Download A Clinical Perspective of Anaesthesia for Pharmacists

A Clinical Perspective of Anaesthesia
An Anaesthetist is a physician who practices anaesthesia. Following medical school, the anaesthesia training
period typically is 6 years. These specialists are known as anaesthesiologists in many countries.
Anaesthesia is: (1) the management and preparation of patients for anaesthesia; (2) the provision of insensibility
to pain during surgical, obstetric, therapeutic and diagnostic procedures; (3) the monitoring and restoration of
homeostasis during the postoperative period and (4) the diagnosis and treatment of painful syndromes. It is a
mix of applied pharmacology, pathophysiology and biotechnology.
Types. Anaesthesia may be general, regional, local, or sedation (“neurolept”). The aims of premedication may
include anxiolysis, sedation, analgesia, drying of secretions, reduction of emesis and reduction of aspiration risk.
In Australia, oral premedication (temazepam is in common use) is more popular than IM premedication, but is
used with diminishing frequency in adults.
Sedative drugs used in anaesthesia include midazolam, lorazepam, diazepam (all 3 are benzodiazepines),
droperidol (popular for its antiemetic properties), and low-dose propofol. Opioids (fentanyl, remifentanil
infusion) maybe co-administered. Premedication is not always necessary whereas preoperative discussion with a
supportive anaesthetist is good practice. This preoperative consultation allows the anaesthetist to assess the
fitness of the patient for anaesthesia and to discuss anaesthetic options. Patients are assigned a numeric physical
status; ranging from 1 (healthy) to 5 (not expected to live 24 hours) and, more recently, 6 (brain-dead organdonor); devised by the American Society of Anesthesiologists.
General Anaesthesia is a readily reversible loss of consciousness with reduced response to painful stimulation
and generalised reduction in muscle tone. Intravenous anaesthetics include thiopentone, propofol,
methohexitone (now discontinued), and ketamine, among others. Inhalational induction of anaesthesia is used
occasionally for adults (e.g. needle phobia, upper airway problems) and more commonly used for children. The
triad of general anaesthesia includes unconsciousness ("hypnosis"), analgesia and muscle relaxation (or loss of
reflexes). General anaesthesia may be inhalational, intravenous, "balanced", or dissociative (ketamine-induced).
General anaesthesia may be achieved by a single agent or by several different agents each selected for a different
effect ("balanced anaesthesia"). During general anaesthesia, ventilation may be spontaneous, controlled or
assisted. General anaesthesia may involve paralysis (achieved with muscle relaxants) or the patient may breathe
spontaneously. During anaesthesia, the airway may be maintained with a face mask, laryngeal mask airway
(LMA), or a tracheal tube (nasal or oral) or a tracheostomy (via a stoma in the neck). Anaesthesia has
traditionally been divided into several phases: the preoperative visit, induction, maintenance, emergence and
recovery (see flow diagram, page 4). Anaesthesia may be induced with intravenous, inhalational, intramuscular
or, very rarely, rectal agents.
General Anaesthesia aims to producing a balance between stimulus and response. Stimuli include pain, moving
a patient, inflating a tourniquet, regurgitation, applying skin antiseptic ("prep"), pungent gases, etc. Patient
response may include hypertension, tachycardia, production of tears, gagging, coughing, bucking, straining,
regurgitation, vomiting, involuntary movements, and laryngospasm. Monitors for depth of anaesthesia, such as
processed EEG monitors, are becoming popular.
General anaesthesia's minor hazards include hypothermia, back and joint injury, dental or airway injury, sore
throat, dehydration, various aches and pains, nausea and vomiting. General anaesthesia's major hazards include
death, brain damage, bone damage, awareness, and nerve palsy. Complications from surgery are at least ten
times more frequent than anaesthetic complications.
1
Thiopentone is an ultra short acting barbiturate first produced as Pentothal in 1945. It is a yellow powder with
a garlic odour that is mixed with water to produce a 2.5 % solution. An intravenous "sleep dose" of 3-5 mg/kg
has made it the gold standard for rapid induction of anaesthesia. Awakening follows redistribution of this drug
from the brain. It produces temporary respiratory depression (usually apnoea) and a dose-related fall in cardiac
output. It is contraindicated in certain porphyrias, respiratory obstruction, and absence of someone who can
manage an airway. There are problems with intra-arterial and peri-venous injections. Propofol is a 1% solution
supplied as an emulsion produced in soy bean and egg phosphatide. Rapid induction of general anaesthesia is
produced with 1-2 mg/kg IV and there are frequent episodes of pain on injection and occasional dystonia. There
is less "hangover" than with barbiturate induction agents as well as antipruritic and antiemetic properties. This is
a favoured drug in the setting of ambulatory ("day case") surgery and has become popular for sedation in the
Intensive Care Unit. It is becoming popular to use with a computerised infusion pump to deliver a targetcontrolled infusion (TCI).
Inhalational anaesthetics include those that are gases (nitrous oxide) or vapours at room temperature
isoflurane, sevoflurane, desflurane). The potency of inhalational anaesthetics is defined by its MAC value (the
Minimum Alveolar Concentration of an anaesthetic agent required to produce lack of reflex response to skin
incision in 50% of subjects). The MAC values for isoflurane = 1.1%, sevoflurane = 2.0%, desflurane = 6.3%
and nitrous oxide = 105%.
Sevoflurane may achieve rapid induction of anaesthesia but there are theoretical problems with its interaction
with soda lime (an agent used to absorb C02 in anaesthesia breathing circuits). Desflurane is the most recent
addition and requires a heated vapouriser. Isoflurane, a volatile anaesthetic, has been associated with a
coronary steal phenomenon in a dog model of coronary artery disease. Methoxyflurane is an older agent that is
no longer used for anaesthesia (renal failure) but is used by ambulance services for prehospital analgesia.
Nitrous oxide (N20, Priestley, 1772) is a weak anaesthetic with good analgesic properties and its insolubility
means that it achieves rapid equilibrium and therefore has rapid onset. Repeated use may lead to some bone
marrow depression and it is contraindicated in patients with pneumothorax. It is used often during labour.
Ketamine (Ketalar 1965) is a phencyclidine analogue and is associated with dreams and hallucinations in some
patients. It is unusual because it stimulates the cardiovascular and respiratory systems and is useful in the field
(war) and for patients with shock. It is useful for postoperative pain control.
Opioids include morphine, pethidine (meperidine, Demerol), fentanyl, alfentanil, hydromorphone and
remifentanil. Sufentanil is not available in Australia. Opioids have analgesic, anti-tussive (cough) properties and
may induce nausea, vomiting, respiratory depression and itch. Remifentanil infusions are now popular for
producing profound "stress-free" analgesia during GA and is rapidly eliminated, with potential for intraoperative
awareness and severe postoperative pain. Tramadol is an older analgesic with oral and IV/IM forms (30% of its
action is due to an opioid effect) and has been released in Australia for acute and chronic pain control.
Naloxone (Narcan, 1972) is a specific opioid antagonist given in increments to reverse respiratory depression
without reversing analgesia. Also given by nurses, paramedics and emergency physicians for treatment of opioid
overdose.
Muscle relaxants include pancuronium, vecuronium, atracurium, cis-atracurium, mivacurium, rocuronium and
suxamethonium. Suxamethonium is the only depolarising neuromuscular blocker with an onset of 1 circulation
time and an average duration of 5 minutes. Its offset is due to hydrolysis (plasma cholinesterase) and its
problems include potassium release, myalgia, bradycardia, increased intraocular and intracranial pressure, dual
block, triggering malignant hyperthermia and prolonged duration, where there are problems with cholinesterase.
Non-depolarising muscle relaxants may be "reversed" with neostigmine (or edrophonium, pyridostigmine) given
with atropine or glycopyrrolate. Vecuronium and rocuronium may be reversed with the new agent
sugammadex.
2
Monitoring during anaesthesia includes devices or techniques to monitor respiration (tidal volume and
frequency), heart rate and rhythm, blood pressure, oxygen saturation (S P02), expired C02 (capnography),
temperature, urine output and other special parameters (such as central venous pressure, cardiac output), as the
patient's medical condition requires. The Australian and New Zealand College of Anaesthetists (ANZCA)
publish guidelines for patient monitoring, as well as equipment and staffing matters. See www.anzca.edu.au.
Local Anaesthesia (LA) is rendering one part of the body insensible to pain and consciousness is maintained.
Common drugs used are lignocaine, bupivacaine, prilocaine; the newest one is ropivacaine. The types of LA
include infiltration, topical, peripheral nerve blocks, major plexus blocks (brachial plexus block), spinal blocks,
epidural blocks (usually lumbar, but also thoracic or caudal) and intravenous regional anaesthesia (Bier block).
The problems include patient reluctance, logistics, failure, local anaesthetic toxicity and the specific
complications of each block.
Sedation (“neurolept anaesthesia” or “conscious sedation”) is a state of altered consciousness where a patient
feels relaxed and drowsy but is cooperative and able to maintain their own airway. It does not in itself; render
the patient insensitive to pain. LA is usually required. Problems include excessive sedation, airway obstruction,
apnoea, loss of airway reflexes, hypotension, inadequate anaesthesia, prolonged recovery. Sedation with
"monitored anaesthesia care" may be employed in the setting of angiography, embolisation, radiography,
lithotripsy, eye surgery and major regional nerve blocks. Midazolam (Hypnovel, Versed) is a water soluble
benzodiazepine that is popular for sedation which may be given by the PO, IM or IV routes of administration. It
is short acting, non irritant and has good cardiovascular stability with production of sedation, amnesia and (in
larger doses) anaesthesia. Flumazenil (Anexate) is a specific benzodiazepine antagonist that is given in 0.1 mg
aliquots to reverse effects of benzodiazepines, with an approximate half life of one hour.
Spinal and Epidural Anaesthesia. The benefits include excellent analgesia, avoiding the hazards of general
anaesthesia, possible reduction in lung and thrombo-embolic complications, and (administered slowly)
cardiovascular stability. The limitations include infection, coagulopathy, time to perform block and onset time,
patient refusal, hypotension. In general, these blocks cannot be used for surgery above the diaphragm. Major
complications, although uncommon, include epidural haematoma, abscess and neurological injury.
Acute Pain Services (APS) are common in major hospitals to improve the poor record of perioperative pain
control. These services are expensive but have been shown to improve the quality of analgesia in patients
following surgery, trauma, burns and other acute pain states (pancreatitis, etc). APS members include
anaesthetists, an RN, and pharmacy support. Opioid infusions (nurse-controlled or patient-controlled analgesia)
and epidural infusions (LA / opioid) are common techniques. Some hospitals use a pharmacy admixture service.
The APS aims to optimise analgesia, treat side-effects (hypotension, nausea, itch, respiratory and CNS
depression) and survey patient monitoring and documentation.
Future. Better use of existing drugs, with occasional introduction of newer agents with low toxicity and rapid
elimination. Total IntraVenous Anaesthesia (TIVA, TCI) with propofol is popular wiht many anaesthetists.
Electrical anaesthesia is not developed adequately. Medical simulators have been developed to enhance
training in many aspects of anaesthesia, resuscitation and critical care.
Richard Riley, Anaesthetist, Royal Perth Hospital
Website: www.anaesthesiawa.org
3