Relevant Concentrations of Inhaled Anesthetics for In Vitro Studies of Anesthetic Mechanisms

acetylcholine of a 4 b 2 acetylcholine receptors expressed in Xenopus oocytes. ( A ) Fractional decrease in a single preparation. The shallowness of the isoflurane concentration–effect curve (R g slightly more than 1) bears no necessary relation to the steepness (P g ) of the population concentration–effect relation that the receptor may mediate. Assuming, for example, that anything less than a 50% (0.5 fraction) depression allows movement in response to incision and that more than 50% prevents movement, the resulting population concentration–effect relation is infinitely steep ( D ; the relation is approximated by a curve with a P g of 100). But if receptors differ among humans and if the boundary between movement–no movement remains at 50%, then the population concentration–effect relation becomes less steep ( B ). At concentration A in ( B ), 100% of patients would move with incision; at B, 67% would move; at C, 33%; and at D, none would move. Although the resulting concentration–effect curve ( D ) is steep (P g of 10, a value found in MAC studies, see fig. 1), it is not infinitely steep. If the receptor concentration–effect curves are distributed more broadly [ C ; points A, B, C, and D have the same meaning as in ( B )], the resulting population concentration–effect curve is still less steep (P g 2.5) ( D ). The points (diamonds, triangles, and circles) were obtained by calculating the fraction of patients that would be immobile at 10, 20, 30, 40, etc. m M isoflurane.