GE HEALTHCARE B20, User'S Reference Manual

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Airway gas
17-17
Automatic agent identification with E-sCAiO, N-CAiO modules
The modules with agent identification option will automatically identify and select Isoflurane,
Desflurane, Sevoflurane, Enflurane and Halothane. The E-sCAiO module is able to identify two
agents simultaneously and displaying them as primary and secondary agents. The N-CAiO
module is able to identify and display one agent. The inspiratory and expiratory concentrations
of the agent are displayed in a numeric parameter window. Minimum concentration for the
identification is 0.15 vol%. The agent selection remains active even if the concentration
decreases below 0.15 vol%. Automatic agent identification is operational after the normal
warm up of the module (approximately five minutes).
•
If rapid agent concentration changes are required, fresh gas flow must be increased.
•
Anesthetic agent concentration in the circuit is affected by patient uptake, breathing
system volume and the fresh gas flow. It quantifies the speed of washin and wash-out
anesthetic agents.
Minimum alveolar concentration (MAC)
MAC and MACage
The minimum alveolar concentration (MAC) concept is based on the assumption that in a
steady state, the alveolar partial pressure of a gas is equal to the partial pressure in the
effector organ of the central nervous system. MAC values are used to estimate the level of
anesthesia caused by volatile anesthetics.
The MAC value can be displayed in digit field: 1 MAC is the alveolar concentration (end-tidal) of
the agent at which 50% of patients do not respond to a noxious or surgical stimulus. The value
is calculated from the actual measured anesthetic agent and N
2
O values with empirical
formulas based on statistical studies with anesthetized patients.
The monitor can display two different MAC values, MAC or MACage, based on different
formulas. The use of MAC or MACage is selected during installation and configuration.
The MAC values correspond to those of healthy adults of about 40 years old, and cannot be
applied to children or elderly patients. Age and some other individual factors influencing the
effect of volatile agents are not taken into account.
The other calculation method, MACage, takes the patient's age into account. The calculation
uses 0 if age is less than 0, and 100 if age is more than 100. In addition, MACage calculations
include the atmospheric pressure and the patient’s (highest measured) temperature values. If
no patient temperature is measured, 37°C is used instead. For volatile agents this calculation
method means about 6.7% decrease of MAC value with each increasing decade of life.
MACage is calculated if it is enabled in the care unit settings and the patient’s age is given on
the monitor. If no age is given, MAC is calculated despite the care unit setting.
References used for MAC and MACage values
The alveolar concentrations of traditional (MAC) and age dependent (MACage) values are based
on the following references:
•
References for anesthetic agent MAC values:
−
Mapleson W.W.: Effect of age on MAC in humans: a meta-analysis. Br. J. of
Anaesthesia 1996; 76: 179-185
− Rampil I.J.; Zwass M.; Lockhart S.; Eger E.I. II; Johnson B.H.; Yasuda N.; Weiskopf R.B.:
MAC of I653 in surgical patients, Anesthesiology. Tram-Rac71 (3A):A269, September
1989
−
Scheller M.S., Partridge B.L., Saidman L.J.: MAC of sevoflurane in humans and the
New Zealand white rabbit. Anesthesiology 1987; 67: A373