Depth of anaesthesia

What “depth of anaesthesia” means (core concept)

• Depth of anaesthesia = how strongly anaesthetic drugs are affecting the brain and spinal cord at a given moment.
• Practical aim: patient is unconscious (no awareness), comfortable (analgesia), and still enough for surgery (immobility), with stable physiology.
• Depth is not a single number: hypnosis (unconsciousness), analgesia (pain control), and muscle relaxation can be dissociated.
• You assess depth using a combination of: clinical signs, vital signs, drug doses/concentrations, and (sometimes) brain monitoring (e.g., processed EEG).

Why it matters (under vs over)

• Too light (under-anaesthesia): risk of awareness, movement, coughing/bucking, sympathetic surge (tachycardia, hypertension), laryngospasm/bronchospasm.
• Too deep (over-anaesthesia): hypotension, bradycardia, reduced cardiac output, delayed wake-up, increased vasopressor/fluids, potential organ hypoperfusion.
• The “right” depth changes with stimulus: induction/intubation and incision need more than quiet periods (e.g., skin closure).

How you assess depth in day-to-day practice

• Look at the whole picture: heart rate, blood pressure, lacrimation/sweating, pupil size/reactivity, breathing pattern (if spontaneous), movement, coughing, ventilator dyssynchrony.
• Check the anaesthetic delivery: vaporiser setting, end-tidal agent concentration, fresh gas flows, circuit connections, IV infusion rates, syringe driver function, IV access patency.
• Consider the surgical stimulus: ask the surgeon what’s happening (incision, traction, peritoneal stretch, tourniquet inflation).
• Remember confounders: beta-blockers, pacemakers, autonomic neuropathy, high spinal/epidural, hypovolaemia, sepsis, pain masked by neuromuscular blockade.

Minimum Alveolar Concentration (MAC) basics (inhalational agents)

• MAC = the end-tidal concentration of an inhaled agent that prevents movement in response to surgical stimulus in 50% of patients (a population measure).
• MAC is mainly about immobility (spinal cord effect), not guaranteed unconsciousness or amnesia.
• MAC decreases with: increasing age, opioids, benzodiazepines, propofol, hypothermia, pregnancy, severe illness. MAC increases with: chronic alcohol use, some stimulants.
• Practical tip: use age-adjusted MAC where available on the machine; interpret alongside clinical signs and analgesia.

Processed EEG depth monitors (e.g., BIS) — when and how to use

• Processed EEG monitors estimate hypnotic depth (brain effect) and can help reduce risk of awareness in selected cases (e.g., TIVA with paralysis, high-risk patients).
• They do not measure analgesia; a patient can be “deep” on EEG but still respond with hypertension/tachycardia if analgesia is inadequate.
• Common targets are manufacturer-specific; treat the number as a trend, not an absolute truth.
• Artifacts are common: poor electrode contact, diathermy, EMG (muscle activity), shivering, and some drugs (e.g., ketamine can raise values despite adequate hypnosis).

TIVA (propofol-based) depth: practical safety points

• With TIVA, you cannot rely on end-tidal agent; vigilance is higher, especially if neuromuscular blockade is used.
• Use a reliable infusion strategy (TCI where available) and confirm: correct drug, correct line, correct pump settings, and that the cannula is working.
• Consider processed EEG monitoring for TIVA with paralysis or other awareness risk factors (local policy dependent).
• If unexpected lightness: check IV patency/extravasation, pump/line disconnection, empty syringe, wrong rate, or drug swap errors.

Neuromuscular blockade: why it changes your assessment

• Paralysis removes movement as a warning sign; you must rely more on haemodynamics, ventilator synchrony, lacrimation/sweating, and monitoring.
• Always ensure adequate hypnosis and analgesia before and during paralysis; document and communicate if concerns arise.
• Use nerve stimulator monitoring (e.g., TOF) to guide blockade and recovery; avoid “masking” awareness with unnecessary paralysis.

Common first-time scenarios and what to do

• Tachycardia/hypertension at incision: first think pain/stimulus → give opioid/analgesic, deepen anaesthetic (increase volatile/propofol), check ventilation/CO2, exclude hypoxia and light anaesthesia.
• Hypotension after induction: often vasodilation + reduced sympathetic tone ± hypovolaemia → reduce agent, give fluid bolus if appropriate, use vasopressor (e.g., metaraminol/phenylephrine) per local practice, reassess.
• Patient “breathing against ventilator”/coughing on tube: may be light, inadequate opioid, or ventilator settings → deepen anaesthetic, give opioid, check tube position, consider additional muscle relaxant if appropriate.
• Delayed wake-up: consider too much anaesthetic/opioid, hypothermia, metabolic issues (hypoglycaemia, hypercarbia), residual neuromuscular block → stop/reduce agents early, warm patient, check gases/glucose, reverse NMB appropriately.

Reducing risk of accidental awareness (introductory approach)

• Higher risk situations: emergency surgery, obstetrics, haemodynamic instability (light anaesthesia intentionally), TIVA with paralysis, difficult airway, previous awareness.
• Before induction: check equipment, drug labels, and plan; consider depth monitoring for high-risk cases (local policy).
• During maintenance: ensure adequate hypnotic dose (volatile end-tidal/age-adjusted MAC or propofol infusion), adequate analgesia, and avoid long periods of paralysis with uncertain hypnosis.
• If awareness is suspected intra-op: deepen anaesthesia promptly, give amnestic agent if appropriate (e.g., benzodiazepine per consultant/local guidance), document events, and escalate to senior help.