The Practical Conduct of Anaesthesia

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The Practical Conduct of Anaesthesia


The conduct of anaesthesia is planned after details concerning the surgical procedure and the medical condition of the patient have been obtained at the preoperative visit. Preoperative assessment and selection of appropriate premedication are discussed in Chapter 17.



PREPARATION FOR ANAESTHESIA


Before starting, consideration should be given to induction and maintenance of anaesthesia, the position of the patient on the operating table, the equipment necessary for monitoring, the use of intravenous (i.v.) fluids or blood for infusion and the postoperative care and recovery facilities that will be required.


The anaesthetic machine must be tested before use for leaks, misconnections and proper function. A checklist, e.g. that published by the Association of Anaesthetists of Great Britain and Ireland (AAGBI 2004), is recommended. This is discussed in Chapter 20. The breathing system to be used should be new for each patient, or a new filter of appropriate size for each patient should be placed between the patient and the system, according to the AAGBI recommendations (2008).


The availability and function of all anaesthetic equipment should be checked before starting (see Table 21.1). The anaesthetist should be satisfied that the correct operation is being performed upon the correct patient and that consent has been given. Surgical Safety checklists are available for all the theatre team. The patient must be on a tilting bed or trolley and the anaesthetist should have a competent, trained assistant.




INDUCTION OF ANAESTHESIA


Anaesthesia is induced using one of the following techniques:



Inhalational Induction


The most common indications for inhalational induction of anaesthesia are listed in Table 21.2.



The proposed procedure should be explained to the patient before starting. A technique using a cupped hand around the fresh gas delivery tube may be preferred for young children, otherwise a face mask is used. The mask or hand is introduced gradually to the face from the side; the use of a transparent perfumed mask can render the procedure less unpleasant. While talking to the patient and encouraging normal breathing, the anaesthetist adjusts the mixture of the fresh gas flow and observes the patient’s reactions. Initially, nitrous oxide 70% in oxygen is used and anaesthesia is deepened by the gradual introduction of increments of a volatile agent, e.g. sevoflurane which can can be increased up to an inspired concentration of 6%. Maintenance concentrations of isoflurane (1–2%) or sevoflurane (2–3%) are used when anaesthesia has been established.


A single-breath technique of inhalational induction has been advocated for patients who are able to cooperate. One vital capacity breath from a prefilled 4 L reservoir bag containing a high concentration of volatile agent (e.g. sevoflurane 8%) in oxygen (or nitrous oxide 50% in oxygen) results in smooth induction of anaesthesia within 20–30 s.


Observation of the colour of the patient’s skin and pattern of ventilation, palpation of the peripheral pulse, ECG and SpO2 monitoring, and measurement of arterial pressure are important accompaniments to the technique of inhalational induction.


If spontaneous ventilation is to be maintained during the procedure, airway patency is ensured by use of an oropharyngeal airway, a laryngeal mask airway or a tracheal tube once anaesthesia has been established.




Intravenous Induction


Induction of anaesthesia with an i.v. agent is suitable for most routine purposes and avoids many of the complications associated with the inhalational technique. It is the most appropriate method for rapid induction of the patient undergoing emergency surgery, in whom there is a risk of regurgitation of gastric contents. All drugs which may be required at induction should be prepared, and a cannula inserted into a suitable vein. The anaesthetist should wear rubber gloves for this and for other procedures such as airway manipulations and insertion of an airway or tracheal tube.


If an existing i.v. cannula is to be used, its function must be checked. Cannulae with a side injection port (‘Venflon’ type) are useful; large cannulae (e.g. 16G, 14G) are necessary for transfusion of fluids or blood. A vein in the forearm or on the back of the hand is preferable; veins in the antecubital fossa should be avoided because of the risks of intra-arterial injection and problems with elbow flexion. After selection of a suitable vein and skin preparation with 2% chlorhexidine in alcohol, subcutaneous local anaesthetic can be used. Alternatively, local anaesthetic cream (‘EMLA’ or ‘Ametop’) may have been applied preoperatively. Intravenous entry is confirmed and the cannula is secured firmly with tape. ‘Opsite’ or other specific cannula dressings may be used when long-term use is anticipated.


Patient monitors, including SpO2, ECG and arterial pressure should be attached before induction of anaesthesia. Preoxygenation of the lungs may begin, using a close-fitting face mask and 100% oxygen delivered by a suitable breathing system for 5 min. Alternatively, three to four large (vital capacity) breaths may be used. Preoxygenation before routine elective induction of anaesthesia avoids transient hypoxaemia before establishment of effective lung ventilation.


Doses of the common i.v. agents are shown in Table 21.3. The induction dose varies with the patient’s weight, age, state of nutrition, circulatory status, pre-medication and any concurrent medication. A small test dose is commonly administered and its effects are observed. Slow injection is recommended in the aged and in those with a slow circulation time (e.g. shock, hypovolaemia, cardiovascular disease) while the effects of the drug on the cardiovascular and respiratory systems are assessed.



A rapid-sequence induction technique is indicated for patients undergoing emergency surgery and for those with potential for vomiting or regurgitation. After i.v. induction, a rapid transition to stage 3 anaesthesia (see below) is achieved; this is maintained by the introduction of an inhalational agent or by repeated bolus injections or a continuous infusion of an i.v. anaesthetic agent. Emergency anaesthesia is discussed fully in Chapter 37.



Complications and Difficulties


Regurgitation and vomiting. If pharyngeal regurgitation occurs, the patient should be placed immediately into the Trendelenburg position and material aspirated with suction apparatus. Should inhalation of gastric contents occur, treatment is with 100% oxygen, bronchodilators and tracheal suction. Steroids and antibiotics are not routinely administered but may be considered. Continued IPPV may be required if the resultant pneumonitis is severe.


Intra-arterial injection of thiopental. This rare complication should nowadays be avoided by the appropriate choice of venous site and by checking the ‘flashback’ of blood on cannulation before injection. Pain and blanching in the hand and fingers occurs as a result of crystal formation in the capillaries. The cannula should be left in the artery and 40 mg papaverine injected with local anaesthetic (e.g. lidocaine 1% 5 mL). Further treatment includes stellate ganglion block, brachial plexus block or sympathetic block with i.v. guanethidine.


Perivenous injection. This causes blanching and pain and may result in a small degree of tissue necrosis. Propofol produces less tissue damage than thiopental. Hyaluronidase may be used to speed dispersal of the drug.


Cardiovascular depression. This is likely to occur particularly in the elderly, the hypovolaemic or the untreated hypertensive patient. Reducing the dose and speed of injection is essential in these patients. Infusion of i.v. fluid (e.g. 500 mL colloid or 1000 mL crystalloid solution) is usually successful in restoring arterial pressure but other agents e.g. ephedrine 3–12 mg i.v. may be required.


Respiratory depression. Slow injection of an induction agent can reduce the extent of respiratory depression. Respiratory adequacy must be assessed carefully and the anaesthetist should be ready to assist ventilation of the lungs if necessary.


Histamine release. Thiopental in particular may cause release of histamine with subsequent formation of typical wheals. Severe reactions may occur to individual agents, and appropriate drugs and fluids should be available in the anaesthetic room for treatment. Guidelines for emergency management of acute major anaphylaxis are available (AAGBI) and may be displayed in the anaesthetic room. This is discussed further in Chapter 43.


Porphyria. An acute porphyric episode may be precipitated by barbiturates in susceptible individuals.


Other complications. Pain on injection (especially with etomidate or propofol), hiccup or dystonic muscular movements may occur. The use of lidocaine 10–40 mg per 20 mL propofol 1% reduces the incidence of pain on injection.



POSITION OF PATIENT FOR SURGERY


After induction of anaesthesia, the patient is placed on the operating table in a position appropriate for the proposed surgery. When positioning the patient, the anaesthetist should take into account surgical access, patient safety, anaesthetic technique, monitoring and position of i.v. cannulae, etc.


Some commonly used positions are shown in Figure 21.1. Each may have adverse effects in terms of skeletal, neurological, ventilatory and circulatory effects.



The lithotomy position may result in nerve damage on the medial or lateral side of the leg from pressure exerted by the stirrups, which must be well padded. Care must be taken to elevate both legs simultaneously so that pelvic asymmetry and resultant backache are avoided. The sacrum should be supported and not allowed to slip off the end of the operating table.


The lateral position may result in asymmetrical lung ventilation. Care is required with arm position and i.v. infusions. The pelvis and shoulders must be supported to prevent the patient from rolling either backwards (with a risk of falling from the table) or forwards into the recovery position.


The prone position may cause abdominal compression which may result in ventilatory and circulatory embarrassment. To prevent this, support must be provided beneath the shoulders and iliac crests. Excessive extension of the shoulders should be avoided. The face, and particularly the eyes, must be protected from external pressure or trauma. The tracheal tube must be secured firmly in place as it is almost impossible to reinsert it with the patient in this position.


The Trendelenburg position may produce upward pressure on the diaphragm because of the weight of the abdominal contents. Damage to the brachial plexus may occur as a result of pressure from shoulder supports, especially if the arms are abducted.


The sitting position requires careful support of the head. In addition, venous pooling and resultant cardiovascular instability may occur.


The supine position carries the risk of the supine hypotensive syndrome during pregnancy (see Ch 35) or in patients with a large abdominal mass.


Positioning during anaesthesia is discussed extensively by Martin & Warner (1997).



MAINTENANCE OF ANAESTHESIA


Anaesthesia may be continued using inhalational agents, i.v. anaesthetic agents or i.v. opioids either alone or in combination. Tracheal intubation with or without muscle relaxants may be used. Regional anaesthesia may be used to supplement any of these techniques to achieve the components of the familiar anaesthetic triad of sleep, neuromuscular relaxation and analgesia.



Inhalational Anaesthesia with Spontaneous Ventilation


This is an appropriate form of maintenance for superficial body surgery, minor procedures which produce little reflex or painful stimulation and operations for which profound neuromuscular blockade is not required.




Minimum Alveolar Concentration


Minimum alveolar concentration (MAC) is the minimum alveolar concentration of an inhaled anaesthetic agent which prevents reflex movement in response to surgical incision in 50% of subjects. MAC values of commonly used inhalational agents are shown in Appendix C. MAC varies little with metabolic factors but is reduced by opioid medication and in the presence of hypothermia. MAC is higher in neonates and is reduced in the elderly (see Ch 2). The effects of inhalational anaesthetics are additive: thus 1 MAC-equivalent could be achieved by producing an alveolar concentration of 70% nitrous oxide (0.67 MAC) and 0.4% isoflurane (0.33 MAC).


The rate at which MAC is attained may be increased by raising the inspired concentration and by avoidance of airway obstruction. Increasing ventilation at a constant inspired concentration produces more rapid equilibration between inspired and alveolar concentrations. The time taken for equilibration increases with the blood/gas solubility coefficient of the agent; those with a high blood/gas solubility coefficient (e.g. halothane) do not reach equilibrium for several hours (see Ch 2). It follows, therefore, that the inspired concentration must be considerably higher than MAC to produce an adequate alveolar concentration when such agents are used.


Control of the depth of anaesthesia to achieve adequacy without overdose by varying the inspired concentration of volatile agent requires constant assessment of the patient’s reaction to anaesthesia and surgery. This rapid control is one of the main advantages of inhalational anaesthesia. The signs of inadequate depth of anaesthesia include tachypnoea, tachycardia, hypertension and sweating.



Signs of Anaesthesia


Guedel’s classic signs of anaesthesia are those seen in patients premedicated with morphine and atropine and breathing ether in air. The clinical signs associated with anaesthesia produced by other inhalational agents follow a similar course, but the divisions between the stages and planes are less precise (Fig. 21.2).



Stage 1: the stage of analgesia. This is the stage attained when using nitrous oxide 50% in oxygen, as used in the technique of relative analgesia (see Ch 29).


Stage 2: stage of excitement. This is seen with inhalational induction, but is passed rapidly during i.v. induction. Respiration is erratic, breath-holding may occur, laryngeal and pharyngeal reflexes are active and stimulation of pharynx or larynx, e.g. by insertion of a Guedel or laryngeal mask airway, may produce laryngeal spasm. The eyelash reflex (used as a sign of unconsciousness with i.v. induction) is abolished in stage 2, but the eyelid reflex (resistance to elevation of eyelid) remains present.


Stage 3: surgical anaesthesia. This deepens through four planes (in practice, three – light, medium, deep) with increasing concentration of anaesthetic drug. Respiration assumes a rhythmic pattern and the thoracic component diminishes with depth of anaesthesia. Respiratory reflexes become suppressed but the carinal reflex is abolished only at plane IV (therefore, a tracheal tube which is too long may produce carinal stimulation at an otherwise adequate depth). The pupils are central and gradually enlarge with depth of anaesthesia. Lacrimation is active in light planes but absent in planes III and IV – a useful sign in a patient not premedicated with an anticholinergic.


Stage 4: stage of impending respiratory and circulatory failure. Brainstem reflexes are depressed by the high anaesthetic concentration. Pupils are enlarged and unreactive. The patient should not be permitted to reach this stage. Withdrawal of the anaesthetic agents and administration of 100% oxygen lightens anaesthesia.


Observation of other reflexes provides a guide to depth of anaesthesia. Swallowing occurs in the light plane of stage 3. The gag reflex is abolished in upper stage 3. Stretching of the anal sphincter produces reflex laryngospasm even at plane III of stage 3.



Complications and Difficulties


Airway obstruction. This is relieved by appropriate positioning and the use of airway equipment (see below).


Laryngeal spasm. This may occur above light-medium planes of stage 3 as a result of stimulation. Treatment is to stop the stimulation and gently deepen anaesthesia. If spasm is severe, 100% oxygen is applied with the face mask held tightly, while the airway is maintained by hand and pressure is applied to the reservoir bag. Attempts to ventilate the patient’s lungs usually result only in gastric inflation. However, as the larynx partially opens, 100% oxygen flows through under pressure. Further gentle deepening of anaesthesia may then take place. In severe laryngeal spasm, i.v. succinylcholine may be required, and after the lungs have been inflated with oxygen it is advisable to intubate the trachea.


Bronchospasm. This may occur if volatile anaesthetic agents are introduced rapidly, particularly in smokers or those with excessive bronchial secretions. Humidification and warming of gases may minimize the problem. Bronchospasm may accompany laryngospasm. Administration of bronchodilators may be required. This complication occurs readily in the presence of or shortly after a respiratory tract infection.


Malignant hyperthermia. Volatile agents, succinylcholine or amide-type local anaesthetic agents may trigger this syndrome in susceptible individuals (see Ch 43).


Raised intracranial pressure (ICP). All volatile agents may produce an increase in ICP and this is accentuated by retention of CO2 which accompanies the use of volatile agents in the spontaneously breathing patient. A spontaneous ventilation technique is therefore contraindicated in patients with an intracranial space-occupying lesion or cerebral oedema.


Atmospheric pollution. The use of the appropriate scavenging apparatus helps to reduce levels of theatre pollution by volatile and gaseous agents (see Ch 20).



Delivery of Inhalational Agents – Airway Maintenance


Maintenance of the airway is one of the most important of the anaesthetist’s tasks. Inhalational agents may be delivered via a face mask, a laryngeal mask airway (LMA) or another supraglottic airway device (SAD), or a tracheal tube.


May 31, 2016 | Posted by in ANESTHESIA | Comments Off on The Practical Conduct of Anaesthesia
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