Monitored Anaesthesia Care & Sedation

Monitored anaesthesia care (MAC) is a specific anaesthesia service in which an anaesthesiologist supervises a patient receiving local/regional anaesthesia or procedural sedation, ready to escalate to general anaesthesia (GA) if needed. The central exam theme is the sedation continuum — minimal → moderate → deep → GA — and the pharmacology (midazolam, propofol, dexmedetomidine, ketamine) plus the monitoring and rescue obligations that go with each level.

Definitions & key concepts

• Monitored Anaesthesia Care (MAC): an anaesthesia provider is present, monitors vitals, administers sedatives/analgesics, and is qualified to convert to GA and manage the airway. MAC is defined by the provider and readiness, not by the depth of sedation reached. A patient under MAC may, at moments, be only minimally sedated or transiently deep.
• Procedural sedation and analgesia (PSA): drug-induced depression of consciousness allowing a patient to tolerate an unpleasant procedure while maintaining cardiorespiratory function.
• Conscious sedation: older term, roughly synonymous with moderate sedation.

High-yield: MAC is distinguished from moderate sedation by the provider's qualification and the explicit readiness/intent to rescue and convert to GA, not by how deep the patient is. This is a favourite "definition" MCQ.

The sedation continuum (ASA classification)

Sedation is a continuum, not discrete states — a patient can drift deeper than intended. The provider must be able to rescue a patient from one level deeper than the level intended.

Parameter	Minimal (anxiolysis)	Moderate ("conscious")	Deep	General anaesthesia
Responsiveness	Normal to verbal	Purposeful to verbal/tactile*	Purposeful after repeated/painful stimulus	Unarousable even with pain
Airway	Unaffected	No intervention needed	Intervention may be required	Intervention often required
Spontaneous ventilation	Unaffected	Adequate	May be inadequate	Frequently inadequate
Cardiovascular	Unaffected	Usually maintained	Usually maintained	May be impaired

*Reflex withdrawal from a painful stimulus is not a purposeful response.

High-yield: "Purposeful response to repeated or painful stimulation" = deep sedation. "Purposeful response to verbal or light tactile" = moderate sedation. Reflex withdrawal does NOT count as purposeful — a frequent trap.

Rescue principle (flow): Intended level → patient drifts one level deeper → provider must independently recognise & manage → open airway, give positive-pressure ventilation, reverse drugs, support circulation. Hence anyone doing moderate sedation must be able to rescue from deep sedation; anyone doing deep sedation must be able to rescue from GA.

Drugs for procedural sedation

Midazolam (benzodiazepine)

• Water-soluble at low pH, lipophilic at physiological pH → rapid CNS entry. Onset 1–2.5 min IV, duration 30–60 min.
• Produces anxiolysis, sedation, amnesia (anterograde), anticonvulsant effect — but no analgesia.
• Acts at GABA-A receptor, increasing frequency of chloride channel opening (barbiturates increase duration).
• Dose: 0.02–0.05 mg/kg IV titrated. Reduce in elderly/hepatic disease.
• Synergistic respiratory depression with opioids — the classic lethal combination.

Propofol

• 2,6-diisopropylphenol in a lipid (soybean/egg lecithin) emulsion. GABA-A agonist.
• Rapid onset (~30 s), rapid offset (redistribution) → ideal for titratable sedation/short procedures. Antiemetic, no analgesia.
• Adverse effects: dose-dependent respiratory depression & apnoea, hypotension (vasodilation + myocardial depression), pain on injection, no reversal agent.
• Propofol infusion syndrome (PRIS): prolonged high-dose infusion → metabolic acidosis, rhabdomyolysis, hyperkalaemia, lipaemia, cardiac failure.
• Egg/soya allergy historically a caution (allergy is usually to egg white; emulsion uses lecithin — modern view is more permissive but still tested as a "contraindication").

Ketamine

• NMDA receptor antagonist → dissociative anaesthesia. Provides profound analgesia + amnesia while preserving airway reflexes and ventilation.
• Bronchodilator (good in asthma); sympathomimetic → ↑HR, ↑BP, ↑CO (useful in shock/hypovolaemia).
• Adverse: emergence delirium/hallucinations (reduced by benzodiazepines), hypersalivation (give an antisialagogue like glycopyrrolate), raised ICP/IOP (traditional caution), laryngospasm in children.
• Excellent for paediatric sedation, burns dressings, and the haemodynamically unstable patient.

Dexmedetomidine

• Highly selective central α2-agonist (α2:α1 ≈ 1600:1; clonidine ≈ 200:1).
• Produces "cooperative/arousable" sedation, anxiolysis, analgesia, sympatholysis with minimal respiratory depression — its signature exam fact.
• Mimics natural NREM sleep (acts on locus coeruleus). Loading dose 1 µg/kg over 10 min, then 0.2–0.7 µg/kg/h.
• Adverse: bradycardia and hypotension (sometimes biphasic hypertension with rapid bolus), dry mouth. Useful for awake fibreoptic intubation and ICU sedation.

High-yield: Dexmedetomidine is the sedative that causes the least respiratory depression and allows an arousable, cooperative patient — drug of choice for awake fibreoptic intubation. Ketamine uniquely preserves ventilation and gives analgesia and bronchodilation.

Drug	Mechanism	Analgesia	Respiratory depression	Cardiovascular	Reversal
Midazolam	GABA-A (↑ frequency)	None	Yes (↑ with opioids)	Mild ↓ BP	Flumazenil
Propofol	GABA-A	None	Marked, apnoea	↓ BP, ↓ CO	None
Ketamine	NMDA antagonist	Profound	Minimal (preserved)	↑ HR, ↑ BP	None
Dexmedetomidine	α2-agonist	Moderate	Minimal	Bradycardia, ↓ BP	None
Fentanyl/opioids	µ-opioid	Yes	Yes	Mild	Naloxone

Sedation scales

Ramsay Sedation Scale (older, 6 levels)

1. Anxious, agitated, restless
2. Cooperative, oriented, tranquil
3. Responds to commands only
4. Brisk response to light glabellar tap/loud noise
5. Sluggish response to glabellar tap/loud noise
6. No response to stimulus

High-yield: Ramsay 1 = agitated; Ramsay 6 = unresponsive. Levels 2–4 generally represent adequate sedation. Limitation: it conflates agitation and sedation on one axis and is not validated for the agitated/deeply sedated extremes well.

Richmond Agitation–Sedation Scale (RASS) — preferred in ICU

Ranges +4 to −5, anchored at 0 = alert & calm.

Score	Term	Description
+4	Combative	Violent, danger to staff
+3	Very agitated	Pulls/removes tubes/catheters
+2	Agitated	Frequent non-purposeful movement
+1	Restless	Anxious, not aggressive
0	Alert and calm	—
−1	Drowsy	Eye contact to voice >10 s
−2	Light sedation	Eye contact to voice <10 s
−3	Moderate sedation	Movement to voice, no eye contact
−4	Deep sedation	No response to voice, movement to physical stimulus
−5	Unarousable	No response to voice or physical stimulus

High-yield: RASS distinguishes −1/−2 by the 10-second eye-contact rule; −3 vs −4 by response to voice vs physical stimulus. Target for most ventilated ICU patients is RASS 0 to −2 (light sedation), which improves outcomes vs deep sedation.

Other scales worth a line: OAA/S (Observer's Assessment of Alertness/Sedation), MOAA/S, and Riker SAS. Bispectral index (BIS) gives a 0–100 processed-EEG number (≈40–60 for GA); used adjunctively but not mandatory for sedation.

Monitoring requirements

Standard ASA monitoring during sedation/MAC:

• Oxygenation: continuous pulse oximetry (SpO₂); supplemental O₂ as needed.
• Ventilation: clinical observation plus capnography (EtCO₂) — now recommended for moderate/deep sedation because it detects apnoea/hypoventilation before desaturation, especially under drapes/dark rooms.
• Circulation: ECG and NIBP at least every 5 minutes.
• Temperature when clinically indicated.
• A dedicated person monitors the patient (for moderate sedation may assist with brief interruptible tasks; for deep sedation/GA must be exclusively dedicated).

High-yield: Capnography (EtCO₂) is the earliest and most sensitive monitor of hypoventilation/apnoea during sedation — it changes before SpO₂ falls (oximetry is delayed, more so with supplemental oxygen). Expect this as the single-best-answer.

Pre-procedure checklist: focused history & airway exam (Mallampati, mouth opening, neck mobility, thyromental distance), ASA physical status, NPO/fasting status, IV access, and immediate availability of suction, oxygen, bag-mask, airway adjuncts and resuscitation drugs.

Fasting (ASA): clear fluids 2 h, breast milk 4 h, light meal/infant formula/non-human milk 6 h, fatty/fried meal 8 h — the "2-4-6-8" rule.

Mnemonic — "SOAPME" for sedation setup: Suction, Oxygen, Airway equipment, Pharmacy (drugs incl. reversal), Monitors, Equipment (special/emergency).

Reversal strategies

Agent reversed	Reversal drug	Dose	Caution
Benzodiazepines	Flumazenil	0.2 mg IV, repeat to 1 mg	Short t½ → re-sedation; precipitates seizures in chronic users/TCA co-ingestion
Opioids	Naloxone	0.04–0.4 mg IV titrated	Short t½ → re-narcotisation; acute withdrawal, pulmonary oedema

• Propofol, ketamine, dexmedetomidine have NO reversal agents — management is supportive (airway, ventilation, fluids/vasopressors, atropine for dexmedetomidine bradycardia).
• Re-dosing/observation: because flumazenil and naloxone are shorter-acting than many agonists, the patient must be monitored for re-sedation after reversal.

Rescue flow for over-sedation: Stop drug → call for help / stimulate patient → open airway (jaw thrust, chin lift) → 100% O₂ + bag-mask ventilation → specific reversal (flumazenil/naloxone) → support circulation → escalate to definitive airway/GA if no improvement.

Discharge criteria

Recovery is assessed with the Modified Aldrete score (activity, respiration, circulation, consciousness, O₂ saturation — each 0–2; ≥9 for discharge) or the Post-Anaesthetic Discharge Scoring System (PADSS) for day-care/ambulatory (vital signs, ambulation, nausea, pain, surgical bleeding; ≥9). Patient must have a responsible escort for ambulatory discharge.

Complications

• Respiratory: hypoventilation, apnoea, airway obstruction, hypoxaemia, laryngospasm (esp. ketamine in children), aspiration. The commonest serious adverse events are respiratory.
• Cardiovascular: hypotension (propofol), bradycardia (dexmedetomidine), arrhythmias.
• Drug-specific: PRIS, emergence delirium (ketamine), paradoxical agitation (midazolam, esp. children/elderly), injection pain (propofol).
• Inadequate sedation / patient movement / awareness during the procedure.

High-yield: The leading cause of sedation-related morbidity & mortality is respiratory depression with inadequate monitoring/rescue — hence the emphasis on capnography and a dedicated observer.

Key differentials & distinctions

• Moderate vs deep sedation: response to verbal/light tactile vs repeated/painful stimulation; airway/ventilation increasingly compromised in deep.
• Deep sedation vs GA: in deep sedation patient is arousable to painful stimulus; in GA, unarousable.
• MAC vs moderate sedation: defined by provider qualification and readiness to convert to GA, not depth.
• Dexmedetomidine vs propofol sedation: dexmedetomidine = arousable, minimal respiratory depression, bradycardia; propofol = deeper, apnoea-prone, faster offset.

Recently asked / exam angle

• Definition-matching: identify the level of sedation from a clinical vignette describing the patient's response to stimuli (verbal vs painful, reflex vs purposeful).
• Which monitor detects hypoventilation earliest? → Capnography (EtCO₂).
• Sedative with least respiratory depression / allows cooperative patient / drug for awake fibreoptic intubation → Dexmedetomidine (α2-agonist; α2:α1 = 1600:1).
• Reversal of midazolam → Flumazenil; of opioids → Naloxone; both with re-sedation caution.
• Ketamine features: NMDA antagonist, dissociative, analgesia + bronchodilation, preserved airway, emergence reactions reduced by benzodiazepines; sympathomimetic — good in shock, caution in raised ICP/IOP.
• Ramsay anchors (1 = agitated, 6 = no response) and RASS range +4 to −5 with 0 = alert/calm; ICU target RASS 0 to −2.
• Propofol infusion syndrome triad and GABA-A mechanism; benzodiazepines increase frequency of Cl⁻ channel opening (vs barbiturates → duration).
• 2-4-6-8 fasting rule and Aldrete ≥9 for discharge.

Rapid revision

1. MAC = anaesthesiologist present, ready to convert to GA; defined by provider/readiness, not depth.
2. Sedation is a continuum; must be able to rescue from one level deeper than intended.
3. Moderate sedation: purposeful response to verbal/light touch; deep: only to repeated/painful stimulus; reflex withdrawal ≠ purposeful.
4. Capnography (EtCO₂) detects apnoea/hypoventilation before SpO₂ falls — earliest ventilation monitor.
5. Midazolam: GABA-A, ↑ channel frequency; amnesia, no analgesia; reversed by flumazenil.
6. Propofol: rapid on/off, apnoea + hypotension, no reversal, watch for PRIS, pain on injection.
7. Ketamine: NMDA antagonist, dissociative, analgesia + bronchodilation, preserved ventilation; emergence delirium ↓ by benzodiazepines.
8. Dexmedetomidine: selective α2-agonist (1600:1), arousable sedation, minimal respiratory depression, causes bradycardia/hypotension; best for awake fibreoptic intubation.
9. Ramsay 1–6 (1 agitated, 6 unresponsive); RASS +4 to −5 (0 alert/calm); ICU target RASS 0 to −2.
10. Naloxone reverses opioids; both reversal agents are short-acting → monitor for re-sedation.
11. NPO 2-4-6-8 (clear fluids–breast milk–light meal–fatty meal); discharge when Aldrete ≥9 / PADSS ≥9.
12. Most sedation-related deaths are due to unrecognised respiratory depression — dedicated observer + monitoring + rescue equipment ("SOAPME") are mandatory.