Figure 31-6 Relationship between opioid and hypnotic drug concentrations and the probability of nonresponsiveness. 1, No chance of nonresponsiveness; 2, no chance of response; 3, hypnotic concentration-versus-response relationship in the absence of opioids; 4, hypnotic concentration-versus-response relationship in the presence of large doses of opioids; 5, area of maximum synergy between opioids and hypnotics; 6, in the absence of hypnotics, even profound levels of opioids cannot suppress the response; red line in the middle of the surface, the 50% isobole; red curve at the top of the surface, 95% isobole.