Question

Enumerate physiological and pharmacological factors affecting MAC.

Hint:

MAC is a critical factor in determining the appropriate dosage of inhaled anaesthetics for different patients, taking into account their age, metabolic state, and other factors.

Answer 1

Step 1: Physiological factors affecting MAC.
Several physiological factors can influence the MAC value of inhaled anaesthetics:
- Age: The MAC decreases with age. Older patients require lower concentrations of anaesthetic to achieve the same effect.
- Body temperature: Hypothermia reduces MAC, while hyperthermia increases MAC.
- Metabolic rate: A higher metabolic rate, as seen in hyperthyroidism, increases MAC, while hypothyroidism decreases MAC.
- Gender: Females generally require lower MAC values compared to males.
- Pregnancy: Pregnancy decreases MAC, especially in the later stages.
- PaCO₂ levels: Elevated carbon dioxide levels in the blood (hypercapnia) decrease MAC, while low levels (hypocapnia) increase MAC.

Step 2: Pharmacological factors affecting MAC.
Pharmacological factors also play a significant role in determining MAC:
- Concurrent medications: Certain drugs, such as opioids and benzodiazepines, can lower MAC by enhancing the effects of anaesthetics.
- Pre-anaesthetic medication: Medications like muscle relaxants and sedatives can reduce the required MAC for anaesthesia.
- Anaesthetic agents: The MAC for different anaesthetic agents varies. For example, halothane has a lower MAC compared to nitrous oxide, indicating it is more potent.

Step 3: Conclusion.
The MAC is influenced by various physiological and pharmacological factors, including age, temperature, gender, metabolic rate, and the use of other medications. Understanding these factors is essential for adjusting anaesthetic doses for optimal patient care.