General Anaesthesia

• Categories of anaesthesia


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


  1. Putting the patient in a state of controlled unconsciousness

  2. Patient will be intubated or have a supraglottic airway device

  3. Breathing will be supported and controlled by a ventilator

  4. Patient will be continuously monitored at all times immediately before, during and after general anaesthesia
• Purpose of fasting before a planned general anaesthetic is to ensure the patient has an empty stomach to reduce the risk of stomach contents refluxing into the oropharynx and being aspirated into the trachea
• Gastric contents in the lungs can cause pneumonitis (inflammation of the lung tissue)
• Risk of aspiration is highest before and during intubation, and when the patient is extubated
• Endotracheal tube protects the airway from aspiration once correctly fitted
• 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


  Patient breathes 100% oxygen before being put under a general anaesthetic to have a reserve of oxygen for the period between losing consciousness and being successfully intubated and ventilated
• Premedication is given before a general anaesthetic to relax the patient, reduce anxiety, reduce pain, and make intubation easier
• Medications for premedication may include

  • Benzodiazepines (e.g., midazolam) to relax muscles and reduce anxiety (also causes amnesia)
  • Opiates (e.g., fentanyl or alfentanyl) to reduce pain and reduce hypertensi
• General anaesthesia


  Process of using medications to relax the patient, reduce anxiety, reduce pain, and make intubation easier
• Medications used in general anaesthesia

  • Benzodiazepines (e.g., midazolam)
  • Opiates (e.g., fentanyl or alfentanyl)
  • Alpha-2-adrenergic agonists (e.g., clonidine)
• Rapid Sequence Induction/Intubation (RSI)


  Used to gain control over the airway quickly and safely in emergency scenarios where detailed pre-planning is not possible
• During RSI


  The biggest concern is the aspiration of stomach contents into the lungs
• Cricoid pressure


  Pressing down on the cricoid cartilage in the neck to prevent the reflux of stomach contents into the pharynx
• Triad of General Anaesthesia

  • Hypnosis
  • Muscle relaxation
  • Analgesia
• Hypnosis in general anaesthesia

  • Hypnotic agents are used to make the patient unconscious, given intravenously or by inhalation
• Intravenous options for general anaesthetic

  • Propofol
  • Ketamine
  • Thiopental sodium
  • Etomidate
• Inhaled options for general anaesthetic

  • Sevoflurane
  • Desflurane
  • Isoflurane
  • Nitrous oxide
• Volatile anaesthetic agents


  Liquid at room temperature, need to be vaporised into a gas to be inhaled
• Vaporiser devices


  Used for inhaled volatile agents to control the depth of anaesthesia by altering the concentration of the vaporised medication
• Common practice in general anaesthesia


  Intravenous medication used as an induction agent, inhaled medications used to maintain anaesthesia
• Total intravenous anaesthesia (TIVA)


  Involves using an intravenous medication for induction and maintenance of general anaesthetic
• Muscle Relaxation in general anaesthesia

  Muscle relaxants block the neuromuscular junction to relax and paralyse the muscles for easier intubation and surgery
• Categories of muscle relaxants

  • Depolarising (e.g., suxamethonium)
  • Non-depolarising (e.g., rocuronium and atracurium)
• Medications to reverse effects of neuromuscular blocking medications

  • Cholinesterase inhibitors (e.g., neostigmine)
  • Sugammadex (specifically for non-depolarising muscle relaxants)
• Analgesia in general anaesthesia

  Opiates are frequently used for pain relief
• Medications to reverse effects of neuromuscular blocking medications


  • Neostigmine
• Medications to reverse effects of non-depolarising muscle relaxants

  • Sugammadex
• Common agents used in anaesthetics

  • Fentanyl
  • Alfentanil
  • Remifentanil
  • Morphine
• Common options for prophylaxis given at the end of the operation

  • Ondansetron (5HT 3 receptor antagonist) – avoided in patients at risk of prolonged QT interval
  • Dexamethasone (corticosteroid) – used with caution in diabetic or immunocompromised patients
  • Cyclizine (histamine (H 1 ) receptor antagonist) – caution with heart failure and elderly patients
• Emergence


  Before waking the patient, the muscle relaxant needs to have worn off to avoid "awareness under anaesthesia". A nerve stimulator may be used to test the muscle responses to stimulation, ensuring the effects have ended. This can be tested on the ulnar nerve at the wrist or the facial nerve at the temple.
• Malignant hyperthermia causes


  Increased body temperature, increased carbon dioxide production, tachycardia, muscle rigidity, acidosis, hyperkalaemia
• Malignant hyperthermia is a rare but potentially fatal hypermetabolic response to anaesthesia. It is treated with dantrolene, which interferes with the movement of calcium ions in skeletal muscle.
• Risks of general anaesthesia include sore throat, post-operative nausea and vomiting, accidental awareness, aspiration, dental injury, anaphylaxis, cardiovascular events, malignant hyperthermia, and death.
• There are genetic mutations that increase the risk of malignant hyperthermia, inherited in an autosomal dominant pattern.
• Last updated August 2021