Abstract
We now know how intravenous anesthetics act but do not understand where and how inhaled anesthetics act.
Generally two or more drugs are often combined to achieve these end-point-produced interactions labeled as “synergistic”, “additive” or “infra-additive” ; additive interactions suggest a common site of action, and synergistic interactions suggest a different site of action.
Lately Eger et al. have paid attention to synergy, additivity and infra-additivity in drug interactions. They published two papers about this research.
First they searched and selected the available data on anesthetic drug interactions for end-points of hypnosis and immobility (MAC) from the entire PubMed database.
They found that most interactions between drug classes were synergistic, except ketamine, which interacted in either an additive or infra-additive manner. Inhaled anesthetics typically showed synergy with intravenous anesthetics but were additive except in the case of nitrous oxide and isoflurane.
Next they studied the additivity of MAC for 11 inhaled anesthetic pairs defined by differences in potency for a channel/receptor in rats. They also studied 4 additional pairs that included nitrous oxide because of previous reports suggesting a deviation from additivity for the combination of nitrous oxide and isoflurane. The results showed that all combinations produced additivity, except for the combination of isoflurane with nitrous oxide.
Such results are consistent with the notion that inhaled anesthetics act on a single site to produce immobility in the face of noxious stimulation.
