General
anaesthesia
Induction
of anaesthesia
Intravenous injection is most common in contemporary practice,
although the recent introduction of nonpungent sevoflurane has led to renewed
use of inhalational induction. Inhalational induction is useful in young
children, or ‘needle phobic’ adults, and may also be used in patients who
are at risk
Maintenance
of anaesthesia
Following the induction of anaesthesia, inhalational volatile or
intravenous anaesthetic agents are continuously administered to maintain an
adequate depth of anaesthesia. Adding nitrous oxide contributes analgesic and
weak anaesthetic effects, which reduce the concentration of volatile anaesthetic
agent required for maintenance. To provide a safety margin, at least 30 per cent
oxygen is added to the inspired mixture. Although still employed in some parts
of the world, ether has generally been replaced by halothane, enflurane and
isoflurane. Desflurane and sevoflurane are the most recently introduced agents,
conferring the advantages of fewer side effects and more rapid recovery.
The
use of nitrous oxide is slowly waning, as oxygen-enriched air with volatile or
intravenous maintenance gains popularity.
If
compressed sources of oxygen, nitrous oxide or air are scarce, then air may be
drawn into the anaesthetic circuit, either by the (unparalysed) patient’s own
respiratory effort or by a mechanical ventilator.
Total
intravenous anaesthesia, a technique undergoing contemporary resurgence, avoids
the use of inhalational anaesthetic agents and is claimed to provide enhanced
quality and rapidity of recovery. It is also used when inhalational anaesthesia
may be impractical, such as during airway laser surgery or endoscopy, and is
popular for cardiopulmonary bypass. It is also indicated in spinal surgery
during neurophysiological monitoring of cord integrity, as evoked potential
signals are suppressed by inhalational anaesthesia. Intravenous anaesthesia
avoids atmospheric pollution, and is usually conducted by infusing propofol and
a short acting opioid analgesic agent, such as fentanyl or alfentanil, in
combination with neuromuscular block and pulmonary ventilation with a mixture of
air and oxygen.
Management
of the airway during anaesthesia
General anaesthesia reduces the tone of the muscles required to preserve
airway patency, and hence the need for manual methods (e.g. jaw thrust), or
devices such as the Guedel or laryngeal mask airways, or endotracheal tubes.
Sir
Ivan Magill developed the endotracheal tube during World War I to facilitate
plastic surgery around the mouth
The
following means of airway control in the anaesthetised or unconscious patient
are used (Fig. 6.1).
Positioning
of the tongue and jaw
The anaesthetist thrusts the jaw forward, from behind the
temporomandibular joints, thereby elevating the tongue off the posterior
pharyngeal wall, which may also be achieved by inserting an artificial
oropharyngeal airway such as the ‘Guedel’. The anaesthetic gases are given
through a face mask.
The
laryngeal mask airway (LMA) (developed by Archie Brain) is also inserted
via the mouth, and is positioned with the mask over the larynx, sealed by an
inflatable cuff. It frees the anaesthetist’s hands from holding the
patient’s jaw or face mask. Its placement is less stimulating than
endotracheal intubation. It has proved to be a reliable means of maintaining a patent airway, and is a technique readily taught to
nonanaesthetists for emergency airway management. It is likely to replace the
face mask for immediate care prior to endotracheal intubation.
The
endotracheal tube may be passed into the trachea via either the mouth or
the nose. It is usually placed by direct laryngoscopy, using a laryngoscope, but
it is occasionally impossible to visualise the larynx. A fibreoptic technique
may be used in which the tracheal tube is ‘rail-roaded’ over the flexible
laryngoscope, once the tip has been steered into the trachea.
A
cuffed endotracheal tube is used to facilitate artificial ventilation or surgery
around the face or airway, and to protect the lungs if there is a risk of
pulmonary aspiration. If fluid may collect in the mouth from above (as in nasal
surgery), a throat pack is placed in the oropharynx.
Although
endotracheal intubation is generally straight forward, complications do occur:
• accidental and unrecognised oesophageal intubation;
• accidental intubation of a main bronchus;
• trauma to larynx, trachea or teeth;
• aspiration of vomitus during neuromuscular blockade for intubation;
• failure to intubate and loss of airway control;
•
disconnection or blockage of the tube;
• delayed tracheal stenosis, in children or after prolonged
Careful
observation of physical signs and constant vigilance, aided by pulse oximetry,
capnography of the expiratory gases, inspiratory oxygen concentration
measurement and ventilator disconnection alarms are mandatory to minimise these
risks.
Tracheostomy
tube
Anaesthesia can safely be conducted through a tracheostomy tube but it
should have an inflatable cuff for airway control. Silver or fenestrated
tracheostomy tubes should therefore be replaced by plastic cuffed tubes at
induction of anaesthesia.
Endobronchial
tube
In pulmonary and open oesophageal surgery, selective intubation of
either bronchus is usual to facilitate deflation of the lung on the operated
side. Its use is essential to protect the normal lung in the presence of a
bronchopleural fistula.
Ventilation
through a bronchoscope
The lungs can be ventilated during bronchoscopy by intermittent jets
of oxygen down a cannula within the bronchoscope. The oxygen entrains air by
the Venturi effect to generate enough pressure and flow to inflate the lungs.
The
technique demands constant observation of the patient’s chest movement.
Neuromuscular
blockade during surgery
Pharmacological blockade of neuromuscular transmission provides
relaxation of muscles to facilitate surgery and mechanical positive pressure
ventilation. Muscle tone may also be reduced by very deep anaesthesia, but may
compromise the circulation. Neuromuscular blockade demands complete control of
the airway and ventilation by the anaesthetist. The depolarising muscle
relaxant, suxamethonium, rapidly provides excellent intubating conditions of
brief duration, but commonly causes postoperative diffuse muscle pains, and
rarely may cause a prolonged block if the patient is deficient in plasma
pseudocholinesterase.
The
competitive neuromuscular blocking agents such as curare and its modern
successors produce a prolonged effect which may persist into the postoperative
period. Atracurium, cisatracurium, vecuronium and rocuronium share more
predictable activity profiles and are less dependent on hepato renal function.
A peripheral nerve stimulator is also used to check for adequate depth of
blockade during surgery, and to confirm satisfactory recovery of neuromuscular
function prior to extubation of the trachea. The advent of neuromuscular
blockade in the 1940s facilitated many advances in abdominal and thoracic
surgery, but introduced the risk of accidental patient awareness during surgery.
Haemostasis
and blood pressure control
Although the dangers of profound hypotension are nowadays well accepted,
a 20—3 0 per cent reduction of mean arterial blood pressure from the awake
preoperative level in fit patients is still deemed acceptable, and can greatly
improve the quality of the operative field and reduce total blood loss.
Reduction of venous pressure at the wound by correct patient positioning and
avoidance of any causes of venous obstruction (Fig. 6.2), and maintenance of
satisfactorily deep anaesthesia and slightly reduced arterial carbon dioxide
tension, further contribute to providing a dry surgical field.
Hypotensive
drugs may be used to produce deliberate controlled hypotension if there is a
clear surgical benefit to be obtained, although preservation of cerebral
perfusion and oxygenation remains paramount. The surgeon must be aware of the
prevailing blood pressure, particularly at the time of ensuring satisfactory
haemostasis prior to wound closure. Usually, the anaesthetist will attempt to
bring the blood pressure back to the normal level at this stage of the
procedure.
Management
of temperature during anaesthesia
Hypothermia develops quickly during anaesthesia and surgery due to
vasodilation, cold infusions of fluid, and loss of body heat by radiation and
fluid evaporation from open body cavities. It is a particular hazard in children
because of the high ratio of body surface area to body mass. .The elderly are
also at particular risk as hypothermia and shivering increase oxygen consumption
and vascular resistance, predisposing to myocardial infarction. In high-risk
patients and for long operations, warm air blowers and warming blankets should
be used, and fluids for intravenous infusion, or irrigation of body cavities or
organs (such as the bladder and renal pelvis), should be warmed to body
temperature. Careful intraoperative temperature control greatly reduces
postoperative morbidity.
Monitoring
during anaesthesia
Accurate monitoring of vital functions during anaesthesia is now
regarded as obligatory in all parts of the world. Surgery should not be
practised where proper facilities for monitoring and cardiopulmonary
resuscitation are not available. The basic parameters monitored are inspiratory
oxygen concentration, oxygen saturation by pulse oximetry, expiratory carbon
dioxide tension measurement, blood pressure and the electrocardiogram. For major
surgery, invasive, direct monitoring of the circulation is used, but the
potential value of information gained must be weighed against the possible
dangers of placing intra arterial or central venous or pulmonary artery
catheters. Hourly observation of urine output via a urinary catheter is most
helpful in assessing renal perfusion. Ventilators should all have airway
pressure monitors and disconnection alarms.
In
the UK, the Association of Anaesthetists recommends the following standards of
routine monitoring in the anaesthetised patient (Fig. 6.3):
• the continuous presence of an adequately trained anaesthetist;
• regular blood pressure and heart rate measurements (recorded);
• continuous monitoring of the electrocardiography (ECG) throughout
anaesthesia;
• continuous analysis of the oxygen content in the inspiratory gas
mixture;
• oxygen supply failure alarm;
• ventilator disconnection alarm;
• pulse oximeter;
• capnography (measurement of end-tidal carbon dioxide content);
• temperature measurement availability;
• neuromuscular monitoring availability.
In
the USA, spirometry during anaesthesia is regarded as necessary.
Recovery
from general anaesthesia
Recovery from general anaesthesia should be closely supervised by
trained nursing staff in an area equipped with the means of resuscitation and
with adequate monitoring devices. An anaesthetist should be readily available.
For the seriously ill patient, a high dependency unit or intensive care unit may
be necessary until the patient’s condition is satisfactory and stable. The
transition from tracheal intubation with ventilatory support to spontaneous
breathing with an unprotected airway is a time of increased risk, when
respiratory
arrest or obstruction may occur.
The
common causes of failure to breathe after general anaesthesia are:
• central sedation from opioid drugs or
anaesthetic agents;
• hypoxia or hypercarbia of any cause;
• hypocarbia from mechanical overventilation;
• persistent neuromuscular blockade;
• pneumothorax from pleural damage during anaesthesia or surgery;
• circulatory failure leading to respiratory arrest.
Management
of blood pressure in the recovery room
Hypotension
This may be due to hypovolaemia, prolonged vasodilation or myocardial
depressant effects of anaesthetic, drugs, cardiac dysrhythmia or hypoxaemia.
Management is by treatment of the cause.
Hypertension
Hypertension is common postoperatively, usually of brief duration and
associated with peripheral vasoconstriction due to pain, fear, cold or
shivering, or pre-existing hypertensive disease. Hypertension predisposes to
cerebral and myocardial damage, and needs active management. If it persists in
the presence of adequate pain relief, sublingual nifedipine and intravenous
labetalol or hydralazine are useful. Rarely, control with more powerful
intravenous drugs such as sodium nitroprusside or glyceryl trinitrate is
necessary, in conjunction with direct intra-arterial blood pressure monitoring.
General
anaesthesia for day-case surgery
Day-case management already accounts for about 40 per cent of procedures
in UK hospitals and is intended to reach over 60 per cent. While the principles
remain the same, it is even more necessary for the day-case patient to recover
rapidly from general anaesthesia and mobilise with the minimum of side-effects.
Longer and more complex operations are now conducted as day cases, demanding
high-quality anaesthesia and effective analgesic strategies.
Careful
selection of patients is essential with regard to coexisting diseases, the
nature of the proposed surgery, the availability of a suitable escort and
transport home, and domiciliary care. Well-controlled nondebilitating chronic
diseases do not preclude day care, but may require a longer period of
postoperative supervision before permitting discharge home.
General
anaesthesia combined with regional anaesthesia, as for inguinal hernia repair,
is often suitable. Anaesthetics which promote rapid recovery such as propofol,
sevoflurane and desflurane are used. Drugs with prolonged depressant central
action, including premedicant drugs, are avoided and, where possible, analgesics
should act peripherally or be of
short duration if centrally acting. Where possible, patients are managed
with a laryngeal mask or face mask, although endotracheal intubation is the
necessary airway control for many day cases such as oral or ear, nose and throat
(ENT) procedures, and is generally uncomplicated.
With
the patient relatively isolated from immediate medical supervision and advice,
postoperative analgesia must be carefully tailored to the procedure, especially
in the case of the more painful procedures (such as hernia repair,
haemorrhoidectomy, tubal surgery, meniscectomy) for which stronger analgesics
and combinations should be provided. Regular postoperative dosing is recommended
to avoid breakthrough pain, as may particularly occur following the initial
benefits of local anaesthetic.
General
anaesthesia and cardiopulmonary bypass
The technique is discussed in Chapter 48 of this book.
Anaesthetic
agents and oxygen cannot be delivered to the circulation through the lungs when
the lungs are bypassed, so all drugs are given directly into a vein or into the
blood while it passes through the oxygenator.