Principles of General Anaesthesia

Aims

  • Triad of anaesthesia
    • Unconscious
    • Paralysis
      • Whilst anaesthetic agents will produce paralysis at high doses, the use of muscle relaxants will allow a lower dose to be used whilst keeping muscles relaxed.
    • Analgesia
      • Noxious stimuli, even in the unconscious patient, can elicit physiological responses.  Analgesic agents are used to reduce the ‘sensation’ and prevent this.

Drugs

  • Volatile Anaesthetic agents
    • Induce unconsciousness and oblivion once adequate depth of anaesthesia has been achieved.  They can inhibit movement to pain and relax muscles partly by suppression of spinal reflexes.  I.e. useful maintenance agents
    • e.g. Sevoflurane; halothane; isoflurane
    • Can cause a drop in systemic vascular resistance and BP, and a reflex tachycardia
    • They do not provide any post-operative pain relief or any inhibition of the sympathetic response to pain (the latter may be achieved with very high doses)
  • IV anaesthetics (namely propofol)
    • Induce unconsciousness, oblivion and amnesia with adequate doses.  They are purely hypnotic, not analgesic (except ketamine).
    • They lack inhibition of spinal reflexes so are poor at inhibiting movement in response to surgery.
    • Examples include
      • Sodium Thiopental (3-7mg/kg)- often preferred for rapid sequence induction
        • Onset 30sec; Offset 5-10mins; Metabolised by the liver to pentobarbital (active) (also an antiepileptic as well as hypnotic and analgesic); Problems include: narrow therapeutic index and overdose may cause cardiorespiratory depression; negative inotropic effect- drop in BP; extremely painful and limb-threatening if given intra-arterially (saline dilution, papaverine to dilate the artery and sympathetic blockade to improve blood flow); NOTE sensitises the larynx and CANNOT be used with laryngeal mask airways (LMAs); Contraindicated in porphyria
      • Propofol (1-2.5mg/kg)
        • Onset 30sec; offset 3-7mins; (can also be used as continuous infusion to maintain anaesthesia); metabolised in the liver; common side effects: Inotropic effect- Myocardial depression and reduction in SVR -> hypotension, little effect on HR, dose dependent respiratory depression, can be painful (often combined with lidocaine); PONV much less likely and less allergies
      • Etomidate (0.25-0.3mg/kg)
        • Onset 30sec; Offset 3-7mins; metabolised in plasma and liver; fewer side effects and better haemodynamic profile but PONV more common and can cause adrenocortical suppression; may be painful (reduced by lipid solution); avoid for sedation in intensive care and porphyria
      • Others include ketamine (which can be a good choice in heart disease/haemodynamic instability as it is not negatively inotropic and will not depress cardiac output)
  • Other drugs commonly used by anaesthetists
    • Benzodiazepines (see also sedation)
      • Mainly due to their amnesic/anxiolytic effect and synergy with opioids and other anaesthetic agents.
      • They also contribute towards muscle relaxation (but not abolish movement)
      • No analgesic effects
    • Muscle relaxants (neuromuscular blockers)
      • Produce muscle relaxation by blocking transmission at the NMJ
      • Two types:
        • Depolarising: maintains muscles in the depolarised (relaxed) state.
          • e.g. suxamethonium
          • quick acting- preferred generally for rapid sequence induction; generally irreversible
          • may cause muscle pain; hyperkalaemia; bradycardia; change in BP; rarely malignant hyperpyrexia
        • Non-depolarising: Slower onset but longer duration
          • e.g. rocuronium – has the advantage that it can be reversed by agents such as sugammadex
      • No anaesthetic or analgesic properties so must be used in conjunction with these (otherwise awareness whilst paralysed)
      • Required if intubation is necessary (not necessarily required if supraglottal airways are used)
    • Simple analgesia
      • Postoperative pain relief; often used with opioids to reduce the dose and to potentiate the analgesic effect
    • Opioids
      • Can reduce the dose requirement for anaesthetic agents (particularly volatile agents), but alone does not produce anaesthetic or muscle relaxant effect, although the reduced pain response may reduce movement in response to surgery.
      • In induction (fentanyl/alfentanyl), main use is to suppress the aiway reflexes (LMA insertion) and provide short lived but profound analgesia for intubation
      • Also useful post-operatively for pain relief (morphine)
    • Regional anaesthesia
      • Produces immobility, muscle relaxation and analgesia within the region.
    • Anticholinergics
      • Dry secretions and prevent reflex bradycardia

Patient response to surgery

  • Surgery induces a number of physiological responses in the patient via
    • sympathetic nervous system mediated response
    • neuro-humoral changes
    • other hormonal and metabolic changes
      • raised ACTH; GH; vasopressin; prolactin; insulin resistance; CO2 production; O2 consumption; circulating catecholamines
  • Can lead to
    • unwanted CVS changes; fluid retention; electrolyte disturbance; metabolic changes (period of catabolism and hyperglycaemia); SIRS response; hypercoagulability

Non-Rapid Sequence Induction

  • Once the patient has taken some deep breaths with 100% oxygen, the following drugs (or similar cohort) can be given in sequence:
    • Benzodiazepine e.g. midazolam (usually 1-3mg)
    • Opiate e.g. Fentanyl (usually 0.5-1μg/kg if spontaneous ventilation and 2-5μg/kg for invasive positive pressure ventilation)
    • IV induction agent e.g. Propofol (usually ~1.5-2.5mg/kg
    • Muscle relaxant (usually non-depolarising) e.g. Atracurium/rocuronium (0.5/0.6 mg/kg (respectively) – generally only if invasive ventilation required); vecuronium is more potent (0.1mg/kg)
  • In non rapid induction, there is time to ensure:
    1. preoperative assessment of the patient e.g. PMHx, Allx
    2. Planning and communication with other skilled members of the team e.g. anaesthetic nurse
    3. Check equipment
    4. Choice/check drugs
    5. Availability of resuscitation facilities

When is IV NRSI not advised?

  • Children- difficulty with IV access and cooperation
  • IV drug user- difficulty with IV access
  • Very needle-phobic patients
  • Stridor- easier to induce via inhalation to avoid obstruction
  • In these patients, gas induction with sevoflurane is recommended (+ NO)

Rapid Sequence Induction

  1. Pre-oxygenate with 100% oxygen
  2. Monitor pulse, BP, ECG and oxygen saturation
  3. Establish IV access
  4. Loosen cervical collar and maintain manual in-line cervical stabilisation from below
  5. Give IV bolus induction agent of choice
  6. Give IV bolus of suxamethonium (1 – 1.5 mg/kg)
  7. Apply cricoid pressure at the time of injection of induction agent
  8. Once muscle fasciculations have stopped, perform laryngoscopy and intubate
  9. Verify tube position by auscultation over both sides of chest and over the stomach
  10. Connect ETCO2 (end-tidal CO2) monitor
  11. Remove cricoid pressure and replace cervical collar
  12. Secure endotracheal tube
  13. Administer muscle relaxant to maintain paralysis for transport
  14. Administer morphine (0.1 mg/kg) and midazolam (0.1 – 0.4 mg/kg) for sedation

Potential adverse effects/complications

  • Rarely- allergies
  • Failure to intubate following induction
  •  CVS
    • Tachy/bradycardia (opioids or induction agents)
    • Hypotension (most anaesthetic agents)
  • Respiratory
    • Laryngospasm and coughing
    • Hypoventilation and apnoea (particularly opioids and induction agents in combination)
  • Regurgitation
  • Movement
  • Malignant hyperpyrexia
    • Rare life threatening complication
    • Familial- Autosomal dominant- caused by rapid influx of Calcium into muscle cells and subsequent muscle rigidity, tachycardia, tachypnoea and DIC, with eventual metabolic acidosis and hyperkalaemia
    • Treatment is with 1mg/kg of Dantrolene Sodium by rapid injection- repeated up to 10mg/kg (i.e. 10 boluses); also with cool IV fluid; cold towels

 

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