General Anaesthesia

There are two main categories of anaesthesia:

  • General anaesthesia – making the patient unconscious
  • Regional anaesthesia – blocking feeling to an isolated area of the body (e.g., a limb)

 

A general anaesthetic involves putting the patient in a state of controlled unconsciousness. It is most often used so that a major surgical operation can be performed. During a general anaesthetic, the patient will be intubated or have a supraglottic airway device, and their breathing will be supported and controlled by a ventilator. The patient will be continuously monitored at all times immediately before, during and after general anaesthesia.

 

Fasting

Before a planned general anaesthesic, the patient will have a period of fasting. The purpose of fasting is to make sure they have an empty stomach, to reduce the risk of the stomach contents refluxing into the oropharynx (throat), then being aspirated into the trachea (airway). Gastric contents in the lungs creates an aggressive inflammatory response, causing pneumonitis (inflammation of the lung tissue). The risk of aspiration is highest before and during intubation, and when they are extubated. Once the endotracheal tube is correctly fitted, the airway is blocked and protected from aspiration. Aspiration pneumonitis and pneumonia are major causes of morbidity and mortality in anaesthetics, although with planned procedures they are very rare.

Fasting for an operation typically involves:

  • 6 hours of no food or feeds before the operation
  • 2 hours of no clear fluids (fully “nil by mouth”)

 

In emergency situations the patient might not be fasted (rapid sequence induction is discussed below). 

 

Preoxygenation

Before being put under a general anaesthetic, the patient will have a period of several minutes where they breathe 100% oxygen. This gives them a reserve of oxygen for the period between when they lose consciousness and are successfully intubated and ventilated (in case the anaesthetist has difficulty establishing the airway). This step may need to be skipped when an emergency general anaesthetic is required.

 

Premedication

Medications are given before the patient is put under a general anaesthetic to relax them, reduce anxiety, reduce pain and make intubation easier. These may include:

  • Benzodiazepines (e.g., midazolam) to relax the muscles and reduce anxiety (also causes amnesia)
  • Opiates (e.g., fentanyl or alfentanyl) to reduce pain and reduce the hypertensive response to the laryngoscope
  • Alpha-2-adrenergic agonists (e.g., clonidine), which can help with sedation and pain

 

Rapid Sequence Induction/Intubation

Rapid sequence induction/intubation (RSI) is used to gain control over the airway as quickly and safely as possible where a patient is intubated in an emergency scenario and detailed pre-planning is not possible. This is considerably more risky, as the patient has often not been fasted (risk of aspiration), and the anaesthetist has not had the chance to plan for individual factors and potential problems (e.g., a difficult airway). It is also used in non-emergency situations where the airway needs to be secured quickly to avoid aspiration, such as in patients with gastro-oesophageal reflux or pregnancy.

The procedure is designed to ensure successful intubation with an endotracheal tube as soon as possible after induction (when the patient is unconscious) to protect the airway. The biggest concern during RSI is the aspiration of stomach contents into the lungs. The bed can be positioned so the patient is more upright to reduce the reflux of contents up the oesophagus. Cricoid pressure (pressing down on the cricoid cartilage in the neck) may be used to compress the oesophagus and prevent the stomach contents from refluxing into the pharynx (this is somewhat controversial and should only be done by someone trained and experienced).

 

Triad of General Anaesthesia 

There is a triad of general anaesthesia:

  • Hypnosis
  • Muscle relaxation
  • Analgesia

 

Hypnosis

Hypnotic agents are used to make the patient unconscious. They can be either given intravenously or by inhalation.

 

Intravenous options for a general anaesthetic include:

  • Propofol (the most commonly used)
  • Ketamine
  • Thiopental sodium (less common)
  • Etomidate (rarely used)

 

Inhaled options for a general anaesthetic include:

  • Sevoflurane (the most commonly used)
  • Desflurane (less favourable as bad for the environment)
  • Isoflurane (very rarely used)
  • Nitrous oxide (combined with other anaesthetic medications – may be used for gas induction in children)

 

Sevoflurane, desflurane and isoflurane are volatile anaesthetic agents. Volatile agents are liquid at room temperature and need to be vaporised into a gas to be inhaled.

Vaporiser devices are used for inhaled volatile agents. The liquid medication is poured into the machine. The machine then turns it into vapour and mixes it with air in a controlled way. During the anaesthesia, the concentration of the vaporised anaesthetic medication can be altered to control the depth of anaesthesia.

Commonly, an intravenous medication will be used as an induction agent (to induce unconsciousness), and inhaled medications will be used to maintain the general anaesthetic during the operation. Inhaled medications need to diffuse across the lung tissue and into the blood, where it takes a while for them to reach an effective concentration. IV agents have a head start, as they are infused directly into the blood and so can quickly reach an effective concentration. 

Total intravenous anaesthesia (TIVA) involves using an intravenous medication for induction and maintenance of the general anaesthetic. Propofol is the most commonly used. This can give a nicer recovery (as they wake up) compared with inhaled options.

 

Muscle Relaxation

Muscle relaxants block the neuromuscular junction from working. Acetylcholine (the neurotransmitter) is released by the axon but is blocked from stimulating a response from the muscle. Muscle relaxants are given to relax and paralyse the muscles. This makes intubation and surgery easier. There are two categories:

  • Depolarising (e.g., suxamethonium)
  • Non-depolarising (e.g., rocuronium and atracurium)

 

Cholinesterase inhibitors (e.g., neostigmine) can reverse the effects of neuromuscular blocking medications.

Sugammadex is used specifically to reverse the effects of certain non-depolarising muscle relaxants (rocuronium and vecuronium).

 

Analgesia

Opiates are the most frequently used medication for analgesia (pain relief). Common agents used in anaesthetics are:

  • Fentanyl
  • Alfentanil
  • Remifentanil
  • Morphine

 

Antiemetics are often given at the end of the procedure by the anaesthetist to prevent post-operative nausea and vomiting. Common options for prophylaxis given at the end of the operation are:

  • Ondansetron (5HT3 receptor antagonist) – avoided in patients at risk of prolonged QT interval
  • Dexamethasone (corticosteroid) – used with caution in diabetic or immunocompromised patients
  • Cyclizine (histamine (H1) receptor antagonist) – caution with heart failure and elderly patients

 

Emergence

Before waking the patient, the muscle relaxant needs to have worn off. It is not good for the patient to regain consciousness whilst still paralysed (“awareness under anaesthesia”). A nerve stimulator may be used to test the muscle responses to stimulation, to ensure the muscle relaxant effects have ended. This is often tested on the ulnar nerve at the wrist, watching for thumb movement (twitches). Alternatively, the facial nerve can be stimulated at the temple while watching for movement in the orbiculares oculi muscle at the eye. This involves a train-of-four (TOF) stimulation, where the nerve is stimulated four times to see if the muscle responses remain strong (indicating it has worn off) or whether they get weaker with additional stimulation (indicating it has not fully worn off). Medication can be used to reverse the effects of the muscle relaxants as discussed above (e.g., sugammadex).

Once the muscle relaxant has worn off, the inhaled anaesthetic is stopped. The concentration of the anaesthetic in the body will fall, and the patient will regain consciousness. They are extubated at the point where they are breathing for themselves. 

 

Risks of General Anaesthesia

Sore throat and post-operative nausea and vomiting are common adverse effects of general anaesthesia. 

Significant risks of general anaesthesia include:

  • Accidental awareness (waking during the anaesthetic) 
  • Aspiration
  • Dental injury, mainly when the laryngoscope is used for intubation
  • Anaphylaxis
  • Cardiovascular events (e.g., myocardial infarction, stroke and arrhythmias) 
  • Malignant hyperthermia (rare)
  • Death

 

Malignant Hyperthermia

Malignant hyperthermia is a rare but potentially fatal hypermetabolic response to anaesthesia. The risk is mainly with: 

  • Volatile anaesthetics (isoflurane, sevoflurane and desflurane) 
  • Suxamethonium

 

There are genetic mutations that increase the risk of malignant hyperthermia. These are inherited in an autosomal dominant pattern. 

Malignant hyperthermia causes:

  • Increased body temperature (hyperthermia)
  • Increased carbon dioxide exhalation
  • Tachycardia
  • Muscle rigidity
  • Acidosis
  • Hyperkalaemia

 

It is treated with dantrolene. Dantrolene is a muscle relaxant that works by interfering with the movement of calcium ions in skeletal muscle.

 

Last updated August 2021
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