Minimum Alveolar Concentration (Mac) Definition; Calculating The Mac Factors - Dräger Apollo Operating Instructions Manual

Chapter 8 - Monitoring
Minimum alveolar concentration
1 MAC is equal to the anesthetic gas concentration
at 760 mmHg at which 50% of all patients no longer
respond to a stimulation of the nerves.
The values listed in the table below are used to
calculate the MAC factor. These values are merely
guideline values and are still subject to debate. The
current MAC values listed in the instructions
enclosed with the anesthetic gases are binding.
1 MAC
Halothane 0.77%
Enflurane 1.7%
Isoflurane 1.15%
Desflurane 6.65%
Sevoflurane 2.1%
N O 105%
2
The MAC value depends on the patient's age. The
above values refer to the age of 40 years.
If there is a mixture of two volatile agents, the
concentration of the secondary agent is displayed if
the MAC factor is 0.1 MAC or greater. The gas with
the higher expiratory MAC factor is displayed above
the secondary gas.
A secondary agent becomes the primary agent if its
MAC factor exceeds the MAC factor of the primary
agent by 0.2 MAC.
A mixture of more than two volatile anesthetic agents
cannot be reliably detected.

Calculating the MAC factors

The MAC value is a simple navigation aid for
determining the dosage of anesthetic agent.
Apollo takes into account age correction, altitude
correction and mixture calculation. The influence of
other medication (opiates or intravenous hypnotics)
is not taken into account when calculating MAC
factors.
The measured values used for calculating the MAC
factor are the end-expiratory concentrations. Partial
pressures are taken into account.
The displayed value is calculated at 760 mmHg as
follows:
exp. conc. agent
1
f =
MAC (age) agent
1
118
(MAC) definition
exp. conc. agent
2
+ +
MAC (age) agent
2
Figure 82. Influence of Age on MAC
Influence of age on the MAC value
exp. conc. N O
2
MAC (age) N O
2
Gas Measurement
Desflurane
0.2*N2O
Sevoflurane
Enflurane
Isoflurane
Halothane
Age (years)
Apollo Operator's Manual