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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 always involves an will help readers to get more from clinical n Local anaesthetic field block agent, usually an and attachments in and anaesthetics or n Peripheral 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 space filled with fat and surgical access requirement and the Anaesthetic practice has evolved from a venous plexi. † extends to S2, containing degree of or stimulation anticipated. need for pain relief and altered conscious- arachnoid mater, CSF, pia mater, 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 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 , computed venous agent . 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 . Delays in inducing anaesthesia may patients. Certain regional procedures ligament ligament represent slow arm– circulation time may take place in pain clinics and out- (e.g. elderly, cardiovascular ), patient settings. In a reversible state Table 1. Characteristics of different central neuraxial blocks of 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 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 causing haemodynamic instability Gradual titration causing less haemodynamic ([email protected]) disturbance

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CTD_C71_C75_anaesthesia.indd 71 26/04/2013 16:24 patient anxiety, or abdominal pathology, an un-fasted patient plate depolarization and propagation of the extravasation. An 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 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 and as offset is not reliant on organ function. As Inhalational induction regurgitation is reduced (Sinclair and the percentage 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 . 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) 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 influx and acetylcho- produce a consistent muscular response. : 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 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 are also sup- Aspiration of nasogastric tube relaxation within 30–60 seconds lasting pressed. Manual manoeuvres and simple Pre-oxygenation Fraction of inspired 100% several minutes. Its relatively short-lived adjuncts such as a chin tilt, jaw thrust and effects are the result of its 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 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. ) 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 1–2 mg/kg Intubation Once tracheal Ventilation Features Sits above vocal cords Passes through vocal cords intubation commenced Maintains airway Inflated cuff confirmed: Cricoid pressure released No airway protection against aspiration Airway protected

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preoperative visit, history, examination and effector site concentration can be dialled systemic effects of anaesthesia vary with the review of investigations and previous up alongside basic patient demographics drugs used so different agents are favoured anaesthetic charts contribute to the assess- (age, sex, weight) and the pump adjusts the in different clinical contexts. In general, ment of the airway and perioperative plan- rate of infusion to achieve the specified intravenous (propofol and thiopentone) ning (Cranshaw and Cook, 2011). drug concentration. This is known as a and volatile agents all reduce pres- target controlled infusion. Effective secure sure as a result of vasodilation, and negative Maintenance intravenous access is crucial. inotropy and chronotropy. Starting posi- Maintenance of anaesthesia refers to keep- The choice of maintenance technique tive pressure ventilation (i.e. ventilating ing a patient unconscious and can be may be determined by surgical and patient someone) can impede venous return to the achieved using inhaled volatile agents or factors and the experience of the anaes- , reducing preload and . continuous infusion of intravenous agents. thetist. Total intravenous anaesthesia is The sympathetic stimulation from surgery Volatile agents are most commonly used, often used in day surgery, or opposes these changes. delivered via vaporisers found on the ‘back if patients get severe postoperative Intravenous (propofol, thiopentone and bar’ of the which feed and vomiting as it avoids emetogenic ) and volatile agents are all respi- into the . The concentra- volatiles and enables rapid recovery with ratory and depress airway tions of the inhaled agents are measured minimal hangover effect (Yuill and reflexes to differing degrees. Propofol is and displayed. Expired end tidal concen- Simpson, 2002). particularly effective at inducing transient tration is equivalent to the alveolar concen- apnoea and depressing airway reflexes tration which in turn represents the con- Systemic effects of general facilitating placement of supraglottic centration at the site of action (CNS). This anaesthesia devices post induction. Of the volatile gives the anaesthetist an idea of the amount General anaesthesia to multi-system agents, is the least irritant to of anaesthetic agent reaching the patient physiological changes (Tables 3 and 4). The airways making it particularly suitable for and the likely depth of anaesthesia. The minimal alveolar concentration is the alve- Table 3. Systemic effects of general anaesthesia olar concentration of a volatile agent which when given alone prevents movement in System Common anaesthetic agents 50% of healthy volunteers to a standard Cardiovascular : mean arterial pressure = ( Normotension or surgical stimulus (e.g. skin incision). The x stroke volume) x systemic vascular resistance minimal alveolar concentration varies Vasodilation (↓systemic vascular resistance) between different volatile agents inversely related to their (as their structures Negative chronotropy (↓heart rate) vary) and is also affected by other pharma- Negative inotropy (↓stroke volume) cological and physiological variables (Yentis Respiratory Loss of airway reflexes and tone Airway reflexes and tone maintained et al, 2009). Bronchodilation Intravenous maintenance of anaesthesia can be achieved with infusions of propofol GastrointestinalI Propofol = Salivation with or without an opioid delivered via a Volatiles = emetogenic Emetogenic pump. Several pharmacokinetic models CNS Hypnosis anaesthesia, analgesia, have been developed which map the theo- retical body compartments among which a From Sasada and Smith (2008) drug distributes. The desired plasma or Table 4. Stages of a general anaesthetic: an A, B, C, D approach

Stage of general anaesthesia Airway Breathing Circulation Drugs Induction Plan for securing, maintaining High flow oxygen at Vasodilation leads to reduced systemic Intravenous: analgesia (opioid co-induction, e.g. and protecting airway as soft induction, consider vascular resistance and mean arterial ) then hypnotic agent (e.g. propofol, tissue tone and reflexes are lost pre-oxygenation pressure, intubation can cause thiopentone) with or without muscle relaxation sympathetic hypertensive response or volatile induction (sevoflurane) Maintenance Maintain airway position and Maintain saturations, Maintain adequate cardiac output Volatile (e.g. sevoflurane, , ) patency ventilatory strategies, and tissue perfusion, fluid balance Intravenous (total intravenous anaesthesia, e.g. lung protection propofol +/- ), analgesia, antiemesis Emergence Suction secretions, as airway Increase fraction of Time of haemodynamic instability Reversal of neuromuscular block tone and reflexes return plan inspired oxygen, ensure for safe removal of supraglottic adequate spontaneous device or extubation tidal volumes

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CTD_C71_C75_anaesthesia.indd 73 26/04/2013 16:24 gaseous induction and paediatrics. Perioperative care for awareness can include Desflurane is irritant and is therefore often Warming clinical observation (such as papillary dila- used once anaesthesia and airway have As homeothermic mammals, our core tation, lacrimation, sweating) and meas- been established. Both sevoflurane and is designed to be around urement (heart rate, , end isoflurane are and may 36.5°C. Patients are susceptible to hypo- tidal volatile concentration and depth of even have a role in the management of thermia under anaesthesia as a result of anaesthesia monitors). Most depth of brittle . vasodilation causing redistribution of heat anaesthesia monitors interpret patterns of Ketamine is an exceptional intravenous from core to periphery, convection, radia- neuronal electrical activity to deduce the agent in that it maintains cardiovascular tion (exposed areas), conduction (contact level of conscious state (Al-Shaikh and stability and preserves muscular tone, air- with cold metal objects), evaporation Stacey, 2007). way patency and bronchodilates in addi- (endotracheal tube bypasses nasopharyn- tion to its analgesic properties. It is termed geal humidification, exposed moist mucos- Other drugs a ‘dissociative anaesthetic’, meaning the al surfaces) and loss of compensatory heat- Aside from the traditional triad of anaes- patient may be unaware and detached from preserving or heat-generating mechanisms, thesia drugs, the cardiovascular system is his/her surroundings but not completely e.g. shivering. often manipulated to offset the effects of unconscious. Its attributes make it useful can cause coagulopathy, anaesthesia or surgical stimulation. Heart in haemodynamically unstable patients, perioperative cardiac events, increased risk rate may be increased by an anti-mus- the developing world and field anaesthesia of postoperative and can pro- carinic (e.g. atropine or glycopyrrolate) (Peck et al, 2008). long recovery and hospital stay. Exposure or a mixed beta-adrenoceptor should be minimized and temperature (e.g. ) or reduced by beta- Emergence and recovery monitored pre-, intra- and postoperative- blockers. Blood pressure can be increased Once anaesthesia is no longer required, ly. Warm air devices and warmed fluids by vasoconstricting with an alpha 1 maintenance agents can be switched off. can be used to offset heat loss and main- adrenoceptor agonist such as metarami- Before emergence, adequate analgesia and tain optimal body temperature (Harper et nol or reduced with an alpha antagonist anti-emesis should be ensured and neuro- al, 2008). (e.g. phentolamine). muscular junction function restored if a has been used. Fluid balance Analgesia Like induction, emergence can be a The anaesthetist needs to be an expert in Pain relief is very important for patients time of physiological disturbance. As fluid resuscitation, using crystalloids, col- and features in the triad of anaesthesia. patients start to wake from anaesthesia or loids and blood products where appropri- Despite a patient being unconscious and ‘lighten’ they may develop agitation, ate. The aim is to ensure good tissue unaware intraoperatively, stimulation (e.g. and breath-holding. perfusion and hence oxygenation. Rather surgery) will still elicit a sympathetic Conventionally extubation is performed than give a fixed fluid dose, monitors response which analgesia can desirably following oropharyngeal suction, once the (e.g. oesophageal doppler) are often used attenuate. Appropriate analgesia is also patient is generating good tidal volumes and fluid challenges given to achieve a set essential for smooth emergence and com- and is awake, ensuring airway reflexes endpoint, aiming to avoid hyper- or fort immediately after surgery. Analgesia is have returned and the patient will protect hypovolaemia (Doherty and Buggy, typically multi-modal with titrated his/her own airway. In certain circum- 2012). to extent of stimulation and predicted stances extubation may be performed postoperative pain. ‘deep’, i.e. with the patient still under Positioning anaesthesia. Under anaesthesia, airway Patients are vulnerable to nerve and pres- Safety reflexes will remain suppressed, reducing sure point injury under anaesthesia and Patient safety is crucially important. The the risk of coughing, laryngospasm and protection of these areas is the responsibil- World Health Organization surgical safe- hypertension associated with extubation. ity of the anaesthetist. Patients should ide- ty checklist is a tool to attempt to make This may be preferable in certain neuro- ally be in a neutral position with padding the perioperative journey safer and surgical and cardiac patients in whom used to support at-risk areas (Knight and enhance team communication. The entire surges in intracranial or systemic blood Mahajan, 2004). team must ensure the correct patient is pressure should be avoided. However, the consented for the correct procedure and airway will be unprotected against aspira- Awareness that any allergies or potential complica- tion until the patient is awake. Awareness is the unplanned recall of events tions are acknowledged and shared among The recovery room is an intermediate under anaesthesia and is often one of the the team. place of safety between theatre and the complications patients fear most. It can be The World Health Organization surgi- ward where immediate surgical or anaes- implicit or explicit, from a vague of cal safety checklist has three components thetic complications can be detected and having been awake through to specific which are completed on arrival to the managed. , pain scores and other memories of events and conversations anaesthetic room, before the start of sur- potential problems such as postoperative respectively. Awareness is distressing and gery or intervention and at the end of the nausea and vomiting are monitored. can to post-traumatic syndromes. procedure (Walker et al, 2012).

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Conclusions P, Kenny G, eds. Basic Physics and Measurement in Care. 3rd edn. Cambridge University Press, Anaesthesia. 5th edn. Butterworth-Heinemann, Cambridge: 99–102 Anaesthesia is an enormous subject and London: 171 Sasada M, Smith S (2008) Drugs in Anaesthesia and this article is merely a tip of the iceberg Doherty M, Buggy D (2012) Intraoperative fluids: Intensive Care. 3rd edn. Oxford University Press, introduction to some types of regional and how much is too much? Br J Anaesth 109(1): Oxford 69–79 Sinclair RCF, Luxton MC (2005) Rapid sequence general anaesthesia. Harper CM, Andrzejowski JC, Alexander R (2008) induction. Contin Educ Anaesth Crit Care Pain Anaesthetists, while developing special- NICE and warm. Br J Anaesth 101(3): 293–5 5(2): 45–8 ized airway skills and a deep understanding King JM, Hunter J (2002) of the Walker A, Reshamwalla S, Wilson I (2012) Surgical neuromuscular junction. Br J Anaesth CEPD safety checklists: do they improve outcomes? Br J of physiology and , need an Reviews 2(5): 129–33 Anaesth 109(1): 47–54 holistic approach and broad knowledge Knight DJW, Mahajan RP (2004) Patient Yentis S, Hirsch N, Smith G (2009) Anaesthesia and base because of the varied nature of their positioning in anaesthesia. Contin Educ Anaesth Intensive Care A-Z. 4th edn. Churchill Crit Care Pain 4(5): 160–3 Livingstone, London: 354 role. Anaesthetists will come into contact Peck TE, Hill S, Williams M (2008) Core drugs in Yuill G, Simpson M (2002) An introduction to total with approximately two thirds of hospital anaesthetic practice. In: Peck TE, Hill S, Williams intravenous anaesthesia. Br J Anaesth CEPD patients in a diverse range of clinical con- M, eds. Pharmacology for Anaesthesia and Intensive Reviews 2(1): 24–6 texts and environments. Hopefully this article has whetted your appetite to know more or given you a fresh insight into a KEY POINTS specialty which is taking place in all cor- n Anaesthesia means loss of sensation and can be divided into regional anaesthesia (blockade of nerve ners of your hospital. BJHM transmission) or general anaesthesia (a reversible state of unconsciousness). n General anaesthesia often comprises a triad of hypnosis, analgesia and muscle relaxation. Conflict of interest: none. n General anaesthesia can be divided into three stages: induction, maintenance and emergence. Al-Shaikh B, Stacey S (2007) Non invasive monitoring. In: Al-Shaikh B, Stacey S, eds. n Under general anaesthesia airway tone and reflexes are lost and the airway must be maintained Essentials of Anaesthetic Equipment. 3rd edn. with manual manoeuvres, adjuncts (Guedel, laryngeal mask airways) or definitive devices which also Churchill Livingstone, London: 151–3 Appiah-Ankam J, Hunter J (2004) Pharmacology of protect the airway from regurgitation and aspiration (e.g. endotracheal tubes). neuromuscular blocking drugs. Contin Educ Anaesth Crit Care Pain 4(1): 2–7 n General anaesthesia leads to multi-system physiological changes particularly at induction and Cranshaw J, Cook T (2011) Airway assessment and emergence. management. In: Allman K, Wilson I, eds. Oxford Handbook of Anaesthesia. 3rd edn. Oxford n Other aspects of perioperative care central to anaesthetic practice include thermal homeostasis, fluid University Press, Oxford: 970–6 balance, positioning, avoidance of awareness, analgesia and patient safety. Davis P, Kenny G (2007) Biological Electrical Potentials: Their display and recording. In: Davis

Real people recounting their stories through the hard times and the road to recovery

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