Case 1.3 – Day of Surgery Care (Balanced General Anaesthesia)

General anaesthesia is a temporarily induced state of depressed consciousness designed to facilitate surgery or other painful procedures.

Balanced anaesthesia refers to the technique of combining drugs from the hypnotic, analgesic and muscle relaxant classes to provide the optimal conditions (of depressed conscious state, amnesia, pain relief and muscle relaxation) required for the proposed surgery or intervention in a given individual patient.

In order to choose this combination, the anaesthetist will take into account:

• Patient factors ie. High aspiration risk requiring secured airway with ETT
• Surgical factors: ie. Requirement for muscle relaxation and IPPV to facilitate pneumoperitoneum during laparoscopic cases
• Anaesthetic factors ie. Duration of drug action, mode of metabolism etc

Intelligent use of drugs providing the elements for balanced anaesthesia requires an understanding of mechanism of action, dose, pharmacokinetics, adverse reactions/toxicity and drug interactions.

The main drug classes are:

• Inhalational anaesthetics
• Intravenous anaesthetics
• Analgesics
• Neuromuscular junction Blockers

HYPNOTICS - INHALATIONAL ANAESTHETICS

• Produce generalised suppression of consciousness historically known as hypnosis
• Mechanism of action incompletely understood
• Advantages: gradual induction via inhaled route with maintenance of spontaneous respiration, reliable maintenance, minimal metabolism in the body
• Disadvantages: pungent/irritant, malignant hyperthermia triggers

MAC (Minimum Alveolar Concentration) - the concentration of agent in 100% oxygen at 1 atm that abolishes motor response to a standardised noxious stimulus in 50% of patients. This is a comparative measure of potency for inhalational agents.

Factors that influence MAC:

Increase MAC	Decrease MAC
Children Hyperthermia Hypernatraemia Chronic alcoholism	Extremes of age Hypothermia Hyponatraemia Opioids / Other drugs

MAC awake = 0.33MAC (50% of patients will regain consciousness/follow simple commands)

Inhalation Agent	MAC (%)	Features
Isoflurane	1.15	Bronchodilator
Sevoflurane	1.7	Pleasant smell, non-irritant
Desflurane	6	Rapid onset/offset, irritant, SNS stimulant
Nitrous oxide	104	Potent analgesic, implicated in PONV, dilutional hypoxia

INTRAVENOUS ANAESTHETICS

• Produce generalized suppression of consciousness
• Convenience of rapid onset via intravenous route
• Advantages: rapid onset and offset of anaesthesia, can be adapted for maintenance of anaesthesia

	Propofol	Thiopentone	Ketamine
Mechanism	GABA mediated inhibition of CNS	GABA mediated inhibition of CNS	NMDA R antagonist
IV Induction Dose	2-3mg/kg	3-5mg/kg	1-2mg/kg
Features	Pain on injection Excitatory Phenomena	Venous Irritant PPTE Porphyria	CVS stability Potent Analgesic Dissociative Anaesthetic Hallucinations

BENZODIAZEPINES

• Sedative, anxiolytic, amnestic and antiepileptic via enhancement of GABA transmission in the CNS
• Negative attributes of this class, such as sleep disruption and addiction potential, should confine their use to specific circumstances only

Midazolam:

• Water soluble, short-acting BZD
• Sedative dose 0.05-0.1mg/kg
• t½ 1-4hrs
• Active metabolites

ANALGESICS - OPIOIDS

• Agonists at endogenous mu opioid receptors to provoke analgesia
• Other mu opioid receptor mediated responses include: sedation, respiratory and cough reflex suppression, miosis, pruritis, nausea and vomiting, decreased GI motility, urinary retention, tolerance and dependence
• Potential side effects via non-opioid receptor actions: central SNS suppression, histamine release, allergy
• Numerous formulations for use via numerous routes ie. intravenous, neuraxial, subcutaneous, oral, transdermal and more

Drug	Morphine	Fentanyl	Alfentanil
Features	Potent Analgesia	Potent Analgesia	Potent Analgesia
Dose	0.1-0.15 mg/kg	2-4mcg/kg induction dose	10-20mcg/kg induction dose
Half Life	2-4hrs adults	3-4hrs	1-1.5hrs
Onset	10-15 min	2-5min	90sec
Duration	3-4hrs	30-60min	5-10min
Metabolites	Active and toxic metabolites	No active metabolites	Active metabolites

NON-OPIOIDS

Tramadol:

• Multimodal moderate potency analgesic agent
• Combined action: ⅓ mu opioid receptor agonist, ⅔ monoamine (5HT and NA) uptake inhibition
• Oral and IV formulation - 100mg IV approximates the analgesic action of 10mg morphine parenterally
• Dose 50-100mg q6h maximum 400mg per 24hrs
• Active metabolite
• Side effects: nausea and vomiting, sedation, dysphoria, lowered seizure threshold, dizziness and postural hypotension
• Drug interactions: TCA's, SSRI's, MAOI's, CBZ, ondansetron

NSAIDS:

• Wide range of chemical classes
• Unifying mechanism: cyclo-oxygenase inhibition, thereby preventing genesis of inflammatory mediators (eg prostaglandins)
• Group adverse reactions: gastric erosion, nephrotoxicity, platelet function inhibition, bronchospasm, headache, confusion, sedation, hepatotoxicity, tocolysis, closure of PDA and hypersensitivity reactions

PARACETAMOL:

• Simple analgesic and antipyretic
• Safe and well tolerated
• Dose adults 10-30mg/kg (1gm) q6h maximum 4gm per 24hrs
• Dose children 10-20mg/kg q4h maximum 90mg/kg per 24hrs
• Adverse reactions: GI upset, hypersensitivity reactions, hepatotoxicity in overdose

MUSCLE RELAXANTS/NEUROMUSCULAR JUNCTION BLOCKERS

• Competitive antagonists at the NMJ that temporarily disable muscle contraction
• Depolarising and non-depolarising types - describes action at muscle end-plate
• Will render patient unable to spontaneously breathe or protect airway
• Must not be administered to a conscious patient

Suxamethonium:

• The only depolarising NMJB
• Stored at 4 degrees to maintain activity
• Intubating dose 1-2 mg/kg
• Rapid onset 45-60secs
• Short duration 5-10min
• These properties make it ideal for use in patients where rapid airway control is required (ie. aspiration risk) or rapid recovery of spontaneous respiration is required (ie. difficult airway scenario)
• Adverse reactions: hyperkalaemia, post-op myalgia, raised intracranial and intraocular pressure, malignant arrhythmias, prolonged apnoea, anaphylaxis, bradycardia, malignant hyperthermia, masseter spasm
• Contraindications/precautions: malignant hyperthermia history, burns/denervation injury/myopathies/muscular dystrophies, plasma cholinesterase deficiency/sux apnoea, renal impairment/hyperK⁺

Non-Depolarizing NMJB's:

• Competitive antagonists at the NMJ
• Short, intermediate and long acting agents with various properties

	Rocuronium	Vecuronium	Cisatracurium
Dose Intubating Dose	0.6mg/kg	0.1mg/kg	0.2mg/kg
Onset	60 sec	2-3 min	2-3min
Duration	35min	30-35min	60-90 min
Features			Organ-independent metabolism

Lisa likely had a rapid sequence induction using suxamethonium as the urgency of the clinical situation dictated that her surgery could not be delayed 6 hours of fasting time. She may also have been nauseated or vomiting prior to induction. Suxamethonium use is associated with post-operative myalgia, particularly in young females.

Other contributors to post-operative nausea and vomiting (PONV) could have included:

• Use of volatile agent
• N₂O
• Opioids
• Tramadol
• Post-operative ileus
• Sepsis and dehydration