1.5 The main physical properties of inhalational agents are the minimum alveolar concentration, volatility and blood gas solubility (Table.1:2).


Volatility of anesthetic agents: An agent with a low boiling point will evaporate readily and enable a higher concentration to be administered than with an agent with a high boiling point. 

The minimum alveolar concentration: The minimum alveolar concentration (MAC) of an inhaled anesthetic is the alveolar concentration that prevents movement in 50% of patients in response to a standardized stimulus (e.g., surgical incision). MAC is a useful measure because it mirrors brain partial pressure, allows comparisons of potency between agents, and showing how much is needed to produce the effect required. This is expressed as percentage vapor strength.

An agent with a low MAC is a potent agent because only a small amount is required to produce anesthesia. A high MAC means the agent is weak because a lot of agent is required to produce anesthesia.


Table 1:3 Factors that affect MAC
Factors that decrease Mac Factors that increase MAC include
  • MAC is age-dependent, being lowest in newborns, reaching a peak in infants, and then decreasing progressively with increasing age. 
  • Anesthetic drugs such as lidocaine, opioids, barbiturates, chlorpromazine, diazepam, ketamine, neostigmine, pancuronium 
  • Metabolic acidosis and hypothermia 
  • Hypotension , anemia 
  • Pregnancy 
  • The addition of nitrous oxide will decrease the MAC of another volatile anesthetic
  • Increased central neurotransmitter levels (monoamine oxidase inhibitors, cocaine, ephedrine) 
  • Hyperthermia 
  • Chronic ethanol abuse (determined in humans) 
  • Hypernatremia

N.B. Various factors increase or decrease (Table 1-3) MAC. Unfortunately, no single mechanism explains these alterations in MAC, supporting the view that anesthesia is the net result of numerous and widely varying physiologic alterations. In general, those factors that increase CNS metabolic activity and neurotransmission, increase CNS neurotransmitter levels, and up-regulate of CNS responses to chronically depressed neurotransmitter levels (as in chronic alcoholism) also seem to increase MAC. Conversely, those factors that decrease CNS metabolic activity, neurotransmission, and CNS neurotransmitter levels, and down-regulate CNS responses to chronically elevated neurotransmitter levels seem to decrease MAC. Many notable factors do not alter MAC, including duration of inhaled anesthetic administration, gender, type of surgical stimulation, thyroid function, hypo- or hypercarbia, metabolic alkalosis, hyperkalemia, and magnesium levels. However, there may be a genetic component influencing MAC.