Intravenous boluses of fentanyl, 1 µg kg−1, and remifentanil, 0 Intravenous boluses of fentanyl, 1 µg kg−1, and remifentanil, 0.5 µg kg−1, give similar maximum ventilatory depression in awake volunteers  J. Gelberg, C. Jonmarker, O. Stenqvist, O. Werner  British Journal of Anaesthesia  Volume 108, Issue 6, Pages 1028-1034 (June 2012) DOI: 10.1093/bja/aes029 Copyright © 2012 The Author(s) Terms and Conditions

Fig 1 Set-up. The subject breathed through a mouthpiece, a heat–moisture exchanger (HME), and a resistance connected to a differential pressure transducer (DPT), which measured airway flow. Airway Pco2 was obtained via a sampling line. An open-ended 30 mm internal diameter flexible tube served as a reservoir. The apparatus deadspace between the point of entry of the FGF and the patient was 95 ml. FGF was initially 225 ml kg−1 min−1 and in order to induce CO2 rebreathing, it was then reduced to 75 ml kg−1 min−1 before injection of opioid. British Journal of Anaesthesia 2012 108, 1028-1034DOI: (10.1093/bja/aes029) Copyright © 2012 The Author(s) Terms and Conditions

Fig 2 Determination of minimum ventilation ( V ˙ nadir). Minute ventilation ( V ˙ ), expressed as percentage of preinjection ventilation ( V ˙ preinj), was plotted against time after injection. A smoothing algorithm was applied, and the value and time of V ˙ nadir (↓) were determined from the smoothed curve. The effect of remifentanil, 0.5 µg kg−1, in Subject 5 is shown. In this case, V ˙ nadir was 48% of V ˙ preinj and occurred 2.5 min after injection. British Journal of Anaesthesia 2012 108, 1028-1034DOI: (10.1093/bja/aes029) Copyright © 2012 The Author(s) Terms and Conditions

Fig 3 Method for finding the remifentanil dose that gave the same maximum ventilatory depression as 1 µg kg−1 of fentanyl. In this example (Subject 5), fentanyl injection resulted in a V ˙ nadir/ V ˙ preinj of 45%. The equidepressant remifentanil dose was 0.48 µg kg−1. British Journal of Anaesthesia 2012 108, 1028-1034DOI: (10.1093/bja/aes029) Copyright © 2012 The Author(s) Terms and Conditions

Fig 4 Mean minute ventilation ( V ˙ ) curves obtained after injection of fentanyl, 1.0 µg kg−1 (F 1.0), placebo (saline), and remifentanil, 0.25, 0.5, and 1.0 µg kg−1 (R 0.25, R 0.5, and R 1.0, respectively). British Journal of Anaesthesia 2012 108, 1028-1034DOI: (10.1093/bja/aes029) Copyright © 2012 The Author(s) Terms and Conditions

Fig 5 Minimum ventilation ( V ˙ nadir) after injection of opioid expressed as percentage of the preinjection value ( V ˙ preinj). Individual findings ordered in respect of the response to fentanyl. British Journal of Anaesthesia 2012 108, 1028-1034DOI: (10.1093/bja/aes029) Copyright © 2012 The Author(s) Terms and Conditions