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 organ-donor); 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 severalphases: 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.

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 target-controlled infusion (TCI).

Inhalational anaesthetics include those that are gases (nitrous oxide) or vapours at roomtemperature isoflurane, sevoflurane, desflurane). The potency of inhalationalanaesthetics 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 C02in 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 andremifentanil. 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 andis 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.

Monitoring during anaesthesia includes devices or techniques to monitor respiration (tidal volume and frequency), heart rate and rhythm, blood pressure, oxygen saturation (SP02), 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 ZealandCollege of Anaesthetists (ANZCA) publish guidelines for patient monitoring, as well as equipment and staffing matters. See

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, RoyalPerthHospital

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