Fig. 5. Examples of data fits of the effect of remifentanil on ventilation in one subject in the awake state (A –C ) and asleep with propofol (D –F ). (A, D ) Measured remifentanil concentration (closed circles ), pharmacokinetic data fits (thin line ), and estimated effect site (thick line ) concentrations. (B , E ) Minute ventilation with each spontaneous breath is depicted by open circles . Artificial breaths are depicted by closed squares . The lines  through the data are the data fits. During sleep (E ), the subject is apneic and requires artificial breathing assistance. (C , F ) End-tidal carbon dioxide values (open circles ) and data fits. (F ) The effect of the artificial ventilation is clearly visible in the data fits.