Estimated reading time: 6 minutes
The DNB June 2026 Paper 1 included several questions that were closely aligned with previously asked concepts. One of the recalled questions focused on Minimum Alveolar Concentration (MAC), its definite.
Let’s break it down the way you would actually revise it before an exam.
First things first: What exactly is MAC?
Minimum Alveolar Concentration (MAC) is the minimum alveolar concentration of an inhalational anesthetic at 1 atmosphere pressure that prevents movement in response to a standard surgical incision in 50% of patients.
That definition can look unnecessarily complicated when you first read it.
The easiest way to understand it is:
MAC tells you how much of a volatile anesthetic is needed to prevent movement during surgery in 50% of patients.
And there is one relationship you should not forget:
Lower MAC = Higher potency
Higher MAC = Lower potency
For example, the MAC of sevoflurane is around 2%, whereas desflurane has a MAC of around 6%.
So, a lower concentration of sevoflurane is needed to achieve the same endpoint.
That means:
Sevoflurane is more potent than desflurane.
This is why MAC is useful when comparing the potency of inhalational anesthetic agents.
Important MAC values to revise
You don’t need to turn this into a memory exercise where you keep staring at numbers for hours. Start by knowing the approximate values and, more importantly, their order.
| Inhalational agent | Approximate MAC |
| Methoxyflurane | 0.16% |
| Halothane | 0.75% |
| Isoflurane | 1.4% |
| Sevoflurane | 2% |
| Desflurane | 6% |
| Nitrous oxide | 104% |
Methoxyflurane has a very low MAC, which means high potency. However, it is not commonly used because of its nephrotoxicity.
For exams, knowing the commonly tested values is more useful than trying to memorise every possible number.
Factors that decrease MAC
Now comes the part that is often tested in MCQs and theory questions.
Think about it this way:
If the patient is already more sedated or more sensitive to anesthetic effects, you need less volatile anesthetic.
So MAC goes down.
Important factors include:
- Increasing age
- Hypothermia
- Hypoxia
- Pregnancy
- Opioids
- Benzodiazepines
- Alpha-2 agonists such as dexmedetomidine and clonidine
- Acute alcohol intoxication
A simple revision line:
Elderly + hypothermia + pregnancy + CNS depressants → ↓ MAC
Pregnancy
Pregnancy is a particularly important association.
The requirement for inhalational anesthetic decreases during pregnancy. Increased progesterone levels are one of the factors associated with this increased sensitivity.
So, for a quick recall:
Pregnancy → ↓ MAC
Acute alcohol vs chronic alcohol: Don’t mix them up
This is one of those small differences that can easily cost you a mark.
Acute alcohol intoxication → ↓ MAC
The patient is already under the sedative effects of alcohol, so less anesthetic is required.
Chronic alcohol use → ↑ MAC
With chronic exposure, tolerance develops. The patient is therefore less sensitive to the sedative effects, and the anesthetic requirement increases.
So remember:
Acute alcohol = decrease
Chronic alcohol = increase
Factors that increase MAC
Now flip the concept.
If the patient is more stimulated, has increased metabolic activity, or has developed tolerance to a sedative substance, the anesthetic requirement can increase.
Important factors include:
- Hyperthermia
- Hypernatremia
- Acute cocaine use
- Chronic alcohol use
- Chronic amphetamine use
- Infancy, particularly around 6 months of age
Again, don’t just memorise the list. Try to understand the pattern.
Age and MAC
Age is an important one.
MAC is relatively high in infancy and reaches its maximum around 6 months of age.
After that, it gradually decreases with age.
So:
Infant → higher MAC
Elderly → lower MAC
This comparison is worth remembering because it can easily appear as an MCQ.
What does not significantly affect MAC?
Not every patient characteristic changes MAC.
The commonly recalled points include:
- Gender
- Thyroid disease
- Duration of anesthesia
These are useful to remember because they can be used as distractors in exam questions.
What about the CNS effects?
The recalled DNB question also asked about the effects of inhalational anesthetic agents on the CNS.
Volatile anesthetics produce dose-dependent CNS depression. Their effects include changes in:
- Consciousness
- Memory
- Cerebral activity
- Immobility
They can also alter cerebral blood flow and intracranial dynamics, which is why their CNS effects are clinically important rather than being just a theory topic.
When preparing this part for theory, don’t learn it as a random list. Try to connect each effect with its clinical relevance.
How should you write this answer in DNB theory?
If you get a similar question in your exam, don’t start writing everything you remember about inhalational anesthetics.
Keep the answer organised.
A simple structure would be:
1. Definition of MAC
2. Relationship between MAC and potency
3. Important MAC values
4. Factors decreasing MAC
5. Factors increasing MAC
6. Factors with little or no effect on MAC
7. CNS effects of inhalational anesthetics
This makes your answer easier to follow and also reduces the chance of leaving out one part of a multi-component question.
Five-minute revision before the exam
If you are revising MAC at the last minute, ask yourself:
Can I define MAC in one sentence?
Can I explain why lower MAC means greater potency?
Do I remember the approximate MAC of sevoflurane and desflurane?
What happens to MAC in pregnancy?
What happens in elderly patients?
Can I differentiate acute and chronic alcohol use?
Which drugs decrease MAC?
Which conditions increase MAC?
If you can answer these without looking at your notes, your basic MAC revision is in good shape.
The bigger lesson from this DNB paper
The important takeaway isn’t just MAC.
The June 2026 Paper 1 recalls show why residents should not ignore previously asked and repeatedly tested concepts while preparing for DNB theory.
A topic that looks simple can still become a full-length theory question when the examiner asks for its definition, clinical factors, numerical values and physiological effects together.
So while revising, don’t just ask:
“Have I read this topic?”
Ask:
“Can I write this topic as a structured answer if it comes in my paper tomorrow?”
That small change in the way you revise can make your preparation much more exam-oriented.
Note: This discussion is based on student recalls of the DNB June 2026 Paper 1. The exact question paper was not available at the time of the discussion.
Watch Video: DNB JUNE 2026 – PAPER 1 | MAC Question Discussion | Dr. Jhanvi Bajaj
