Preparation for anaesthesia
Recognition of general medical and specific
anaesthetic risk factors facilitates the implementation of pre-emptive measures
and improves patient safety. Early assessment, liaison with the anaesthetist and
appropriate investigations avoid unnecessary delays. In any case, the
anaesthetist who is to be present during the operation should assess the patient
preoperatively and participate in the preparation for surgery.
Preoperative
evaluation and management
Investigation of the general condition of the
patient before surgery should be specific according to the general history and
clinical signs. Investigations in fit people are unnecessary and uneconomic, but
indicated tests should be performed as early as possible, preferably before
admission. Routine haematological and biochemical screens, with
electrocardiography
and chest radiography, are prudent investigations in elderly people receiving
general anaesthesia for all but minor surgery. The saving of a serum sample for
transfusion cross-match, a check for hepatitis antigen and a sickle-cell screen,
if indicated, should not be forgotten.
Cardiovascular
disease
Uncontrolled hypertension and angina,
dysrhythmias and cardiac failure are common reasons for postponement of elective
procedures. Correction of hypertension and ischaemic heart disease is
essential and needs to be continued through the operative period, even though
the patient may be unable to take oral drugs. Fast atrial fibrillation needs to
be controlled before anaesthesia. Symptomatic disorders of sinoatrial
conduction require pacemaker insertion before anaesthesia, as do all cases of
either Mobitz type 2 second-degree block or third-degree heart block. In an
emergency, transvenous temporary pacing wires or external pacing can be used.
Modern variable-rate demand pacemakers may require resetting to fixed rate mode,
but are generally stable during anaesthesia. However, a cardiological opinion
should be sought, bipolar diathermy employed if possible and the.
Recent
myocardial infarction is a strong contraindication to elective anaesthesia.
There is a significant mortality from anaesthesia within 3 months of infarction,
and elective procedures should ideally be delayed until at least 6 months have
elapsed.
Patients
with valvular disease will need corrective treatment of any preoperative
infections, and appropriate perioperative prophylactic antibiotic cover, to
avoid subacute bacterial endocarditis.
Patients
with cardiac disease need careful preoperative evaluation. Much can be derived
from a detailed history including exercise tolerance and drug history.
Echocardiography has enabled noninvasive assessment of cardiac function. Any
electrolyte abnormality (especially hypokalaemia) or anaemia should be corrected
and the circulatory volume should be maintained at normal level. Perioperatively,
the presence of an adequate urine output is a useful indicator of adequacy of
the circulating volume.
Operative
procedures create an increased demand for oxygen due to pain, surgical stress
and temperature loss. Patients with cardiac disease may need a period of
elective postoperative mechanical pulmonary ventilation after surgery, until
the period of raised oxygen consumption has passed. The careful anaesthetist and
surgeon plan such care before surgery.
Respiratory
disease
Thoracic surgical procedures demand specific
preoperative tests of respiratory function including spirometry and blood gas
analysis. In general surgical practice, respiratory infection and asthma are the
common problems needing treatment before anaesthesia. In chronic respiratory
failure, careful attention should be given to perioperative physiotherapy, early
mobilisation and treatment of infection. Measurement of oxygen saturation and
blood gas tensions preoperatively give a very useful guide to future values on
recovery. The need for postoperative ventilatory support should be anticipated.
Regional anaesthesia as appropriate is advantageous in respiratory disease.
Upper abdominal and thoracic procedures are unsuited to regional anaesthesia
alone, as positive pressure ventilation under general anesthesia is necessary.
G
Aspiration of gastric contents carries a high
risk of acid pneumonitis, pneumonia and death. Regurgitation in the presence of
a hiatus hernia, or from ‘the full stomach’, may result from emergency (nonstarved)
cases, bowel obstruction or paralytic ileus and indicates mandatory precautions
during anaesthesia. A rapid sequence induction is conducted, in which the
patient is ‘preoxygenated’ and cricoid pressure is applied from loss of
consciousness until the lungs are protected by tracheal intubation. Bowel
obstruction requires preoperative nasogastric aspiration and careful correction
of fluid and electrolyte balance before anaesthesia is induced.
H2-receptor
blocking agents such as ranitidine are administered if there is an increased
risk of regurgitation, ideally at least 2 hours preoperatively.
Anaesthesia
in the presence of jaundice carries a high risk of renal damage. The
anaesthetist should ensure that no hypovolaemia occurs and that a good urine
output is present before induction, by the preoperative infusion of intravenous
crystalloid solutions. A diuretic agent should only be used if the circulating
volume is first assessed to be adequate.
Metabolic
disorders
Familial porphyria and hyperpyrexia are
hereditary metabolic disorders associated with high anaesthetic risks.
Phaeochromocytoma
is also associated with severe anaesthetic complications. The presence of these
disorders requires highly specific preanaesthetic planning. Diabetes and adrenal
suppression from steroid therapy are also common metabolic disorders which
complicate anaesthesia.
Non
insulin-dependent
diabetic patients on diet and oral hypoglycemic agents will need blood sugar
measurement during anaesthesia. An intravenous infusion of glucose may be
required if the long-acting hypoglycemic effects persist even if the agent was
omitted on the day of surgery.
Except
for minor surgery, an intravenous infusion of glucose with soluble insulin is
likely to be necessary with close monitoring and control of blood sugar levels.
Insulin-dependent diabetes always needs preoperative conversion to control with
rapidly acting soluble insulin by intravenous infusion on the operative day, and
this is continued until the patient has recovered from the operation. In
practice, for maintenance of blood sugar levels, it is best to keep a constant
infusion of 5—10 per cent glucose with potassium supplementation through a
separate intravenous channel at about 2 litres/24 hours. Soluble short-acting
insulin is given continuously by intravenous syringe pump, with the rate
indicated by frequent (1—4-hourly) measurement of blood glucose concentration.
The plasma potassium level needs careful control. The circulating volume should
be manipulated independently via a separate infusion of normal saline, blood
or colloid. In this way a steady control of blood glucose concentration can be
easily achieved by an
Coagulation
disorders
Whether iatrogenic (including therapeutic) or
pathological in origin, coagulation disorders need careful assessment before
surgery with a coagulation screen, or clotting factor and platelet measurements.
In acquired disorders, such as
Neurological
disease
In cerebral disease and trauma, hypoxia,
hypercarbia and respiratory obstruction raise intracranial pressure and can
cause cerebral damage. In the presence of deteriorating consciousness,
management of the airway and ventilation is of prime importance, and especially
so in traumatic injury in which early endotracheal intubation and pulmonary
ventilation should precede supine positioning for computed tomography (CT) of
the brain. Particular care of the neck during intubation is necessary if a
cervical fracture is suspected. Skull traction and awake intubation under local
anaesthesia are sometimes used.
Anticonvulsant
drugs must be continued during surgery on epileptic patients, and this may
necessitate using intravenous administration.
In
peripheral neuropathies and myopathies, the need for prolonged periods of
postoperative ventilation should be anticipated.
Anaesthesia and
psychiatric disease
General, rather than regional, anaesthesia is
usually necessary. Tricyclic antidepressants and monoamine oxidase inhibitor
drugs potentiate sympathomimetic agents so adrenaline and cocaine must be
avoided. Pethidine can also cause hypertension with these drugs. Other narcotic
analgesic agents can be used but caution is necessary as their side effects can
be potentiated, especially with monoamine oxidase inhibitors.
Starvation
before surgery
Standard practice for many years has been 6
hours’ abstinence from food and 4 hours’ abstinence from fluids. Recently,
there has been a shift to permit clear, nonfizzy fluids up to 2 hours
preoperatively. These rules apply whenever loss of protective laryngeal reflexes
may pertain, as during regional anaesthesia and sedation. Small children are
usually given a glucose drink about 4
hours
preoperatively to prevent perioperative hypoglycaemia.
Consent
for surgery and anaesthesia
Informed consent should be obtained by the surgical team, preferably the
operating surgeon, before any sedation is given, but the anaesthetist should
still explain anaesthetic procedures, especially regional and spinal techniques,
and discuss potential sequelae.
Preoperative
drugs and treatment
Preoperative sedative and analgesic medication is becoming much less
common. Heavy sedative, antiemetic, antitussive, amnesic medication was
previously used for the relatively unpleasant inductions of anaesthesia with
pungent inhalational agents. Except for patients who are already in pain,
opioid analgesic agents are generally first given during induction of
anaesthesia, administered intravenously for rapid onset of action prior to
surgery. For reduction of anxiety, oral short-acting benzodiazepines are now
more commonly used 1—2 hours preoperatively, especially for children. Oral
trimeprazine is also still popular for children.
For
the increasing numbers of day-case procedures, preoperative sedation is avoided
so as to promote rapid emergence from anaesthesia and mobilisation.
The
anticholinergic agents, atropine, glycopyrronium and hyoscine, are used to
reduce respiratory and oral secretions. They are not essential with modern
anaesthetic agents, but still useful for airway surgery and endoscopy. Atropine
and glycopyrronium also protect against vagal dysrhythmias, for which
administration at induction is just as effective, and can cause alertness and
tachycardia. Hyoscine is pleasantly sedative without the cardiac effects of
atropine, but it can cause excessive sedation in infants or the elderly.
Antithrombotic
prophylaxis is usually initiated preoperatively in major surgery, commonly by
subcutaneous heparin injection. Particular attention must be given to higher
risk patients such as women taking contraceptive and hormone replacement drugs,
and those undergoing pelvic, hip, knee and cancer surgery. Low dose progesterone
preparations may be effectively covered by subcutaneous heparin, but other less
commonly prescribed forms of contraceptive hormone treatment may need to be
stopped 1 month before major surgery.
Preoperative
chest physiotherapy, possibly with bronchodilator treatment, may be indicated.
If
indicated, prophylactic antibiotic agents are given by the anaesthetist in
concert with the surgeon, either with the premedication or intravenously at
induction of anaesthesia.