An Introduction to Anaesthesia
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What You Need to KNoW about An introduction to anaesthesia Introduction divided into three stages: induction, main- n Central neuraxial block, e.g. spinal or Anaesthetic experience in the undergradu- tenance and emergence. epidural (Figure 1 and Table 1). ate timetable is often very limited so it can In regional anaesthesia, nerve transmis- remain somewhat of a mysterious practice sion is blocked, and the patient may stay Components of a general well into specialist training. This introduc- awake or be sedated or anaesthetized dur- anaesthetic tion to the components of an anaesthetic ing a procedure. Techniques used include: A general anaesthetic always involves an will help readers to get more from clinical n Local anaesthetic field block hypnotic agent, usually an analgesic and attachments in surgery and anaesthetics or n Peripheral nerve block may also include muscle relaxation. The serve as an introduction to the topic for n Nerve plexus block combination is referred to as the ‘triad of novice or non-anaesthetists. anaesthesia’. Figure 1. Schematic vertical longitudinal section The relative importance of each com- Types and sites of anaesthesia of vertebral column and structures encountered ponent depends on surgical and patient The term anaesthesia comes from the when performing central neuraxial blocks. * factors: the intervention planned, site, Greek meaning loss of sensation. negative pressure space filled with fat and surgical access requirement and the Anaesthetic practice has evolved from a venous plexi. † extends to S2, containing degree of pain or stimulation anticipated. need for pain relief and altered conscious- arachnoid mater, CSF, pia mater, spinal cord The technique is tailored to the individu- ness to allow surgery. Early anaesthetics above L1/2 and spinal nerves. al situation. used plant derivatives with later introduc- Ligamentum Epidural space* tion of ether, inhaled gases and chloro- flavum Induction form. Modern anaesthesia has been devel- (tough) The induction of anaesthesia refers to the oped and refined to enable surgery, inter- transition from an awake to an anaesthe- Spinous ventions, pain relief and stabilization, and Vertebral tized state. This end point can be ill defined process organ support. body and the process of induction is a time of Various forms of anaesthesia are con- Dural Intervertebral physiological disruption with multi-system ducted throughout the hospital and sac† disc effects. beyond. The operating theatres are the most common venue but anaesthetics are Standard induction delivered on the labour ward, day surgery, Intravenous intensive care, the emergency room, The standard induction is with the intra- interventional radiology, computed venous agent propofol. A calculated by tomography and magnetic resonance Dura mater weight dose is delivered and the effects Supraspinous Interspinous imaging, and on the wards during emer- Posterior Anterior reviewed before further titration of the ligament ligament gency care and transfer of acutely unwell longitudinal longitudinal drug. Delays in inducing anaesthesia may patients. Certain regional procedures ligament ligament represent slow arm–brain circulation time may take place in pain clinics and out- (e.g. elderly, cardiovascular disease), patient settings. In general anaesthesia a reversible state Table 1. Characteristics of different central neuraxial blocks of unconsciousness is achieved. It can be Dr Ciara Donohue is Specialist Registrar Subarachnoid (spinal) Epidural in Anaesthesia in the Centre for Anaesthesia, University College London Hospitals, London NW1 2BU, Mr Ben Hobson is Medical Student at University College London, London, and Dr Robert CM Injection through dura into CSF Catheterization of potential space outside dura Stephens is Consultant Anaesthetist, Low volume (up to 3 ml) High volume (>10 ml) University College London Hospitals and High concentration local anaesthetic 0.5% bupivicaine Variable concentration local anaesthetic, analgesia Honorary Senior Lecturer in the Centre for 0.1% bupivicaine, anaesthesia up to 2% lignocaine Anaesthesia, University College London, London Rapid onset dense sensorimotor block Gradual titration of block density, may be motor sparing Correspondence to: Dr C Donohue Profound vasodilation causing haemodynamic instability Gradual titration causing less haemodynamic ([email protected]) disturbance British Journal of Hospital Medicine, May 2013, Vol 74, No 5 C71 CTD_C71_C75_anaesthesia.indd 71 26/04/2013 16:24 patient anxiety, recreational drug use or abdominal pathology, an un-fasted patient plate depolarization and propagation of the extravasation. An opioid is often given to in an emergency or trauma situation, impulse. Atracurium undergoes spontane- reduce the dose of induction agent needed obstetric emergency or a strong history of ous degradation in the plasma known as and smooth the induction process. A mus- reflux. Pre-oxygenation plus rapid induc- ‘Hoffman’ degradation, while some is cle relaxant is usually given if intubation is tion and paralysis obviate the need for bag hydrolysed by esters so it is a useful agent in required. mask ventilation before securing the air- patients with hepatic and renal impairment way, so the risk of gastric insufflation and as offset is not reliant on organ function. As Inhalational induction regurgitation is reduced (Sinclair and the percentage acetylcholine receptor occu- An alternative method of inducing anaes- Luxton, 2005). pancy falls, competitive antagonism is lost thesia is with a volatile agent, e.g. sevoflu- and acetylcholine can once again bind to rane. The concentration of volatile deliv- Muscle relaxation receptors to generate an end plate potential ered is gradually increased with the patient If intubation is required, it may be neces- and reach the threshold for transmission. spontaneously breathing. Common uses sary to paralyse the patient using: Neuromuscular function is restored include paediatric practice, cases of diffi- n Depolarizing muscle relaxants (e.g. sux- (Appiah-Ankam and Hunter, 2004). cult airway, difficult venous access or amethonium) Neuromuscular junction function inhaled foreign body where maintaining n Non-depolarizing muscle relaxants should be monitored using a peripheral spontaneous ventilation is preferable. (benzylisoquinoloniums, e.g. atracuri- nerve stimulator and observing response to Intubation of the trachea can be achieved um, or aminosteroids, e.g. rocuronium). stimulations over a peripheral nerve (e.g. under deep inhalational induction without Normally, an action potential reaching the ulnar). The stimulation is supramaximal in muscle relaxation. nerve terminal of the neuromuscular junc- order to stimulate all the nerve fibres and tion causes calcium influx and acetylcho- produce a consistent muscular response. Rapid sequence induction: line to be released pre-synaptically. The number and strength of resultant when and why? Acetylcholine crosses the cleft and binds to muscle twitches gives information about A specifically adapted induction process is postsynaptic nicotinic acetylcholine recep- the recovery of the neuromuscular junction used when rapid intubation of the trachea tors causing opening of these ion channels (Davis and Kenny, 2003). In order to is required to minimize risk of regurgita- and depolarization of the motor end plate. enhance neuromuscular recovery post non- tion and aspiration (Table 2). Such instanc- If a sufficient end plate potential is depolarizing relaxation at the end of sur- es include intestinal obstruction or intra- achieved, an action potential is generated gery, the amount of acetylcholine in the leading to muscle contraction (King and synapse is increased by inhibiting the ace- Table 2. Rapid sequence induction Hunter, 2002). tylcholinesterase enzyme using a reversal A depolarizing agent such as suxametho- agent such as neostigmine. Preparation Trained staff nium (biochemically two acetylcholine Emergency drugs and equipment molecules) binds to the postsynaptic ace- Airway maintenance tylcholine receptors, resulting in transient Under anaesthesia the soft tissues of the Tipping trolley receptor agonism and muscle contraction airway relax and patency may be lost. Suction on under pillow followed by a refractory period of muscle Protective airway reflexes are also sup- Aspiration of nasogastric tube relaxation within 30–60 seconds lasting pressed. Manual manoeuvres and simple Pre-oxygenation Fraction of inspired oxygen 100% several minutes. Its relatively short-lived adjuncts such as a chin tilt, jaw thrust and effects are the result of its metabolism by Guedel airway are used as soon as the 3 minutes regular breathing or plasma cholinesterase. patient begins to lose airway tone to pre- five vital capacity breaths Non-depolarizing agents are competitive vent obstruction. Conventionally the Cricoid pressure Pressure over cricoid cartilage antagonists of acetylcholine at the post- options for maintaining the airway of an Compression of underlying synaptic nicotinic receptor and are used for anaesthetized patient are a supraglottic oesophagus more prolonged paralysis. Blocking the ion device (e.g. laryngeal mask airway) or Prevents regurgitation of gastric channel, their main action is to prevent end endotracheal intubation (Figure 2). At the contents soiling oropharynx or airway Figure 2. Supraglottic and endotracheal airways. Release pressure if vomiting Device Supraglottic Endotracheal Drugs No co-induction opioid Thiopentone 3–5 mg/kg Suxamethonium