Monitored Anaesthesia Care and Sedation
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Direct answer
Monitored anaesthesia care (MAC) is anaesthesiologist-directed sedation and analgesia during a procedure under local anaesthesia, graded on the ASA continuum: minimal sedation (anxiolysis, patient responds to voice), moderate or conscious sedation (purposeful response to verbal or light tactile stimulation, airway maintained), deep sedation (purposeful response only to repeated or painful stimuli, airway may need support) and general anaesthesia (unrousable, airway intervention required) — defined by responsiveness and airway, never by the drug chosen. Typical agents are midazolam 0.02-0.05 mg/kg titrated in 1-2 mg increments, fentanyl 25-50 microgram, propofol 25-100 microgram/kg/min, dexmedetomidine loaded at 1 microgram/kg over ten minutes then 0.2-0.7 microgram/kg/h, and ketamine in subanaesthetic boluses. Standards match general anaesthesia — ECG, oximetry, blood pressure and, since the ASA amendments, waveform capnography during moderate and deep sedation because apnoea is detected minutes before desaturation — with flumazenil 0.2 mg increments (to 1 mg) and naloxone titrated from 40 microgram as the antagonists to have drawn.
What you must remember
- The continuum definitions: minimal — responds to voice, airway and ventilation unaffected; moderate — purposeful response to verbal or tactile, no airway support needed; deep — purposeful to repeated painful stimulus, may need airway; general anaesthesia — unrousable, airway intervention usually required.
- Drug doses by level: midazolam 0.02-0.05 mg/kg IV titrated; fentanyl 0.5-1 microgram/kg; propofol 25-100 microgram/kg/min infusion or 0.5 mg/kg boluses for moderate, higher for deep; dexmedetomidine 1 microgram/kg over 10 minutes then 0.2-0.7 microgram/kg/h; ketamine 0.2-0.5 mg/kg for analgesic-sedative effect.
- Monitoring standard: identical vigilance to GA — ECG, pulse oximetry, non-invasive blood pressure and waveform capnography for moderate and deep sedation; oxygen by nasal cannulae at 2-4 L/min with the capnography sampling line tucked beside them.
- Capnography rationale: apnoea and airway obstruction show on the trace long before the saturation falls, especially true under propofol, whose respiratory depression outlasts sedation appearance.
- Antagonist rescue: flumazenil 0.2 mg IV over 15 seconds, repeated to 1 mg total (onset under 2 minutes, shorter half-life than midazolam — resedation risk); naloxone 0.04-0.4 mg titrated in 40-microgram steps (also shorter than most opioids).
- Depth-assessment scales: Ramsey 1-6 (1 anxious to 6 unresponsive), the Observer's Assessment of Alertness/Sedation for midazolam titration, and procedure-specific targets — cooperative for colonoscopy, immobile for cataract or cardioversion.
- High-risk settings: obesity, sleep apnoea, the elderly (half doses) and prone positioning, where moderate sedation drifts to deep unannounced.
Sedating for an endoscopy, minute by minute
A 55-year-old man for colonoscopy lies in the left lateral position; monitoring attached, oxygen at 2 L/min, capnography line under the nares, suction checked. The target is moderate sedation: rousable and cooperative. Midazolam 2 mg goes in, then fentanyl 50 microgram; two minutes later he is drowsy but answers his name, and the endoscope passes. Halfway through, pain at the splenic flexure prompts a further 25 microgram fentanyl; his respiratory rate falls to eight and the capnography trace flattens — the moment the monitor earns its place, because the saturation is still 97%. A jaw thrust and a verbal prompt, and the trace resumes.
Had propofol been the technique, the same scenario runs on an infusion of 50-75 microgram/kg/min, with the technician's roles fixed: watch the trace and keep the antagonists — flumazenil and naloxone — drawn and labelled before the case starts, not after the airway is lost. The MAC case ends not when the scope is out but when orientation is documented and the unit's discharge criteria are met; resedation from a longer-acting benzodiazepine than its antagonist is the classic late recovery trap.
How the exam frames it
Sedation questions test the continuum before they test the drugs: a scenario describes a patient who "moves purposefully when the shoulder is shaken" and the answer must be deep sedation with the implication that airway equipment and a person able to rescue the airway are required — the phrase "rescue the airway" is itself the credentialing concept the ASA uses, and quoting it earns marks. The second fixed question is the capnography mandate for moderate and deep sedation, with the physiology (apnoea precedes desaturation, more so with propofol) as justification. Dose traps include dexmedetomidine's ten-minute loading limit, ketamine's preserved airway reflexes with salivation needing an antisialagogue, and the antagonist half-life mismatch behind resedation.
Frequently asked questions
What are the four levels of the sedation continuum?
Minimal sedation (responds to voice), moderate or conscious sedation (purposeful response to verbal or tactile stimuli, airway intact), deep sedation (purposeful response only to repeated or painful stimuli, airway may need support), and general anaesthesia (unrousable).
Which monitoring is required during moderate and deep sedation?
Continuous ECG, pulse oximetry, blood pressure, and waveform capnography per current ASA guidance, plus immediately available airway equipment and a practitioner competent to rescue the airway.
What are the standard sedative drug doses in MAC?
Midazolam 0.02-0.05 mg/kg titrated in 1-2 mg increments, fentanyl 25-50 microgram, propofol 25-100 microgram/kg/min, dexmedetomidine 1 microgram/kg over 10 minutes then 0.2-0.7 microgram/kg/h, and ketamine 0.2-0.5 mg/kg.
How is midazolam oversedation reversed?
Flumazenil 0.2 mg intravenously over 15 seconds, repeated to a total of 1 mg; because flumazenil's duration is shorter than midazolam's, the patient is observed for resedation.
Why is capnography added to sedation monitoring?
Apnoea and partial obstruction appear on the waveform several minutes before oxygen saturation falls — time in which a jaw thrust and stimulation prevent deterioration into an emergency.