General anaesthesia

Safe general anaesthesia (GA) has developed in tandem with increasingly complex operations and procedures. Without advances in anaesthetic practice many surgical advances would have been impossible. GA has become more complex but has improved to become safer and more routine than ever before.

However, regional or spinal anaesthesia may be even safer and it is important that the safest and most appropriate forms of anaesthesia be selected. There is risk with even a brief anaesthetic because, for example, of risk during induction and recovery. The drive for safer anaesthetics means that GA is seldom administered outside hospitals which have 24-hour anaesthetic cover and an intensive care unit.

Pre-operative assessment¹

Pre-operative assessment by the anaesthetist is essential for safe anaesthesia. This involves assessing the fitness of the patient for operation but also assessment of which anaesthetic drugs and techniques can be used to complete the operation as safely as possible. A comprehensive assessment requires full understanding of the operation being undertaken, an assessment of the patient and full knowledge of the available drugs and techniques.

A multitude of factors can be present in an individual patient which will affect administration of safe GA. Consideration of these factors and the requirements of the particular operation requires a systematic approach to assessment of the patient. This is achieved through careful history, examination and, if necessary, further investigation.

History

• Pre-existing conditions affecting operation and anaesthesia. For example:
  

  • Heart disease (including recent myocardial infarction, heart failure and hypertension).

  • Liver disease.

  • Blood disorders (for example, anaemia and coagulopathy).

  • Diabetes mellitus

  • Respiratory disease (especially chronic obstructive pulmonary disease and asthma).

  • Neuromuscular disease.

  
  See separate articles Precautions with Patients with Diabetes Undergoing Surgery and Precautions for Patients on Steroids Undergoing Surgery.

• Medication, particularly that which can alter the response to an anaesthetic or surgery, should be considered. For example:
  

  • Steroids, neuroleptics, antihypertensives, antidepressants and barbiturates are all important. Even steroids taken in the fairly recent past are important if there is a risk that they have suppressed the adrenal-hypothalamic-pituitary axis.

• Experience of previous anaesthetics should be taken into account. For example:
  

  • Any adverse reactions to anaesthetic or other drugs.

  • Previous anaphylaxis or idiosyncratic reactions.

  • Particular fears or concerns about anaesthesia. There are many such fears (for example, of awareness, choking, vomiting, 'masks', 'needles', etc.). These can be addressed if identified beforehand and techniques tailored to the patient where possible.

Examination
This will be influenced by the history and the operation to be performed:

• Examine particularly cardiovascular and respiratory systems. Check heart rate, blood pressure, heart murmurs, carotid bruits and any pleural effusions or respiratory disease.

• Check for potential problems with the airway, such as a short neck or impaired mobility of the temporomandibular joint (TMJ). There are also problems of the neck in rheumatoid arthritis, as hyperextension may lead to snapping of the odontoid peg.

• Check mouth and dentition. Loose teeth or crowns can present problems.

• Check the veins for ease of intravenous (IV) access.

Investigations
These are only requested if there is an indication:

• Haemoglobin estimation is indicated if there is history of blood loss, anaemia, etc.

• U&E and creatinine if on diuretics or if there is history of previous abnormalities, renal impairment, etc.

• Further investigations such as LFTs may be indicated. A history of easy bleeding or bruising should lead to a coagulation screen being performed.

• Patients of Afro-Caribbean origin must have haemoglobin electrophoresis to exclude sickle cell trait unless this is already on record.

• ECG may be required in anyone with existing heart disease, or those with high risk of heart disease - including those with type 2 diabetes or in older patients.

• Existing lung disease or older age requires spirometry.

• CXR may be indicated from history and examination or routine for age.

• Blood may be taken for group and cross-match if this is thought necessary.

Premedication²

• This is now only used in anxious patients, generally given 30 minutes to 2 hours before surgery. Temazepam is a commonly used anxiolytic, or midazolam is used in the anaesthetic room, both as an anxiolytic and to reduce the amount of induction agent used.

• Drugs may also be used to reduce gastric acidity - generally ranitidine but, for rapid sequence, sodium citrate may be given pre-induction. See separate Mendelson's Syndrome article.

• Risk factors for postoperative nausea and vomiting are not well established but, if risk is considered to be high then additional components may be added to the premedication to help alleviate the problem.³ Postoperative nausea and vomiting are more common with procedures involving the middle ear and with gynaecological, bowel, gallbladder or ophthalmic surgery.

Intravenous anaesthetics²

Intravenous anaesthetics may be used either to induce anaesthesia or for maintenance of anaesthesia throughout surgery.

Total intravenous anaesthesia

• This is a technique in which major surgery is carried out with all drugs given intravenously. Respiration can be spontaneous, or controlled with oxygen-enriched air.

• Neuromuscular blocking drugs can be used to provide relaxation and prevent reflex muscle movements. The main problem to be overcome is the assessment of depth of anaesthesia.

• Target Controlled Infusion (TCI) systems can be used to titrate intravenous anaesthetic infusions to predicted plasma-drug concentrations in ventilated adult patients.

Drugs used for intravenous anaesthesia

• Propofol, the most widely used intravenous anaesthetic, can be used for induction or maintenance of anaesthesia in adults and children. It is associated with rapid recovery and less hangover effect than other intravenous anaesthetics.

• Thiopental sodium is a barbiturate that is used for induction of anaesthesia, but has no analgesic properties. Induction is generally smooth and rapid, but dose-related cardiovascular and respiratory depression can occur.

• Etomidate is an intravenous agent associated with rapid recovery without a hangover effect.

• Ketamine is used rarely. It is used mainly for paediatric anaesthesia, particularly when repeated administration is required (eg, serial burns dressings).

Inhalational anaesthetics²

• Inhalational anaesthetics include gases and volatile liquids. Gaseous anaesthetics require suitable equipment for storage and administration. Volatile liquid anaesthetics are administered using calibrated vaporisers, using air, oxygen, or nitrous oxide-oxygen mixtures as the carrier gas.

• Volatile liquid anaesthetics can be used for induction and maintenance of anaesthesia, and following induction with an intravenous anaesthetic. Isoflurane is the preferred inhalational anaesthetic for use in obstetrics. Other volatile liquid anaesthetics include desflurane and sevoflurane.

Nitrous oxide

• Nitrous oxide is used for maintenance of anaesthesia and, in sub-anaesthetic concentrations, for analgesia.

• Nitrous oxide is unsatisfactory as a sole anaesthetic owing to lack of potency, but is useful as part of a combination of drugs since it allows a significant reduction in dosage.

• If nitrous oxide is used in patients with an air-containing closed space, it diffuses into such a space with a resulting increase in pressure. This effect may be dangerous in conditions such as pneumothorax, which may enlarge to compromise respiration, or in the presence of intracranial air after head injury, entrapped air following recent underwater dive, or recent intra-ocular gas injection.

Airway

• Extreme care is required in surgery of the mouth, pharynx, or larynx where the airway may be difficult to maintain (eg, in the presence of a tumour in the pharynx or larynx).

• To facilitate tracheal intubation, induction is usually followed by a neuromuscular blocking drug or a short-acting opioid.

Monitoring¹

During induction and maintenance of anaesthesia - clinical observations, supplemented by the following equipment:

• Pulse oximeter.

• Non-invasive blood pressure monitor.

• Electrocardiograph.

• Airway gases: oxygen, carbon dioxide and vapour.

• Airway pressure.

The following must also be available:

• A nerve stimulator (whenever a muscle relaxant is used).

• A means of measuring the patient's temperature.

During recovery - clinical observations, supplemented by:

• Pulse oximeter.

• Blood pressure monitor.

The following must also be immediately available:

• Electrocardiograph.

• Nerve stimulator.

• Means of measuring temperature.

• Capnograph.

Complications of general anaesthesia

Complications of GA are varied and can be fatal but, thankfully, are rare.⁴ Complications of anaesthesia remain an important contributor to the statistics on maternal mortality.

• Damage to the mouth or pharynx, including damage to teeth and artificial crowns during intubation (uncommon - those with pre-existing poor dentition are most at risk).

• Minor idiosyncratic/allergic reaction to agents, producing nausea and vomiting (uncommon).

• Major idiosyncratic/allergic reaction to agents, inciting cardiovascular collapse, respiratory depression and jaundice (uncommon).

• Slow recovery from anaesthetic due to poor cardiac, hepatic or renal function, drug interactions, incorrect drug or dosage and inadequate reversal (uncommon).

• Malignant hyperpyrexia caused by anaesthetic gas or suxamethonium (rare).

• Prolonged apnoea after suxamethonium caused by pseudocholinesterase deficiency (rare).

• 'Awareness' during surgery can occur when the patient is paralysed but without effective anaesthetic.

Because of the aim to give a fairly light anaesthetic, aided by muscular relaxation, it is easy to let the patient become aware but, being paralysed, unable to respond. Sometimes patients can accurately recount the conversation that the operating team was having. The paralysed patient must be closely monitored to detect any signs of awareness or pain. If there is a suggestion of intraoperative awareness, IV midazolam can induce amnesia. New methods for monitoring anaesthesia are being devised.⁵

See also separate Important Complications of Anaesthesia article.

Recovering from general anaesthesia

The time of recovery is a time of risk. All patients are observed on a one-to-one basis by an anaesthetist or recovery nurse until they have regained airway control and cardiovascular stability and are able to communicate.⁶ Patients are kept under clinical observation at all times and all measurements recorded:

• Level of consciousness.

• Oxygen saturation and oxygen administration.

• Blood pressure, respiratory rate, heart rate and rhythm.

• Pain intensity - eg, verbal rating scale (none, mild, moderate, severe).

• IV infusions, drugs administered.

• Other parameters (depending on circumstances) - eg, temperature, urinary output, central venous pressure, end-tidal CO₂, surgical drainage.

Patients are only discharged to the ward when:

• The patient is fully conscious, able to maintain a clear airway and exhibiting protective airway reflexes.

• Respiration and oxygenation are satisfactory.

• The cardiovascular system is stable with no unexplained cardiac irregularity or persistent bleeding, with a pulse and blood pressure at an acceptable level and adequate peripheral perfusion.

• Pain and nausea or vomiting should be controlled and suitable analgesic and anti-emetic regimens prescribed.

• Temperature should be within acceptable limits (ie not significantly hypothermic).

There is considerable pressure these days for day case surgery and so anaesthetics are often arranged for rapid recovery so that the patient may be discharged just a few hours later. However, effects will last and the patient must be discharged into the care of another who will drive home. Driving must be avoided until 24 hours after the anaesthetic and even important decisions such as signing legal documents should be avoided for this time as judgment is impaired.

Safety

The potential dangers of general anaesthesia should not be underestimated, although serious problems are rare.

Acknowledgements: EMIS would like to thank Lee Varney for his contributions and suggestions, which have been included in this article.