title: "Anaesthetics" description: "Induction agents, neuromuscular blockers, volatiles, and local anaesthetic dosing — focused for ED/ICU." rotation: "critical-care" week: 1 backLink: "/critical-care/week/1" tags:

"airway"
"rsi"
"anaesthetics"
"induction agents"
"neuromuscular blockade"
"local anaesthetics"

Anaesthetics

1. Overview

General Anaesthesia (GA): Three components of a balanced GA:

Unconsciousness
Analgesia
Lack of movement (paralysis)

GA vs Regional Anaesthesia

General Anaesthetic	Local/Regional Anaesthetics
Systemic drugs producing unconsciousness	Block voltage-gated Na channels
Requires airway management	Patient awake or sedated
Systemic side effects	Localised effects

2. Stages of General Anaesthesia

Stage	Key Actions
Induction	IV access, positioning, preoxygenation, monitoring, IV anaesthetic agent
Maintenance	Propofol infusion OR volatile gas, opioids for analgesia, antibiotics, antiemetics
Emergence	Turn off agent, reverse paralysis, remove airway device
Recovery	30-60 min close monitoring, check consciousness, analgesia, PONV

Clinical Pearl

Preoxygenation: Fill functional residual capacity (FRC) with 100% O2. This provides oxygen reserve during apnoea period of intubation.

Source: CC Bible; local anaesthetic airway teaching.

3. Induction Agents

Drug	Mechanism	Dose (mg/kg)	Onset	Duration	Key Features
Propofol	Enhances GABA	1.5-2.5	15-45 sec	5-10 min	Smooth induction, hypotension, pain on injection
Thiopentone	Barbiturate, GABA	3-5	15-30 sec	5-10 min	Less hypotension, groggy recovery
Ketamine	NMDA antagonist	1.5-2	30-60 sec	20-40 min	CVS stable, bronchodilator, emergence delirium
Etomidate	GABA	0.2-0.3	15-45 sec	5-10 min	Most haemodynamically stable

Cardiovascularly Stable Induction Agents:

Ketamine
Etomidate
Thiopentone

Use in haemodynamically unstable patients or trauma.

Clinical Pearl

Ketamine advantages:

Maintains BP (sympathetic stimulation)
Bronchodilator (good for asthmatics)
Potent analgesic
Preserves airway reflexes

Disadvantages: Emergence phenomena (hallucinations, delirium)

4. Neuromuscular Blocking Agents

Classification

Type	Mechanism	Example
Depolarising	ACh agonist → persistent depolarisation → refractory state	Suxamethonium
Non-depolarising	Competitive ACh antagonist at NMJ	Rocuronium, vecuronium, atracurium

Commonly Used Agents

Drug	Onset	Duration	Notes
Suxamethonium	30-60 sec	5-10 min	Fastest onset/offset, used for RSI
Rocuronium	60-90 sec	30-60 min	Can be reversed with sugammadex
Vecuronium	2-3 min	30-40 min	Minimal cardiovascular effects
Atracurium	2-3 min	20-35 min	Hofmann degradation (good in renal failure)

Warning

Suxamethonium Contraindications:

Hyperkalaemia or risk factors
Burns >24 hours old
Denervation injuries (spinal cord, stroke)
Prolonged immobilisation
Malignant hyperthermia susceptibility
Myopathies

Risk: Massive potassium release due to upregulated ACh receptors

Reversal Agents

Agent	Target	Dose	Mechanism
Sugammadex	Rocuronium, Vecuronium	2-16 mg/kg	Encapsulates aminosteroid NMBAs
Neostigmine	Non-depolarising NMBAs	0.05 mg/kg + glycopyrrolate	Anticholinesterase

Sugammadex advantage: Can reverse even deep neuromuscular blockade within minutes. Far more reliable than neostigmine.

5. Volatile Anaesthetics

Agent	Notes
Sevoflurane	Most commonly used, good for gas induction (paediatrics)
Desflurane	Fast offset, pungent (not for induction)
Isoflurane	Cheaper, more cardiovascular depression

Clinical Pearl

Gas induction indications:

No IV access
Difficult IV access (paediatrics)
Needle phobia
Patient preference

Warning

Volatile anaesthetic risks:

Post-operative nausea/vomiting (worst of all agents)
Malignant hyperthermia trigger
Environmental impact

6. Airway Management

Airway Devices

Device	Procedures	Key Features
Facemask	Very short procedures	Least invasive, requires jaw thrust
LMA	Most procedures	Easy insertion, no need for paralysis
ETT	Aspiration risk, long procedures	Best protection, requires laryngoscopy

ETT Sizing

Patient	ETT Size
Adult female	7.0 mm ID
Adult male	8.0 mm ID
Paediatric	(Age/4) + 3.5 mm (cuffed)

Preoperative Airway Assessment

LEMON for Difficult Intubation

L - Look externally (obesity, facial hair, trauma)
E - Evaluate 3-3-2 rule
M - Mallampati score (III-IV = difficult)
O - Obstruction (tumour, abscess, oedema)
N - Neck mobility (cervical spine injury)

Mallampati Classification

Class	Visibility
I	Tonsillar pillars visible
II	Uvula partially obscured
III	Only soft palate visible
IV	Hard palate only

Difficult airway predictors:

Thyromental distance <6 cm
Mouth opening <3 cm (LMA), <4 cm (laryngoscopy)
Mallampati III or IV
Reduced neck extension

7. Neuraxial Anaesthesia

Spinal vs Epidural

Feature	Spinal	Epidural
Space	Subarachnoid (CSF)	Epidural
Location	Below L1 only	Anywhere
Endpoint	CSF flow (reliable)	Loss of resistance
Onset	Seconds-minutes	15-30 minutes
Duration	Single shot	Catheter for repeat dosing
Hypotension	More	Less

Local Anaesthetics

Drug	Onset	Duration	Use
Lignocaine	Fast (5-10 min)	Short (1-3 hr)	Infiltration, nerve blocks
Bupivacaine	Slow (10-20 min)	Long (2-12 hr)	Epidural, spinal
Ropivacaine	Slow (10-20 min)	Long (2-12 hr)	Epidural, nerve blocks

Lignocaine Dosing

Q: Max safe lignocaine dose (plain vs with adrenaline)? A: 3 mg/kg (plain); 7 mg/kg (with adrenaline)

1% lignocaine = 10 mg/mL Source: CC Bible; local anaesthetic dosing reference.

Local Anaesthetic Toxicity

Warning

LAST (Local Anaesthetic Systemic Toxicity):

Symptoms (progressive):

Perioral tingling, tinnitus
Confusion, seizures
Arrhythmias, cardiovascular collapse
Cardiac arrest

Treatment:

Stop injection
Call for help
ABCDE
Benzodiazepines for seizures
Intralipid 20% (1.5 mL/kg bolus)

Source: Local LAST (lipid emulsion) protocol; CC Bible.

Anticoagulant Cessation

Drug	Time Before Neuraxial
Clopidogrel	7 days
Dabigatran	5 days
Apixaban/Rivaroxaban	3 days (72 hours)
Therapeutic LMWH	24 hours
Prophylactic enoxaparin	12 hours

8. Preoperative Assessment

ASA Classification

Class	Description
ASA 1	Healthy patient
ASA 2	Mild systemic disease (well-controlled HTN, obesity)
ASA 3	Severe systemic disease (poorly controlled DM, COPD)
ASA 4	Life-threatening disease (recent MI, severe valve disease)
ASA 5	Moribund (not expected to survive without surgery)
ASA 6	Brain-dead organ donor
E suffix	Emergency surgery

Fasting Guidelines

2-4-6-8 Fasting

2 hours: Clear fluids
4 hours: Breast milk
6 hours: Formula, light meal
8 hours: Heavy meal, meat

Medications Perioperatively

Continue	Potentially Withhold
Cardiac medications	ARBs (hypotension risk)
Antihypertensives (except ARBs)	Anticoagulants
Anti-reflux medication	Oral diabetic agents
Opioids, chronic pain meds	SGLT2i (3 days - euglycaemic DKA)
Inhalers

9. Monitoring

4 Standard Monitors

Minimum monitoring in GA:

ECG
Blood pressure (NIBP)
Pulse oximetry (SpO2)
End-tidal CO2 (capnography)

10. Adverse Events

Common (Not Life-Threatening)

Event	Risk	Prevention
PONV	20%	Prophylactic antiemetics
Sore throat	20%	Careful airway management
Dental damage	Rare	Careful laryngoscopy

Rare, Serious

Event	Risk
Awareness under GA	1:8200 (with muscle relaxant)
Major adverse cardiac/cerebrovascular event	Varies
Death	1:57,000