Key Topics in Otolaryngology (eBook)

3 Anaesthesia—General

General anaesthesia (GA) for ear, nose and throat (ENT) surgery accounts for about 5% of anaesthetic practice in the United Kingdom. While many of the operations are straightforward from the anaesthetist's point of view, others may present significant challenges. The nature of the pathology found in ENT patients may make airway management and intubation difficult. In many cases, specific consideration will be needed as to how best to provide optimal surgical conditions and access to the ‘shared airway’, while maintaining safe and effective anaesthetic conditions. This requires communication and cooperation between the surgeon and the anaesthetist so that each understands the concerns, aims and objectives of the other and a mutually acceptable plan of action can be agreed.

3.1 Techniques for General Anaesthesia

General anaesthesia can be classified according to whether ventilation is spontaneous or controlled.

• Spontaneous ventilation: The patient is permitted to continue breathing spontaneously throughout the operation. This technique is commonly employed when there is no need to intubate the patient to facilitate surgery, for example during grommet insertion, removal of simple skin lesions under GA and other simple surgery. The patient is rendered unconscious before a laryngeal mask airway is inserted to maintain airway patency.

• Controlled ventilation: Positive pressure ventilation is applied to the airway to oxygenate the patient. Controlled ventilation is commonly used following tracheal intubation which, in adults, usually follows the administration of a neuromuscular blocking drug, which relaxes the muscles and renders the patient paralysed. Tracheal intubation (either oral or nasal) is chosen to manage the airway for a host of different reasons depending on the situation, for example a cuffed tracheal tube provides protection against airway soiling; it provides a more ‘secure airway’ which is less susceptible to being dislodged or moved during surgery; it allows for more reliable delivery of positive pressure ventilation with negligible leakage of anaesthetic gases.

Induction, maintenance and emergence describe the inevitable sequence of all general anaesthetics.

3.2 Induction of Anaesthesia

Propofol is the drug most commonly used to induce anaesthesia. It is presented as a white lipid emulsion for intravenous administration and its effects are dose dependent; at low plasma concentrations, it causes sedation, which becomes deeper with increasing plasma concentrations until consciousness is lost entirely.

Induction of anaesthesia may be achieved using other hypnotic agents, which might be chosen in place of propofol because of a specific desirable characteristic. For example, ketamine may be useful in the hypotensive patient.

‘Gas induction’ of anaesthesia is done when a patient inhales a mixture of oxygen/air or oxygen/nitrous oxide and sevoflurane which is a volatile anaesthetic agent (see later). Gas induction is most commonly, although not exclusively, used in children who are resistant to having an intravenous cannula sited.

3.3 Neuromuscular Blocking Drugs

If tracheal intubation is required, it is commonplace to give a neuromuscular blocking drug (NMBD) (muscle relaxant) to cause abduction of the vocal cords and allow placement of the endotracheal tube (ETT). Various drugs are available for this purpose and the anaesthetist chooses according to the desirable properties and side effects of each drug, patient factors and the clinical situation. Table 3.1 lists a few examples.

The effects of a full intubating dose of atracurium, rocuronium or vecuronium last for approximately 30 minutes. It is generally not possible to reverse profound neuromuscular blockade by administering neostigmine, the anti-cholinesterase which is traditionally used as a ‘reversal’ agent. Instead, sufficient time must pass to allow the concentration of the NMBD to fall at the neuromuscular junction. Once a sufficient number of receptors are left unoccupied by the NMBD, it is possible to reverse its effects by giving neostigmine. Neostigmine increases the availability of acetylcholine at the neuromuscular junction. The acetylcholine can then bind to the available receptors at the junction and facilitate muscle contraction once again. There are ways around this; for example, the anaesthetist might choose to give a smaller dose of muscle relaxant to begin with. Sugammadex binds to and inactivates rocuronium and vecuronium molecules to reverse their effects. It is, however, expensive enough for many departments to restrict its use.

3.4 Maintenance of Anaesthesia

Anaesthesia is most commonly maintained using inhalational anaesthesia. Volatile anaesthetic vapours are stored in ‘vaporisers’ which are attached to the anaesthetic machine. Oxygen + air, or oxygen + nitrous oxide, flow through the vaporising chamber where they are mixed with the volatile agent. The mixture is subsequently delivered to the patient via the anaesthetic breathing system. The anaesthetist controls the amount of volatile agent delivered to the patient as a percentage of the total volume of gas delivered, and the depth of anaesthesia increases in a dose-dependent fashion. Commonly used volatile agents are listed below. Again, each might be chosen because of the specific advantages it has to offer:

• Sevoflurane (yellow vaporiser): Faster onset and offset of action than isoflurane, not irritant to the airways and so can be used for gas induction.

• Isoflurazne (purple vaporiser): Slower onset and offset than sevoflurane, irritates the airways and so less suitable for gas induction. Cheaper than sevoflurane.

• Desflurane (blue vaporiser): Very fast onset and offset of action, irritant to the airways and so unsuitable for gas induction, useful when rapid emergence is desirable, for example in morbidly obese patients and those with sleep apnoea.

Anaesthesia can also be maintained using total intravenous anaesthesia (TIVA). This usually consists of a continuous infusion of propofol and remifentanil (a very short-acting analgesic drug). Each agent is delivered by an infusion pump and controlled by pharmacological drug algorithms based on the individual patient demographics. This would be used in cases such as those at risk of malignant hyperthermia from inhalational anaesthesia.

3.5 Emergence

To reverse the patient at the end of surgery, the anaesthetist needs to discontinue any inhalational agents, reverse any residual neuromuscular block, and most importantly, provide timely and adequate analgesia appropriate to the surgical situation. It is obviously important not to be giving ‘top-up’ doses of NMBDs just prior to reversal and communication with the surgeon is needed to prevent this. Care of the airway at reversal is important and referred to in the next section.

3.6 Special Anaesthetic Considerations

3.6.1 Airway Assessment

Airway assessment forms part of the standard anaesthetic pre-operative assessment. Anaesthetic teaching relies on simple ‘bedside tests’ (e.g. the Mallampati score, range of movement of neck flexion/extension, mouth opening, etc.). However, their predictive accuracy is poor. If a patient has had surgery before, his or her previous anaesthetic notes can provide invaluable information about any difficulties encountered and how these were resolved.

3.6.2 Anticipated Difficult Intubation

Securing the airway in patients for certain types of ENT surgery might prove especially challenging, for example those with tumours obstructing/encroaching on the airway, following surgery or radiotherapy to the head and neck which has caused deformity/restricted neck movement or mouth opening, abscesses restricting mouth opening, inhaled foreign bodies or conditions causing airway soiling such as posttonsillectomy bleeding. When managing these high-risk patients, clear communication between the surgeon and the anaesthetist is vital. Information should be sought and shared, for example radiological imaging, nasendoscopy findings, clinical drawings or photo graphs of the airway/tumour morphology, and a clear, stepwise management plan agreed upon and communicated to the whole theatre team. More recent emphasis has been placed on the objective of decreasing the risk of hypoxia during difficult intubations either by continuous oxygen delivery by nasal catheters during laryngoscopy or the application of high-flow ventilatory exchange systems (‘Thrive’). The final common pathway of any airway plan might be emergency surgical airway creation, and so the anaesthetist may request that the ENT surgeon be present in theatre, scrubbed and ready to perform an emergency tracheostomy should attempts at oral/nasal intubation fail. In a small number of cases, it may be decided that it is safest to secure the airway by performing an elective awake surgical tracheostomy.

3.7 Hypotensive Anaesthesia

In the past, it was fashionable to induce severe hypotension during anaesthesia to improve the surgical field by rendering it bloodless. This has fallen out of favour, however, because of the risk of critically reducing cerebral perfusion resulting in permanent brain injury. When it is considered necessary, however, there may be an agreement to induce modest hypotension, targeting a mean blood pressure appropriate for each individual patient. This can usually be...