💨 Minimum Alveolar Concentration (MAC) — Key Concepts😊

2025年10月12日

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Contents

♦️ Introduction

Inhaled anesthetics are still the cornerstone of general anesthesia, but how do we quantify their potency?

That’s where the Minimum Alveolar Concentration (MAC) comes in — a concept every anesthesiologist and perioperative provider should understand.

MAC allows us to compare the strength of different volatile anesthetics and adjust anesthetic depth precisely and safely.

♦️ What is MAC ?? 🤔

🔷 Definition

MAC is defined as the alveolar concentration of an inhaled anesthetic at 1 atmosphere that prevents movement in response to a surgical (painful) stimulus (such as skin incision) in 50% of subjects.
Each inhaled anesthetic has its own specific value, and its potency is roughly inversely proportional to 1/MAC (the Meyer–Overton hypothesis).
MAC reflects immobility of spinal origin, while loss of consciousness and amnesia are achieved at lower concentrations.

👉 In simpler terms…

If 100 patients are anesthetized with a volatile agent alone, and half of them move when the surgeon starts cutting the skin, that anesthetic concentration is defined as 1 MAC.
So in short, MAC represents the approximate potency (required dose) of an inhaled anesthetic (Lower MAC = stronger potency.).

🔷 Representative MAC Values of Common Inhaled Agents

Since MAC decreases with age, the following are approximate values for a 40-year-old adult under inhalational anesthesia without supplemental opioids (such as fentanyl).

Agent	MAC (vol%)	Approximate
Sevoflurane	~1.8–2.0%	≈ 2%
Desflurane	~6.0–6.6%	≈ 6–7%
Isoflurane	~1.15–1.2%	≈ 1%+
Nitrous oxide (N₂O)	~104%	(appears >100%, explanation below)

💬 Additional Notes

When a patient regains consciousness and can respond to verbal commands (e.g., “open your eyes”), the alveolar concentration of inhaled anesthetic is called MACawake, typically around 0.3–0.5 MAC.
The concentration that causes amnesia (no recall of stimuli) is termed MACamnesia, usually about 0.25–0.4 MAC.
At roughly 1.3 MAC, most patients show complete immobility to surgical stimulus.

🔷 MAC and Age：The Most Significant Determinant 👶👴

Age is known as the single strongest factor affecting MAC.

In general:

Infants and young children require higher MAC values (peak around 6 months).
After that, MAC gradually decreases, and beyond age 40, it declines by about 6–7% per decade.

In other words,

“The younger the patient, the more anesthetic concentration is needed for the same effect.”

Formula (Mapleson Equation)：Applicable from age ≥1 year (No need to memorize this one.😅)

Example: Estimated MAC Values by Age (approx. ±10% variation)

Agent	20 yr	40 yr	60 yr	80 yr
Sevoflurane	2.04	1.80	1.59	1.41
Desflurane	7.47	6.60	5.83	5.15
Isoflurane	1.32	1.17	1.03	0.91

♦️ Factors Other Than Age That Affect MAC

MAC is influenced by many physiological and pharmacologic factors —some lower the required concentration, others increase it.

There are also factors with little to no effect, such as gender or anesthesia duration.

🔷 Factors That Decrease MAC (lower required concentration → higher potency)

Aging
Hypothermia (≈4–5% decrease per 1°C drop)
Pregnancy
Co-administration of sedatives (benzodiazepines), opioids, α₂-agonists
Anemia, severe hypotension
Hypoxemia (<40 Torr)
Hyponatremia
Chronic central depressant use (barbiturates, etc.)

🔷 Factors That Increase MAC (higher required concentration → lower potency)

Younger age (especially infants and children)
Hyperthermia
Chronic alcohol consumption
Acute administration of CNS stimulants (cocaine, amphetamines)
Hypernatremia

🔷 Factors with Little or No Effect

Unlike age or temperature, some variables have minimal influence on MAC.

Gender: No consistent difference between male and female patients.
Duration of anesthesia: MAC remains essentially constant over time; “anesthetic fade” is not clinically significant.
Thyroid function: Although hyperthyroidism is sometimes listed as a MAC-raising factor in textbooks, most reviews suggest its direct effect is minimal — secondary effects such as sympathetic tone or body temperature matter more.
Metabolic acidosis / alkalosis: Generally little effect on MAC, though severe acidosis may decrease it.
Arterial CO₂ (PaCO₂): Normally minimal effect, but MAC decreases markedly when PaCO₂ > 80–90 mmHg.

In short, gender and time do not meaningfully change MAC.

🔷 Summary Table of MAC – Modifing Factors

Category	Decrease MAC (↑ potency)	Increase MAC (↓ potency)	Minimal or Biphasic Effect
Age / Temperature	Aging, Hypothermia (−4–5% per 1 °C)	Children (peak at 6 mo), Hyperthermia	—
Physiologic factors	Pregnancy, Anemia, Hypoxia, Hypotension, Hyponatremia	Hypernatremia	Metabolic acidosis/alkalosis: little effect overall, but severe acidosis decreases MAC.
Respiratory / CO₂	—	—	Arterial CO₂ (PaCO₂): generally little effect, but PaCO₂ > 80–90 mmHg → MAC decreases.
Pharmacologic factors	Acute alcohol, Opioids, Benzodiazepines, Propofol, α₂-agonists	Chronic alcohol, Acute CNS stimulants (amphetamine, cocaine)	—
Other factors	Regional anesthesia (apparent MAC ↓), Combined anesthesia	Strong surgical stimulation (temporary MAC ↑)	Gender, Duration of anesthesia, Mild acid–base imbalance

(Compiled from StatPearls, NYSORA, OpenAnesthesia, and e-SAFE.)

🔷 Combination and Additivity of MAC

In general, two volatile anesthetics are not administered simultaneously, but nitrous oxide (N₂O) can be safely combined with volatile agents such as sevoflurane.

Before the introduction of remifentanil, this combination was a common anesthetic technique for balanced anesthesia.

The MAC of nitrous oxide is approximately 100%. For safety reasons, modern anesthesia machines include a built-in mechanism that prevents delivery of N₂O concentrations higher than about 70%.

Accordingly, a mixture of 50% nitrous oxide (for example, O₂ 2 L/min + N₂O 2 L/min) corresponds to roughly 0.5 MAC.

👉 For a 40-year-old adult, administering sevoflurane around 1% (≈0.5 MAC) in combination with 50% N₂O yields a total anesthetic depth equivalent to approximately 1.0 MAC.

This demonstrates the additive nature of MAC:
when multiple inhaled agents are used together, their fractional MAC values can be summed to estimate the overall anesthetic effect.
However, remember that MAC represents immobility, not analgesia or unconsciousness— additional monitoring of awareness and pain response remains essential.

♦️ “MAC = Immobility” — Not Depth of Anesthesia

Volatile inhaled anesthetics such as sevoflurane and desflurane have little to no analgesic effect. In contrast, nitrous oxide, a gaseous anesthetic, does possess analgesic properties.

Therefore, when volatile agents are used at high concentrations, the patient may appear immobile, yet this merely reflects suppressed movement despite ongoing pain— in other words, “the patient is not moving, but may still be in pain.”

Remember, MAC is only an index of immobility; it does not guarantee adequate anesthesia in terms of analgesia or unconsciousness.

In actual anesthetic management, it is essential to maintain a proper balance among multiple components: strong analgesia through opioids or regional anesthesia sufficient hypnotic depth with volatile agents for amnesia and unconsciousness, and, when needed, reliable immobility using neuromuscular blocking agents.

📝 Summary：Take Home Points

MAC = the alveolar concentration preventing movement in 50% of patients — reflects spinal immobility.
Decreases with age (~6 % per decade after 40); peaks around 6 months in infants.
Lowered by hypothermia, pregnancy, opioids, sedatives; increased by children, hyperthermia, sympathetic stimulation.
MACawake (0.3–0.5) = return of consciousness; MACamnesia (0.25–0.4) = loss of memory.
> 1.2 MAC → hemodynamic risk; adjust slowly and titrate to effect.