Fig. 5. Isobolograms for the combination of propofol and remifentanil on (  A ) mean phrenic nerve activity (PNA) and reflexly evoked changes in (  B ) mean heart rate (ΔHR) and (  C ) mean arterial pressure (ΔMAP). The solid diagonal lines are the additive lines constructed by joining the ED⁵⁰values (95% confidence intervals indicated on the axes) for each drug. The points on the lines are the predicted ED⁵⁰infusion rates of remifentanil causing depression of mean PNA (  open triangles ), ΔHR (  open squares ), and ΔMAP (  open circles ) if its interactions with propofol are additive. The symbols (  filled triangles , PNA;  filled squares , mean ΔHR;  filled circles , ΔMAP) are the observed ED⁵⁰values (95% confidence intervals indicated) for the effects of remifentanil in combination with propofol. The observed mean ED⁵⁰s for PNA (  A ) and ΔMAP (  C ) when remifentanil is combined with propofol are below and to the left of the additive line, whereas for mean ΔHR (  B ), it is on this line, indicating that their interactions on PNA and ΔMAP are synergistic, whereas for ΔHR they are additive.