Minimum Alveolar Concentration (MAC)

Definition of MAC

Minimum alveolar concentration (MAC) is the end-tidal alveolar concentration of an inhalational anaesthetic, at 1 atmosphere pressure, that prevents purposeful movement in 50% of subjects exposed to a standard noxious stimulus, classically surgical skin incision. It is therefore a quantal ED₅₀ for immobility, not a measure of hypnosis, analgesia, amnesia or haemodynamic stability. The original work by Eger, Saidman and Brandstater in the 1960s established MAC as a reproducible potency index for volatile agents under controlled conditions, and it remains central to FRCA pharmacology because it links physics, pharmacodynamics and clinical dosing.

How MAC is measured

MAC is measured experimentally rather than calculated. Subjects are held at a fixed end-tidal concentration for at least 15 minutes so that alveolar, arterial and brain partial pressures approximate one another, a standard noxious stimulus is applied (surgical skin incision in humans, tail clamp in the original animal work), and gross purposeful movement is recorded as a simple yes or no. Concentrations are varied between subjects, or by an up-and-down (Dixon) sequence, and quantal dose-response modelling identifies the concentration at which half of subjects move. Because the response is quantal, MAC is a property of a population and has no meaning in one patient: what is measured in theatre is the end-tidal agent concentration on the gas analyser, read as a fraction of the age-adjusted MAC for that agent.

MAC is expressed as volume percent at end-expiration, but its biological determinant is partial pressure in the CNS. Alveolar partial pressure is used as a clinically accessible surrogate for brain partial pressure once equilibration has occurred. Hence MAC is conventionally quoted at sea level; at altitude the volume percent required changes, but the anaesthetic partial pressure required is broadly unchanged. MAC values are additive in partial-pressure terms: for example, 0.5 MAC nitrous oxide plus 0.5 MAC sevoflurane approximates to 1.0 MAC total immobility potency.

MAC and potency

MAC is an inverse measure of potency: the lower the MAC, the more potent the agent. Potency tracks lipid solubility, expressed as the oil:gas partition coefficient (the Meyer-Overton correlation), whereas speed of onset and offset tracks the blood:gas partition coefficient. The two properties are independent, which is why desflurane is both the least potent of the modern volatiles and the fastest in wash-in and wash-out, while halothane is the most potent and the slowest.

Agent	Approximate MAC in adults aged 30–55 years	Blood:gas coefficient	Oil:gas coefficient	Key implication
Desflurane	6.0%	0.42	18.7	Low potency, very rapid wash-in/wash-out
Sevoflurane	2.0%	0.65	47	Rapid onset; suitable for inhalational induction
Isoflurane	1.15%	1.4	91	More soluble; slower kinetics than sevoflurane/desflurane
Halothane	0.75%	2.4	224	High potency; slow kinetics; historical use
Nitrous oxide	104%	0.47	1.4	MAC cannot be achieved at 1 atm without hypoxia
Xenon	63–71%	0.115	1.9	Very rapid kinetics; NMDA antagonism; limited availability

The statistical basis is important. MAC is the median effective concentration for movement suppression. The concentration–response curve for volatile immobility is relatively steep; MAC₉₅, the concentration preventing movement in 95% of subjects, is approximately 1.2–1.3 MAC. Conversely, at 1.0 MAC, 50% of patients may still move if no other antinociceptive or hypnotic drugs are present. This explains why clinical anaesthesia commonly uses balanced techniques rather than volatile monotherapy.

Several derived MAC endpoints are examined frequently:

• MAC-awake: end-tidal concentration at which 50% of patients respond to verbal command; approximately 0.3–0.5 MAC, depending on agent and study methodology.
• MAC-BAR: concentration blocking adrenergic response to skin incision in 50% of patients; typically 1.5–2.0 MAC without adjuncts.
• MAC-intubation: concentration preventing movement/coughing with tracheal intubation in 50%; greater than MAC incision and markedly reduced by opioids.
• Age-adjusted MAC: commonly estimated using the Mapleson relationship: MAC_age = MAC₄₀ × 10^{−0.00269(age−40)}, approximating to a 6% decrease per decade after age 40.

MAC-awake

MAC-awake is the end-tidal concentration at which 50% of subjects respond appropriately to verbal command, conventionally quoted as approximately 0.3-0.5 MAC for the modern volatiles. It is lower on emergence than on induction because of hysteresis between alveolar and effect-site partial pressure. This is the endpoint relevant to consciousness rather than movement, and it explains why a paralysed patient whose end-tidal agent has drifted towards 0.3 MAC is at real risk of awareness even though movement is impossible. Nitrous oxide and xenon have relatively high MAC-awake fractions, so an equivalent MAC fraction supplied by nitrous oxide does not protect against awareness as reliably as the same fraction supplied by a potent volatile. Related reading: sleep and consciousness.

MAC-BAR

MAC-BAR is the concentration blocking the adrenergic response (rise in heart rate, blood pressure and circulating catecholamines) to skin incision in 50% of subjects, at approximately 1.5-2.0 MAC with volatile alone. That concentration is rarely tolerable haemodynamically, so MAC-BAR is reached pharmacologically rather than by turning up the vaporiser: opioids reduce it steeply, with fentanyl around 1.5-3 micrograms/kg producing most of the available effect before a ceiling is reached, and alpha-2 agonists, beta blockade and neuraxial or regional block reduce it further. The endpoints therefore rank MAC-awake < MAC < MAC₉₅ < MAC-BAR. Related reading: effects of anaesthesia and drugs on the cardiovascular system.

Why volatile agents produce immobility

Mechanistically, MAC is not explained by a single molecular target. The historical Meyer–Overton correlation demonstrated that volatile anaesthetic potency correlates strongly with lipid solubility: agents with higher oil:gas coefficients have lower MAC values. This remains a useful potency rule, but exceptions, stereoselectivity and non-immobilising compounds indicate that anaesthesia is mediated primarily by interactions with protein targets, not nonspecific lipid dissolution.

Immobility, the endpoint defining MAC, is generated predominantly within the spinal cord, especially through effects on dorsal horn nociceptive transmission, interneuronal integration and ventral horn motor output. Transection and differential delivery studies show that spinal volatile concentration predicts movement suppression better than cerebral concentration. In contrast, hypnosis and amnesia involve cortical, thalamic and hippocampal networks; hence processed EEG indices such as BIS correlate imperfectly with MAC and should not be regarded as measures of immobility.

At the receptor and ion-channel level, volatile agents enhance inhibitory signalling and suppress excitatory transmission. Sevoflurane, isoflurane and desflurane potentiate GABA_A and glycine receptor currents, activate background potassium conductances including TREK-1, TASK and other two-pore-domain K⁺ channels, reduce presynaptic neurotransmitter release, and inhibit nicotinic acetylcholine, glutamate and some voltage-gated channels. Nitrous oxide and xenon differ mechanistically, with prominent NMDA receptor antagonism and less GABAergic potentiation; this partly explains their analgesic properties and distinct EEG effects.

MAC should therefore be conceptualised as a robust population pharmacodynamic index for one component of anaesthesia—spinal immobility—derived from end-tidal partial pressure under standardised conditions. It is influenced by pharmacokinetic equilibration, physicochemical potency, receptor-level actions and patient biology, but it is not synonymous with depth of anaesthesia in the broad clinical sense.

Factors that increase MAC

• Infancy: MAC is highest in infants around 1-6 months of age (sevoflurane approximately 3.0-3.3%), having been lower in preterm and term neonates.
• Hyperthermia: MAC rises with temperature up to about 42 degrees Celsius, beyond which it falls again.
• Chronic alcohol use and chronic exposure to central nervous system stimulants, through tolerance and altered catecholaminergic tone.
• Acute sympathomimetic states: amphetamine and cocaine intoxication, ephedrine, monoamine oxidase inhibitors and levodopa, all of which raise central catecholamine concentrations.
• Hypernatraemia (raised cerebrospinal fluid sodium).
• Red hair: a modest increase associated with melanocortin-1 receptor variants; a favourite viva aside rather than a clinically large effect.

Equally examinable is what does not change MAC: sex, body weight and height, duration of anaesthesia, thyroid function, potassium and magnesium concentrations, hypertension, metabolic alkalosis, and PaCO₂ across a wide middle range of roughly 15-95 mmHg. Altitude does not change the anaesthetic partial pressure required, although the volume percent needed to reach it does change.

Factors that decrease MAC

• Increasing age beyond early adulthood, by approximately 6% per decade after 40.
• Hypothermia, roughly 2-5% per degree Celsius fall.
• Pregnancy, by around 30-40% at term.
• Extremes of physiology: PaO₂ below about 50 mmHg, mean arterial pressure below about 50 mmHg, profound anaemia with low arterial oxygen content, and PaCO₂ above roughly 95 mmHg (carbon dioxide narcosis).
• Hyponatraemia, and acute alcohol intoxication.
• Central depressant drugs: opioids, benzodiazepines, propofol, ketamine, alpha-2 agonists such as clonidine and dexmedetomidine, intravenous lidocaine, lithium, and acute administration of many antihypertensives that deplete central catecholamines.
• Neuraxial and regional blockade, which reduces the afferent input that MAC is defined against.
• Nitrous oxide, which does not so much reduce MAC as contribute additively to it in partial-pressure terms.

Effect of age

Age is the single most useful correction in daily practice. MAC is relatively low in preterm and term neonates, peaks in infancy at around 1-6 months, and then declines steadily. After 40 years the accepted rule is a fall of approximately 6% per decade, formalised by the Mapleson relationship MAC_age = MAC₄₀ x 10^{-0.00269(age-40)}. An 80-year-old therefore needs roughly 78% of the concentration required by a 40-year-old, which is why modern anaesthetic machines display age-adjusted MAC and why a fixed vaporiser setting produces relative overdose in the elderly and relative underdose in the young adult.

Effect of temperature

MAC falls with hypothermia by approximately 2-5% per degree Celsius, reflecting reduced cerebral metabolic rate and altered anaesthetic solubility: as temperature falls, blood:gas solubility rises, so more agent dissolves in blood for a given partial pressure. Prolonged surgery with passive cooling therefore produces a patient who needs progressively less volatile, and failure to notice this contributes to hypotension and delayed emergence. Above roughly 42 degrees Celsius the relationship reverses and MAC falls again. Related reading: heat production, conservation and loss and temperature measurement.

Pregnancy

MAC falls by approximately 30-40% at term, an effect appearing as early as the first trimester and attributed to progesterone and to increased endogenous opioid activity. The pharmacokinetic changes of pregnancy compound the pharmacodynamic one: minute ventilation rises by around 50% while functional residual capacity falls by about 20%, so the alveolar concentration rises faster and equilibration is quicker. The practical consequence at caesarean section under general anaesthesia is a narrow margin - too much volatile causes uterine relaxation, haemorrhage and neonatal depression, too little produces awareness in a group already recognised as high risk, so an end-tidal target of at least 0.7-1.0 age-adjusted MAC is usually maintained and documented.

Drugs and MAC

Almost all balanced anaesthesia is an exercise in moving MAC pharmacologically. Opioids produce the largest and steepest reduction in the volatile requirement, with a clear ceiling: successive increments buy progressively less MAC sparing while adding respiratory depression and haemodynamic effect. Alpha-2 agonists reduce MAC substantially through central sympatholysis. Benzodiazepines, propofol, ketamine and intravenous lidocaine all reduce it; nitrous oxide contributes additively rather than sparing. In the other direction, chronic alcohol use, chronic stimulant use and acute sympathomimetic states raise the requirement, and this is where an apparently light patient at a conventional vaporiser setting is usually explained. Related reading: neostigmine and sugammadex and suxamethonium for the neuromuscular drugs that mask the movement endpoint entirely.

Clinical relevance

MAC matters clinically for three reasons. It allows agents of very different volume percent to be compared and combined on a single scale, so 0.5 MAC sevoflurane plus 0.5 MAC nitrous oxide can be reasoned about as roughly 1.0 MAC of immobility potency. It provides the target that awareness-prevention strategies are built on: age-adjusted end-tidal anaesthetic concentration monitoring with low-agent alarms performs at least as well as processed EEG in the large trials, and sustained concentrations below about 0.7 age-adjusted MAC in a paralysed patient should always be a deliberate, justified decision. And it frames titration during the case, where the required concentration is not constant but moves with age, temperature, blood pressure, blood loss, opioid dosing and the intensity of surgical stimulus.

Clinical assessment and investigations

Clinical presentation: interpreting “depth” through the lens of MAC

In clinical anaesthesia, MAC is not directly “measured” as a pharmacodynamic endpoint; it is inferred from the end-tidal alveolar partial pressure of volatile agent and interpreted against the patient’s predicted MAC requirement. The classical endpoint is movement in response to a supramaximal surgical stimulus; haemodynamic or EEG responses are related but non-identical endpoints. Thus, a patient may not move at 1.0 MAC yet still exhibit sympathetic activation; conversely, neuromuscular blockade may abolish movement despite inadequate hypnosis.

Features suggesting inadequate volatile effect include movement, lacrimation, sweating, tachycardia, hypertension, coughing, ventilator dyssynchrony, and—most importantly—postoperative explicit recall. Excess effect presents as dose-related hypotension from reduced systemic vascular resistance and myocardial depression, bradycardia, delayed emergence, hypoventilation, and potentiation of neuromuscular blockade. In the paralysed patient, autonomic signs are insensitive and non-specific; absence of movement is uninterpretable.

Differential diagnosis of apparent inadequate or excessive MAC

Clinical problem	Differential diagnosis	Exam-relevant discriminators
Movement at “adequate” MAC	Low alveolar delivery, circuit leak, exhausted agent/vaporiser off, high stimulus, opioid deficiency, hyperthermia, chronic alcohol/amphetamine use	Compare inspired versus end-tidal agent; check fresh gas flow, vaporiser, circuit integrity, capnography; assess neuromuscular monitoring and analgesic dosing
Tachycardia/hypertension	Light anaesthesia, hypovolaemia, hypercarbia, hypoxia, pain, anticholinergics, ketamine, pheochromocytoma, thyroid storm, malignant hyperthermia	ETCO2, temperature trend, airway pressures, SpO2, arterial pressure waveform, recent drugs; volatile MAC alone is not diagnostic
Hypotension at low MAC	Hypovolaemia, sepsis, neuraxial block, anaphylaxis, myocardial ischaemia, pulmonary embolism, adrenal insufficiency, drug interaction	Assess preload responsiveness, ECG, capnography, echocardiography where indicated, lactate/base deficit; do not attribute automatically to volatile overdose
Delayed emergence	High context exposure to volatile, residual opioid/benzodiazepine, residual neuromuscular blockade, hypothermia, hypoglycaemia, hypercarbia, stroke/seizure	End-tidal agent should fall to near zero; TOF ratio >0.9; check glucose, ABG, temperature and neurological status

Investigations and monitoring

The key “investigation” is continuous respiratory gas analysis. The end-tidal volatile concentration approximates alveolar partial pressure once equilibration occurs; the discrepancy between inspired and end-tidal concentrations is greatest during uptake, high cardiac output states, low solubility transitions, or circuit leaks. MAC is conventionally quoted at 1 atmosphere in adults aged approximately 40 years: isoflurane 1.15–1.17%, sevoflurane 1.8–2.1%, desflurane 6.0–6.6%, halothane 0.74–0.75%, nitrous oxide approximately 104%. MAC fractions are broadly additive: for example, 0.5 MAC sevoflurane plus 0.5 MAC nitrous oxide approximates 1.0 MAC, although additivity is not perfect at extremes.

Endpoint	Approximate threshold	Clinical interpretation
MAC-awake	0.3–0.5 MAC	Response to verbal command lost or regained; relevant to emergence and awareness risk
MAC	1.0 MAC	Prevents movement to incision in 50% of subjects; ED50, not a guarantee
MAC95	Approximately 1.2–1.3 MAC	Prevents movement in about 95% of unpremedicated patients
MAC-BAR	Approximately 1.5–2.0 MAC	Blocks adrenergic response in 50%; markedly reduced by opioids and neuraxial blockade

Interpretation must be age-adjusted. MAC decreases by approximately 6% per decade after early adulthood. A commonly used correction is: MAC_age = MAC₄₀ × 10^{−0.00269(age−40)}. Thus, an 80-year-old requires roughly 78% of the MAC of a 40-year-old. Premature neonates have lower MAC; infants have relatively high MAC, with sevoflurane MAC peaking around 3.0–3.3% in early infancy before declining.

Processed EEG, such as BIS, is an adjunct rather than a MAC measurement. BIS values of 40–60 are commonly targeted for general anaesthesia, but volatile concentration and EEG can diverge with ketamine, nitrous oxide, hypothermia, cerebral pathology, artefact, and neuromuscular blockade. Large trials, including B-Aware and BAG-RECALL, showed that BIS-guided protocols can reduce awareness in high-risk populations, but end-tidal anaesthetic concentration protocols are similarly effective when rigorously applied. Contemporary practice therefore emphasises audible low-agent alarms and age-adjusted ETAC monitoring, particularly when neuromuscular blocking drugs are used.

Interpretation of factors affecting MAC

When the observed clinical response is discordant with measured ETAC, actively search for modifiers. MAC is reduced by hypothermia, pregnancy, ageing, acute alcohol intoxication, opioids, benzodiazepines, propofol, alpha-2 agonists, lithium, severe hypotension, anaemia, and hypoxaemia. Clinically important thresholds include PaO₂ <50 mmHg, MAP <50 mmHg, and temperature reduction of roughly 1°C producing an approximate 5% MAC reduction. MAC is increased by hyperthermia, chronic alcohol use, and some sympathomimetic states; hypernatraemia increases MAC, whereas hyponatraemia decreases it.

Additional investigations should be directed by context: arterial blood gas for hypoxaemia, hypercarbia, acidosis and sodium; temperature monitoring for hypothermia or malignant hyperthermia; glucose for delayed emergence; quantitative neuromuscular monitoring with TOF ratio >0.9 before extubation; and equipment checks including vaporiser filling/position, agent analyser calibration, fresh gas flow, circle system leaks, and scavenging. A measured low age-adjusted ETAC, especially <0.7 MAC during paralysis, should be treated as a significant warning for accidental awareness unless deliberately justified by haemodynamic instability or balanced anaesthetic technique.

Management, pharmacology and procedures

Using MAC clinically: dosing, titration and monitoring

MAC is a population-derived pharmacodynamic endpoint: 1.0 MAC prevents movement to a supramaximal surgical stimulus in 50% of subjects; approximately 1.3 MAC prevents movement in 95%. It is therefore a guide to immobility, not reliably hypnosis, analgesia or autonomic suppression. In practice, volatile delivery should be titrated to age-adjusted end-tidal concentration, haemodynamics, surgical stimulus, neuromuscular blockade, co-administered hypnotics/opioids and processed EEG where appropriate. The end-tidal agent concentration is the clinically relevant surrogate for brain partial pressure once equilibration is approached; inspired concentration may substantially overestimate effect during uptake or low cardiac output states.

Concept	Approximate value/clinical implication
MACawake	~0.3–0.4 MAC; 50% obey command/open eyes. Emergence usually occurs below this range, modified by opioids and sedatives.
MAC for intubation	~1.5–2.0 MAC without adjuncts; markedly reduced by opioids, propofol and neuromuscular blockade.
MAC-BAR	~1.5 MAC to obtund adrenergic response in 50%; poor endpoint if beta-blocked, neuraxial blockade or major haemorrhage.
95% immobility	~1.3 MAC; often excessive when combined with remifentanil, regional anaesthesia or elderly physiology.

Age adjustment is mandatory in examination answers and clinical practice. MAC peaks at around 6 months and decreases thereafter by approximately 6% per decade after age 40. A pragmatic formula is:

Age-adjusted MAC = MAC₄₀ × 10^{−0.00269(age−40)}.

Typical MAC₄₀ values in oxygen are sevoflurane 2.0%, isoflurane 1.17%, desflurane 6.0%, halothane 0.75%, nitrous oxide 104% and xenon 63–71%. MACs are broadly additive: for example, 50% nitrous oxide contributes roughly 0.5 MAC and reduces the volatile requirement accordingly, although additivity is less predictable for intravenous agents.

Pharmacological manipulation of MAC

Acute management of volatile anaesthesia is largely management of MAC requirement. Factors that reduce MAC should prompt lower vaporiser settings and closer haemodynamic/EEG surveillance; factors that increase MAC may require higher end-tidal targets, additional analgesia or correction of the underlying stimulus.

Intervention/factor	Effect on MAC	Mechanistic/exam point
Opioids	Dose-dependent reduction with ceiling effect; fentanyl 1–3 micrograms/kg or remifentanil 0.05–0.2 micrograms/kg/min commonly halves volatile requirement.	Spinal and supraspinal antinociception reduces movement and autonomic responses; does not guarantee amnesia.
Propofol/benzodiazepines/barbiturates	Reduce MAC; midazolam 0.02–0.05 mg/kg produces clinically relevant sparing.	GABAA potentiation; synergistic hypnosis and respiratory depression.
Dexmedetomidine	Reduces MAC by ~20–50%; loading 0.5–1 microgram/kg over 10 min, infusion 0.2–0.7 micrograms/kg/h.	Alpha-2 agonism, sympatholysis; bradycardia and hypotension limit use.
Neuraxial/regional block	Reduces MAC and MAC-BAR substantially.	Deafferentation reduces noxious input; beware awareness if volatile reduced excessively during paralysis.
Hypothermia	~5% MAC reduction per °C below 37°C.	Reduced CNS metabolic activity and altered membrane/channel kinetics.
Severe hypotension, anaemia, hypoxaemia	Reduce MAC.	Pathological depression of CNS function; do not interpret low anaesthetic requirement as adequate anaesthesia alone.
Acute alcohol, amphetamine or cocaine use; hyperthermia	Increase MAC.	Enhanced catecholaminergic arousal and nociceptive responsiveness.
Chronic alcohol or sedative use	Often increases MAC via tolerance; acute intoxication reduces MAC.	Important distinction for viva questions and perioperative risk assessment.
Pregnancy	MAC reduced by ~25–40%, apparent by early gestation and persisting briefly postpartum.	Progesterone, endorphins and altered CNS sensitivity; aspiration/airway risk dominates management.

Procedural application and safety

During induction, a high inspired concentration may be required to overcome circuit volume, uptake and functional residual capacity, particularly with sevoflurane. Maintenance should be guided by end-tidal age-adjusted MAC, commonly 0.7–1.0 MAC when combined with opioids, and higher values only when clinically justified. Low-flow anaesthesia requires appreciation of agent-specific pharmacology: desflurane permits rapid titration because of low blood:gas solubility (~0.42), sevoflurane is intermediate (~0.65) and isoflurane slower (~1.4). Nitrous oxide accelerates uptake of volatile agents by the concentration and second-gas effects, but expands closed gas spaces and is avoided in pneumothorax, bowel obstruction, middle ear surgery and recent intraocular gas.

Depth monitoring is particularly important when MAC is deliberately reduced, neuromuscular blockade prevents movement, or total intravenous adjuncts are used. BIS values of 40–60 are often targeted for general anaesthesia, but processed EEG is imperfect with ketamine, nitrous oxide, artefact, hypothermia and neurological disease. The B-Aware trial demonstrated reduced awareness in high-risk patients using BIS guidance, whereas B-Unaware and BAG-RECALL showed that end-tidal anaesthetic concentration protocols were at least comparable when rigorously applied. A defensible strategy is to avoid sustained end-tidal volatile concentrations below approximately 0.5 age-adjusted MAC in paralysed patients unless supplemented by robust hypnotic dosing and monitoring.

Complications, escalation and follow-up

Inadequate MAC manifests as movement, tachycardia, hypertension, lacrimation or processed EEG activation, but these signs lack specificity. Management includes verifying vaporiser filling/position, fresh gas flow, circuit integrity, agent analyser calibration, endotracheal position and neuromuscular blockade status; then increasing end-tidal volatile, administering opioid or hypnotic bolus, and treating the surgical stimulus. Suspected awareness requires immediate deepening of anaesthesia, communication with the team, postoperative structured interview, documentation, explanation, apology where appropriate and referral for psychological support.

Excess MAC causes dose-dependent myocardial depression, systemic vasodilatation, hypotension, respiratory depression and delayed emergence. Management is reduction or cessation of volatile delivery, increased fresh gas flow, airway/ventilatory support, vasopressors such as metaraminol 0.5–1 mg IV boluses or phenylephrine 50–100 micrograms IV, and treatment of contributory hypovolaemia, sepsis or anaphylaxis. Malignant hyperthermia is not a MAC problem but is a volatile-triggered emergency: discontinue trigger, give 100% oxygen, hyperventilate, change circuit/activate charcoal filters if available, administer dantrolene 2.5 mg/kg IV repeated until control, actively cool and monitor potassium, CK, acidosis, coagulopathy and renal injury in critical care for at least 24 hours.

Exam controversies and advanced synthesis

MAC is a population statistic, not a patient-specific “depth of anaesthesia”

In viva examinations, the most important conceptual pitfall is equating MAC with consciousness. MAC is a quantal ED₅₀: the end-tidal alveolar partial pressure at 1 atmosphere preventing purposeful movement to surgical incision in 50% of subjects. It is derived from binary responses and probit/logistic modelling; approximately 1.3 MAC prevents movement in about 95% of patients, assuming a typical population dose-response slope. This does not mean that a patient at 1.3 MAC is necessarily unconscious, amnesic, haemodynamically stable, or protected from awareness.

The immobility component of MAC is mediated predominantly at the spinal cord, whereas hypnosis, amnesia and perception involve thalamo-cortical and hippocampal networks. This explains why MAC correlates imperfectly with processed EEG indices and why spinal cord transection or regional anaesthesia can markedly reduce volatile requirement for movement without equivalently altering cortical drug effect.

MAC subtypes and clinically useful thresholds

Endpoint	Approximate value	Viva interpretation
MAC	1.0 MAC: no movement in 50%	Population ED50 for somatic response to incision
MAC95	About 1.3 MAC	Suppresses movement in most patients; not a guarantee
MAC-awake	About 0.3–0.5 MAC	Response to command/eye opening; closer to consciousness than incision MAC
MAC-BAR	About 1.5–2.0 MAC	Blocks adrenergic response; strongly reduced by opioids and neuraxial block
MAC-intubation	Often >1.5 MAC without adjuncts	Laryngoscopy is a more intense stimulus than skin incision

Age-adjusted MAC and monitoring guidance

Age adjustment is an examination favourite. In adults, MAC falls by approximately 6% per decade after early adulthood and may be estimated by:

MAC_age = MAC₄₀ × 10^{−0.00269(age−40)}.

Approximate MAC₄₀ values at 1 atmosphere are: sevoflurane 2.0%, isoflurane 1.15%, desflurane 6.0%, halothane 0.75%, nitrous oxide 104%, and xenon about 63–71%. A 75-year-old receiving 1.0% sevoflurane is therefore not receiving “0.5 MAC” in an age-adjusted sense; the age-corrected fraction is higher.

Modern anaesthetic monitoring standards from bodies such as the Association of Anaesthetists and ASA require inspired and expired volatile agent concentration monitoring when vapours are used. The practical controversy is not whether end-tidal agent monitoring matters—it does—but whether processed EEG is superior to an end-tidal age-adjusted MAC strategy for preventing awareness. NAP5 reported accidental awareness during general anaesthesia at approximately 1:19,000 overall, with substantially higher risk when neuromuscular blockade is used, and emphasised volatile concentration alarms and vigilance during induction, transfer and emergence.

Awareness trials: what they actually show

Trial	Design	Key message
B-Aware	High-risk patients; BIS-guided versus routine care	BIS reduced definite awareness, about 0.17% versus 0.91%; benefit was against non-protocolised routine practice
B-Unaware	BIS 40–60 versus end-tidal anaesthetic concentration protocol	BIS was not superior to maintaining adequate volatile concentration with alarms
BAG-RECALL	High-risk cohort; BIS protocol versus ETAC protocol, commonly targeting about 0.7–1.3 age-adjusted MAC	Processed EEG did not outperform an end-tidal volatile strategy; awareness events were rare in both arms

The exam synthesis is that processed EEG may be useful, particularly during TIVA with neuromuscular blockade, but it is not a substitute for pharmacological reasoning, end-tidal volatile measurement, haemodynamic interpretation, and clinical context. Conversely, end-tidal MAC is of limited value during TIVA, profound hypothermia, cardiopulmonary bypass, severe low-flow states, or when alveolar-brain equilibration is delayed.

Additivity, opioid interaction and common viva traps

• Partial pressure is the active variable. At altitude, a given vaporiser volume percent produces a lower partial pressure; the volume percent corresponding to 1 MAC rises as barometric pressure falls.
• Volatile agents are approximately additive in MAC fractions. For example, 0.5 MAC nitrous oxide plus 0.5 MAC sevoflurane approximates 1 MAC immobility, although additivity is not perfect for all endpoints.
• Opioids reduce MAC in a non-linear, ceiling fashion. Remifentanil may reduce volatile MAC by roughly 50–70%, but further dose escalation does not abolish the need for hypnotic/amnesic drug.
• Nitrous oxide has a MAC above 100%. It cannot produce MAC as a sole agent at normobaric pressure without hypoxia, despite being analgesic and MAC-sparing.
• Low MAC does not equal light anaesthesia in all patients. Elderly patients, hypothermia, pregnancy, severe anaemia, hypotension, sedatives, opioids, lithium, alpha-2 agonists and acute alcohol intoxication reduce MAC; chronic alcohol use, hyperthermia and stimulant states may increase it.

A high-scoring FRCA answer therefore frames MAC as a useful pharmacodynamic comparator and safety monitor, but explicitly separates immobility, unconsciousness, amnesia, autonomic stability and awareness prevention. That separation is the essence of advanced understanding.

Common Primary FRCA exam pitfalls

• Quoting MAC without the full definition. Examiners expect end-tidal alveolar concentration, at 1 atmosphere, preventing purposeful movement to a standard noxious stimulus, in 50% of subjects.
• Calling MAC a measure of depth of anaesthesia. It is an ED₅₀ for spinal immobility; hypnosis and amnesia are cortical and are described by MAC-awake, not MAC.
• Confusing potency with speed. Oil:gas coefficient predicts potency (and therefore MAC); blood:gas coefficient predicts onset and offset.
• Forgetting altitude. The volume percent required changes with ambient pressure; the partial pressure required does not.
• Muddling the direction of temperature and age. Both hypothermia and increasing age reduce MAC; infancy and hyperthermia increase it.
• Claiming MAC varies with sex, weight, height or duration of anaesthesia. It does not - this is the classic list of non-factors.
• Treating nitrous oxide as MAC sparing. It is additive in partial-pressure terms, and its high MAC-awake fraction means it is a poor guarantor against awareness.
• Assuming an immobile paralysed patient is adequately anaesthetised. Neuromuscular blockade abolishes the endpoint MAC is defined by.

Summary table

Factor	Effect on MAC	Approximate magnitude or threshold
Age over 40 years	Decrease	About 6% per decade (Mapleson correction)
Infancy (1-6 months)	Increase	Peak MAC; sevoflurane about 3.0-3.3%
Hypothermia	Decrease	About 2-5% per degree Celsius
Hyperthermia	Increase	Up to about 42 degrees Celsius, then falls
Pregnancy at term	Decrease	About 30-40%
Opioids	Decrease	Large, with a ceiling effect
Alpha-2 agonists, benzodiazepines, propofol, ketamine, lidocaine, lithium	Decrease	Dose dependent
Acute alcohol intoxication	Decrease	Dose dependent
Chronic alcohol or stimulant use	Increase	Tolerance
Acute sympathomimetics (amphetamine, cocaine, ephedrine, MAOIs, levodopa)	Increase	Raised central catecholamines
Hypoxaemia	Decrease	PaO2 below about 50 mmHg
Hypotension	Decrease	MAP below about 50 mmHg
Severe anaemia	Decrease	Low arterial oxygen content
Hypernatraemia / hyponatraemia	Increase / decrease	Cerebrospinal fluid sodium
Sex, weight, height, duration of anaesthesia, thyroid function, potassium, PaCO2 15-95 mmHg	No change	Classic list of non-factors

Practice SBA questions

1. A 78-year-old is anaesthetised with sevoflurane. Compared with a 40-year-old, the expected MAC is closest to which of the following?

• A. Unchanged
• B. About 90% of the 40-year-old value
• C. About 78% of the 40-year-old value
• D. About 50% of the 40-year-old value
• E. About 25% of the 40-year-old value

Answer: C. MAC falls by approximately 6% per decade beyond 40 years. Applying MAC_age = MAC₄₀ x 10^{-0.00269(age-40)} to a 38-year age difference gives roughly 0.78 of the MAC₄₀ value, so an age-adjusted target rather than a fixed vaporiser setting is required.

2. Which endpoint is normally reached at the highest end-tidal concentration?

• A. MAC-awake
• B. MAC
• C. MAC₉₅
• D. MAC-BAR
• E. MAC-intubation

Answer: D. MAC-BAR, at approximately 1.5-2.0 MAC, blocks the adrenergic response to incision in 50% of subjects and sits above MAC-awake (0.3-0.5 MAC), MAC (1.0) and MAC₉₅ (about 1.2-1.3 MAC). It is normally achieved with opioids or regional blockade rather than by volatile alone.

3. Which of the following increases MAC?

• A. Pregnancy at term
• B. Core temperature of 34 degrees Celsius
• C. Chronic alcohol use
• D. Acute alcohol intoxication
• E. Clonidine premedication

Answer: C. Chronic alcohol use raises the anaesthetic requirement through tolerance, whereas acute intoxication lowers it. Pregnancy, hypothermia and alpha-2 agonists all reduce MAC.

4. Regarding the physicochemical properties of volatile agents, which statement is correct?

• A. A high blood:gas partition coefficient predicts a low MAC
• B. A high oil:gas partition coefficient predicts a low MAC
• C. MAC expressed in volume percent is independent of ambient pressure
• D. Desflurane has a higher oil:gas coefficient than isoflurane
• E. Nitrous oxide MAC can be achieved at 1 atmosphere without hypoxia

Answer: B. Potency correlates with lipid solubility (Meyer-Overton), so a high oil:gas coefficient means a low MAC. Blood:gas solubility governs onset and offset, not potency. Volume percent varies with ambient pressure while the required partial pressure does not, desflurane is less lipid soluble than isoflurane, and nitrous oxide MAC of about 104% is unattainable at 1 atmosphere without a hypoxic mixture.

Related Primary FRCA tutorials

• Sleep and consciousness - the cortical endpoints behind MAC-awake.
• Effects of anaesthesia and drugs on the cardiovascular system - the haemodynamic cost of high MAC.
• Heat production, conservation and loss and temperature measurement - why MAC drifts during long cases.
• Capnography and anaesthetic machine components - how end-tidal agent is measured and delivered.
• Neostigmine and sugammadex and suxamethonium - the drugs that abolish the movement endpoint.
• Oxygen transport and the oxyhaemoglobin dissociation curve - the physiology behind hypoxaemia lowering MAC.

Test this topic: Primary FRCA question bank - or read the Primary FRCA pass mark guide and the Primary FRCA preparation guide.