the volume and nature of the aspirate, its distribution and the host defence mechanisms. The consequences can vary from relatively benign to fulminant acute respiratory failure and death. Aspiration of solids or semi-solids may cause airway obstruction and hypoxia. Aspiration of acidic gastric contents can cause a pneumonitis with bronchospasm, pulmonary oedema and hypoxia (Mendelson’s syndrome). Of those patients who aspirate, 64% have no respiratory sequelae, 20% require ventilation in ICU for more than 6 hours and 5% die. Mortality may be as high as 50% if acute respiratory distress syndrome (ARDS) develops. If no symptoms or signs are present 2 hours after an episode of aspiration, respiratory sequelae are unlikely.
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b Confirm correct placement by free aspiration of air
T T2
T1
H
C Laryngeal inlet
E
E, epiglottis H, hyoid bone T, thyroid cartilage C, cricoid cartilage T1, T2, tracheal rings
5
Predicting risk of aspiration Patients presenting for elective surgery who have not eaten for 6 hours or drunk for 2 hours (and do not fall into the high-risk groups below) are at low risk of aspiration and no precautions need to be taken to prevent the pulmonary aspiration of gastric contents. The risk of morbidity from the precautions outweighs the risks of aspiration in these patients. Patients with a higher risk of aspiration during anaesthesia are those: • who are not fasted when they present for surgery • with oesophageal strictures • with delayed gastric emptying (bowel obstruction, upper gastrointestinal cancer, critical illness, acidosis, pain, taking opioids, diabetes mellitus with an autonomic neuropathy or high blood glucose) • with an incompetent lower oesophageal sphincter (oesophagitis, previous oesophageal surgery, women more than 20 weeks pregnant). These patients require precautions to prevent regurgitation and aspiration pneumonitis.
intravenous cannula or Tuohy needle has been described. The limitations of the technique are as follows. • The needles are of such narrow gauge that they require high pressure to inject oxygen into the airway. This requires a jet ventilator, which is seldom immediately available in the standard anaesthetic room or operating theatre. Alternatives to jet ventilation have been described, using combinations of green oxygen tubing and syringes or tracheal tubes and syringes attached to the breathing system. Assembling this equipment is fraught with difficulty in the emergency situation. • Non-reinforced needles (e.g. intravenous cannulae) are liable to collapse and cause obstruction once passed through the cricothyroid membrane. • Needle cricothyrotomy allows injection of oxygen only under high pressure. If the upper airway is completely obstructed, there is no way for gases to leave the lungs. Thus, there is no route to allow elimination of carbon dioxide and there is a high risk of causing barotrauma.
KEY REFERENCE 1 Conacher I D, Curran E. Local anaesthesia and sedation for rigid bronchoscopy for emergency relief of central airways obstruction. Anaesthesia 2004; 59: 290–2.
Richard Vannerr is Consultant Anaesthetist at Gloucester Hospitals NHS Trust. He trained in anaesthesia at the Middlesex, St Thomas’ and Great Ormond Street Hospitals, London, and in Bristol. His research interests are in the function of the upper oesophageal sphincter, cricoid pressure, and the difficult airway. He is a generalist anaesthetist but also does obstetrics and paediatrics.
FURTHER READING Mason R, Fielder C P. The obstructed airway in head and neck surgery. Anaesthesia 1999; 54: 625–8.
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