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. imageTable 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

Mar 31, 2020 | Posted by in OPHTHALMOLOGY | Comments Off on Anaesthesia—General
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