science

Minimum alveolar concentration

Minimum alveolar concentration is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Minimum alveolar concentration rather than just read about it. In short: Minimum alveolar concentration (MAC) is the concentration, often expressed as a percentage by volume, of a vapour in the alveoli of the lungs that is needed to prevent movement in 50% of patients in response to pain. MAC is used to compare the potency (dose required to induce a specific effect) of anaesthetic vapours.

Key takeaways

Minimum alveolar concentration belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Minimum alveolar concentration to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Minimum alveolar concentration from memory before moving on to harder problems.

Reference excerpt

Minimum alveolar concentration (MAC) is the concentration, often expressed as a percentage by volume, of a vapour in the alveoli of the lungs that is needed to prevent movement in 50% of patients in response to pain. MAC is used to compare the potency (dose required to induce a specific effect) of anaesthetic vapours. The concept of MAC was first introduced in 1965. "Minimum alveolar concentration" is a misnomer, as MAC is representative of a median value. The original paper proposed MAC as the minimal alveolar concentration, which was shortly thereafter revised to minimum alveolar concentration. A lower MAC value represents a more potent volatile anesthetic. Other uses of MAC include MAC-BAR (1.7–2.0 MAC), which is the concentration required to block autonomic reflexes to nociceptive stimuli, and MAC-awake (0.3–0.5 MAC), the concentration required to block voluntary reflexes and control perceptive awareness.

Formal definition The MAC is the concentration of the vapour (measured as a percentage at 1 atmosphere, i.e. the partial pressure) that prevents patient movement in response to a supramaximal stimulus (traditionally a set depth and width of skin incisions) in 50% of subjects. This measurement is done at steady state (assuming a constant alveolar concentration for 15 minutes), under the assumption that this allows for an equilibration between the gasses in the alveoli, the blood and the brain. MAC is accepted as a valid measure of potency of inhalational general anaesthetics because it remains fairly constant for a given species even under varying conditions.

Meyer-Overton hypothesis

The MAC of a volatile substance is inversely proportional to its lipid solubility (oil:gas coefficient), in most cases. This is the Meyer-Overton hypothesis put forward in 1899–1901 by Hans Horst Meyer and Charles Ernest Overton. MAC is inversely related to potency, i.e. a higher MAC equals a lower potency, as a greater concentration of the anaesthetic is required to suppress movement. The hypothesis correlates lipid solubility of an anaesthetic agent with potency (1/MAC) and suggests that onset of anaesthesia occurs when sufficient molecules of the anaesthetic agent have dissolved in the cell's lipid membranes, resulting in anaesthesia. Exceptions to the Meyer-Overton hypothesis can result from:

convulsant property of an agent specific receptor (various agents may exhibit an additional effect through specific receptors) co-administration of α2 agonists (such as dexmedetomidine) or opioid receptor agonists (morphine/fentanyl) can decrease the MAC Mullin's critical volume hypothesis Positive modulation of GABA at GABAA receptors by barbiturates or benzodiazepines

Factors affecting MAC Certain physiological and pathological states may alter MAC. For example, MAC increases with hyperthermia and hypernatremia. Conversely, anemia, hypercarbia, hypoxia, hypothermia, hypotension (MAP less than 40mmHg), and pregnancy seem to decrease MAC. Duration of anesthesia and biological sex seem to have little effect on MAC. Age has been shown to affect MAC. MAC begins to rise at one month of age with a peak at approximately 6 months of age (i.e., greater anaesthetic concentration is required to reach the effective anaesthetic dose). There is a subsequent steady decline in MAC with increasing age, with the exception of another peak during puberty. There is a linear model that describes the change in MAC of approximately 6% per decade of age. Medications, illicit drugs, and prior substance use history have also been found to affect MAC. For example, acute use of amphetamines, cocaine, ephedrine, and chronic use of alcohol increase MAC. Whereas, administration of propofol, etomidate, barbiturates, benzodiazepines, ketamine, opiates, local anesthetics, lithium, verapamil, and alpha 2-agonists (dexmedetomidine, clonidine) decrease MAC. Acute alcohol intoxication and chronic amphetamine use have also been found to decrease MAC. MAC values are additive. For instance, when applying 0.3 MAC of drug X and 1 MAC of drug Y the total MAC achieved is 1.3 MAC. In this way nitrous oxide is often used as a "carrier" gas to decrease the anesthetic requirement of other drugs.

Common MAC values Values are known to decrease with age and the following are given based on a 40-year-old (MAC40):

Nitrous oxide – 104 Xenon – 72 Desflurane – 6.6 Ethyl ether – 3.2 Sevoflurane – 1.8 Enflurane – 1.63 Isoflurane – 1.17 Halothane – 0.75 Chloroform – 0.5 Methoxyflurane – 0.16

Notes

References

Worked examples

Example 1 — a first encounter with Minimum alveolar concentration

Start with the simplest possible case. Write down what Minimum alveolar concentration claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Minimum alveolar concentration before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Minimum alveolar concentration ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Minimum alveolar concentration

In research: Minimum alveolar concentration appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Minimum alveolar concentration in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Minimum alveolar concentration is common in secondary-school and first-year university syllabi. It links to neighbouring topics Anesthesia, so understanding it makes those chapters shorter.
In everyday life: Look for Minimum alveolar concentration outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Minimum alveolar concentration in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Minimum alveolar concentration means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Minimum alveolar concentration out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Minimum alveolar concentration in simple terms?

Why does Minimum alveolar concentration matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Minimum alveolar concentration?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Minimum alveolar concentration.